Science : Chapter 2 : Force and Pressure
TEXTBOOK EXERCISES
I. Choose the best answer.
1. If we apply force against the direction of motion of the body, then the body will
a. stop moving
b. move with an increased speed
c. move with a decreased speed
d. move in a different direction
[Answer: (a) stop moving]
2. Pressure exerted by a liquid is increased by
a. the density of the liquid
b the height of the liquid column
c. Both a and b
d. None of the above
[Answer: (c) Both (a) & (b)]
3. Unit of pressure is
a. Pascal
b. Nm–2
c. Poise
d. Both a and b
[Answer: (d) Both (a) & (b)]
4. The value of the atmospheric pressure at sea level is
a. 76 cm of mercury column
b. 760 cm of mercury column
c. 176 cm of mercury column
d. 7.6 cm of mercury column
[Answer: (a) 76 cm of mercury column]
5. Pascal’s law is used in
a. hydraulic lift
b. brake system
c. pressing heavy bundles
d. All the above
[Answer: (d) All the above]
6. Which of the following liquids has more viscosity?
a. Grease
b. Water
c. Coconut oil
d. Ghee
[Answer: (a) Grease]
7. The unit of viscosity is
a. Nm2
b. poise
c. kgms–1
d. No unit
[Answer: (b) poise]
II. Fill in the blanks.
1. The pressure of a liquid column increases with the depth of the column.
2. Hydraulic lift works under the principle of Pascal’s Law.
3. The property of surface tension of a liquid surface enables the water droplets to move upward in plants.
4. A simple barometer was first constructed by Torricelli.
III. State true or false. If false, correct the statement.
1. Force acting on a given area is called pressure. [Answer: True]
2. A moving body comes to rest due to friction alone. [Answer: True]
3. A body will sink if the weight of the body is greater than the buoyant force. [Answer: True]
4. One atmosphere is equivalent to 1,00,000 newton force acting on one square metre. [Answer: True]
5. Rolling friction is slightly greater than the sliding friction. [ Answer: False]
6. Friction is the only reason for the loss of energy. [Answer: True]
Correct statement: Rolling friction is slightly lesser than the sliding friction.
7. Liquid pressure decreases with the decrease of depth. [Answer: True]
8. Viscosity depends on the pressure of a liquid. [Answer: True]
IV. Match the following.
a.
Static friction – Viscosity
Kinetic friction – Least friction
Rolling friction – Objects are in motion
Friction between the liquid layers – Objects are sliding
Sliding friction – Objects are at rest
[Answer: i-e, ii-c, iii-b, iv-a, v-d]
i. Static friction (e) objects are at rest
ii. Kinetic friction (c) objects are in motion
iii. Rolling friction (b) least friction
iv. Friction between the liquid layers (a) viscosity
V. Sliding friction (d) objects are sliding
b.
Barometer – reduce friction
Increasing area of contact – Atmospheric pressure
Decreasing area of contact – cause of friction
Lubricants – increases friction
Irregular surface – decreases friction
[Ans : i – b, ii – d, iii – e, iv – a, v – c]
i. Barometer (b) atmospheric pressure
ii. Increase friction (d) increasing area of contact
iii. Decrease friction (e) decreasing area of contact
iv. Lubricants (a) reduce friction
v. Irregular surface (c) cause of friction
V. Complete the analogy.
1. Knot in a thread : Static friction :: Ball bearing : rolling friction
2. Downward force : Weight :: Upward force offered by liquid : Buoyant force
VI. Numerical Problem.
1. A stone weighs 500 N. Calculate the pressure exerted by it, if it makes contact with a surface of area 25 cm2.
Answer:
Given : Weight of a stone F = 500 N
Area A=25 cm2 = 25 × 10-4 m2
To find : Pressure P =?
Formula : Pressure P = F/A
= 500 / [25×10-4]
Solution:
Pressure P = 20 × 104 Nm-2 (or) 20 × 104 Pa
2. In a hydraulic lift, the surface area of the input piston is 10 cm2. The surface area of the output piston is 3000 cm2. A 100 N force applied to the input piston raises the output piston. Calculate the force required to raise the output piston.
Answer:
Solution :
Pressure input on piston,
P = F/A
= 100/ [10×10-4] = 105 N
According to Pascal’s law
P= F/A
105 = F / [3000×10-4 ] = [ F×104 ] / 3000
104 × F = 105 × 3000
F = 3000 × 101
F = 3 × 104N
VII. Consider the statements given below and choose the correct option.
a. Both assertion and reason are true and reason is the correct explanation of assertion.
b. Both assertion and reason are true, but reason is not the correct explanation of assertion.
c. Assertion is true, but reason is false.
d. Both assertion and reason are false.
1. Assertion: Sharp knives are used to cut the vegetables.
Reason: Sharp edges exert more pressure.
[Answer: (a) Both assertion and reason are true and the reason is the correct explanation of the assertion]
2. Assertion: Broad straps are used in bags.
Reason: Broad straps last for long.
[Answer: (b) Both assertion and reason are true, but the reason is not the correct explanation of the assertion]
Correct explanation: The weight of the bags falls on larger area of shoulder. So lesser pressure is produced.
3. Assertion: Water strider slides easily on the surface of water.
Reason: Water strider experiences less buoyant force.
[Answer: (b) If both assertion and reason are true, but reason is not the correct explanation of assertion]
Correct explanation: It is due to the surface tension of water.
VIII. Answer very briefly.
1. Give two examples to verify that a force changes the shape of a body.
Answer:
Force can change the static condition of a body.
(i) If you squeeze a sponge, its shape changes.
(ii) If you pull a rubber band, it becomes longer.
2. Give two examples to verify that a force tends to change the static condition of a body.
Answer: Force can change the static condition of a body.
(i) A rest rubber ball begins to move, when a force applied on it.
(ii) Player applies a force on the stationary football while taking a penalty kick in football match. The force applied by player makes the football move towards the goal.
3. How do you feel when you touch a nail immediately after it is hammered into a wooden plank? Why?
Answer: The nail becomes hot due to friction. Friction changes kinetic energy to heat.
4. How does the friction arise between the surfaces of two bodies in relative motion?
Answer: The force of friction is arised by the interlocking of the irregularities of the two surfaces.
5. Name two instruments which help to measure the pressure of a fluid.
Answer:
(i) Manometer
(ii) Pressure gauge.
6. Define one atmosphere.
Answer: The pressure exerted by the mercury column is considered as the pressure of magnitude ‘one atmosphere’ (1 atm).
7. Why are heavy bags provided with broad straps?
Answer: Broader straps are provided on a back-pack for giving less pressure on the shoulders by providing a larger area of contact with the shoulder.
8. How does surface tension help a plant?
Answer: Water molecules rise up due to surface tension. Xylem tissues are very narrow vessels present in plants. Water molecules are absorbed by the roots and these vessels help the water to rise upward due to “capillarity action” which is caused by the surface tension of water.
9. Which has greater viscosity, oil or honey? Why?
Answer: Honey has greater viscosity.
Reason: Thicker liquids are more viscous than thinner liquids. As honey has greater viscosity, more frictional force will be acting on it.
IX. Answer briefly.
1. Define friction. Give two examples of the utility of friction in day to day life.
Answer:
Friction : Friction is a force that slows down moving objects or prevents stationary objects from moving.
Examples of the utility of friction in day to day life.
(i) Cars and buses are able to move safely on the road because of friction between the treaded tyres and the surface of the road.
(ii) We are able to write on paper only with the help of friction between the pencil or pen and paper.
2. Mention any three ways of minimising friction.
Answer:
(i) By using lubricants : These are applied to surfaces to reduce the friction between the surfaces.
Ex : Oil, wax, grease and castor oil.
(ii) With the help of polishing the surface :
We sprinkle fine powder on the carrom board and then we polish its surface to make smooth so that the striker slides easily on the surface.
(iii) By using ball bearing :
We use leadshots in bearing of a cycle hub because rolling friction is smaller than sliding friction.
3. State Pascal’s law and mention its applications.
Answer:
(i) In an automobile service station, the vehicles are lifted upward using the hydraulic lift, which works as per Pascal’s law.
(ii) The automobile brake system works according to Pascal’s law.
(iii) The hydraulic press is used to make the compressed bundles of cotton or cloth so as to occupy less space.
4. Why is a ball bearing used in a cycle hub?
Answer: The rolling friction is smaller than sliding friction, sliding is replaced by rolling with the usage of ball bearings. So lead shots are used in the bearing of a cycle hub to reduce the friction.
X. Answer in detail.
1. Friction is a necessary evil – Explain.
Answer:
Friction is a necessity in most of our day to day activities. It is desirable in most situations of our daily life.
(i) We can hold any object in our hand due to friction.
(ii) We can walk on the road because of friction. The footwear and the ground help us to .walk without slipping.
(iii) Writing easily with a pen on paper is due to friction.
(iv) Automobiles can move safely due to friction between the tyres and the road. Brakes can be applied due to frictional resistance on brake shoes.
(v) We are able to light a matchstick, sew clothes, tie a knot or fix a nail in the wall because of friction.
Though it is giving a negative effect, in most of our day to day life friction helps us to make our life easy. So, it is called as “necessary evil”.
Disadvantages of friction :
(i) Friction wears out the surfaces rubbing with each other, like screws and gears in machines or soles of shoes.
(ii) To overcome the friction an excess amount of effort has to be given to operate a machine. This leads to wastage of energy.
2. Give the different types of friction and explain each with an example.
Answer:
Friction can be classified into two basic types:
(i) Static friction
(ii) Kinetic friction.
(i) Static friction: The friction experienced by the bodies, which are at rest is called static friction. (E.g : All the objects rigidly placed to be at rest on the Earth, a knot in a thread.)
(ii) Kinetic friction: Friction existing during the motion of bodies is called kinetic friction.
Further, kinetic friction can be classified into two:
(i) Sliding friction
(ii) Rolling friction.
(i) Sliding friction: When a body slides over the surface of another body, the friction acting between the surfaces in contact is called sliding friction.
(ii) Rolling friction: When a body rolls over another surface, the friction acting between the surfaces in contact is called rolling friction.
Rolling friction is less than sliding friction. That is why wheels are provided in vehicles, trolleys, suitcases etc.
3. Describe an experiment to prove that friction depends on the nature of a surface.
Answer:
To understand about the frictional force between the layers of liquid in motion.
Materials required: Different kinds of liquid (coconut oil, honey, water, ghee), glass plates – 4 nos.
Procedure :
(i) Take a small quantity of different kinds of liquid like coconut oil, honey, water and ghee etc., in a cup.
(ii) Place one drop of each liquid on a separate glass plate.
(iii) Next, gently raise one end of the glass plate, one by one, so as to allow the liquid to slide down the smooth surface of the plate.
(iv) Observe the speed of each liquid.
Observation: Each liquid moves with a different speed. Water flows faster than other liquids. Coconut oil flows with a moderate speed. Ghee flows very slowly.
Inference: Between the layers of each liquid, in motion, there is a frictional force parallel to the layers of the liquid. This frictional force opposes the motion of the liquid layers while they are in motion.
4. Explain how friction can be minimised.
Answer:
(i) Using lubricants :
(1) A substance, which reduces the frictional force, is called a lubricant.
E.g : Grease, coconut oil, graphite, castor oil, etc.
(2) The lubricants fill up the gaps in the irregular surfaces between the bodies in contact. This provides a smooth layer thus preventing a direct contact between their rough surfaces.
(ii) Using ball bearing :
Since, the rolling friction is smaller than sliding friction, sliding is replaced by rolling with the usage of ball bearings. We can see lead shots in the bearing of a cycle hub.
5. Describe an experiment to prove that the pressure in a liquid increases with depth.
Answer: Take a plastic bottle. Punch three holes on its side in the same direction, but at different heights. Now pour some water into it and let it flow through the holes. Observe the flow of water.
Inference : The water comes out from all the holes with a different force and falls on the table at points that are at variable distances from the bottle. Water from the lowest hole comes out with the greatest force and falls at a point that is at the maximum distance from the bottle. Water from the topmost hole comes out with the least force and falls at the point that is at the minimum distance from the bottle.
Reason: This activity confirms that the pressure in a liquid varies with the depth of the point of observation in it.
XI. Higher Order Thinking Questions.
1. Why is it not advisable to use a fountain pen while travelling in an aeroplane?
Answer: Fountain pens are built in such a way that the pressure inside them balances the atmospheric pressure at sea level. Since atmospheric pressure decreases with an increase in height above sea level, the pressure inside the pen turns out to be much greater than the air pressure in an aeroplane and the pen stalls leaking.
2. Is there any possibility of making a special device to measure the magnitude of friction directly?
Answer: Yes. Tribometer is a special device to measure the magnitude of friction directly.
3. Vidhya feels that mercury is costly. So, instead of mercury she wants to use water as a barometric liquid. Explain the difficulty of constructing a water barometer.
Answer:
(i) Mercury is commonly used in barometers because of its high density means the height of the column can be a reasonable size to measure atmospheric pressure.
(ii) A barometer using water, for instance, would need to be 13.6 times taller than a mercury barometer to obtain the same pressure difference.
(iii) This is because mercury is 13.6 times more dense than water.
XII. Project Work.
Observe the devices, gadgets or things around you. List out the types of friction involved in each device. How would you minimise the friction? Record your observations and discuss your results with your classmates.
Numerical problems
Problem 1
The average weight of an elephant is 4000 N. The surface area of the sole of its foot is 0.1m2. Calculate the pressure exerted by one foot of an elephant.
Solution
Average weight of the elephant = 4000 N
Weight of one leg = Force exerted by one leg
= 4000/4 = 1000 N
Area of the sole of one foot = 0.1 m2
Pressure = Force/Area
= 1000/0.1 = 10000 N/m2 = 104 Nm-2
Pressure exerted by one leg of the elephant is 10,000 newton on one square metre.
1. A stone weighs 500 N. Calculate the pressure exerted by it, if it makes contact with a surface of area 25 cm2.
Answer:
Given : Weight of a stone F = 500 N
Area A=25 cm2 = 25 × 10-4 m2
To find : Pressure P =?
Formula : Pressure P = F/A
= 500 / [25×10-4]
Solution:
Pressure P = 20 × 104 Nm-2 (or) 20 × 104 Pa
2. In a hydraulic lift, the surface area of the input piston is 10 cm2. The surface area of the output piston is 3000 cm2. A 100 N force applied to the input piston raises the output piston. Calculate the force required to raise the output piston.
Answer:
Solution :
Pressure input on piston,
P = F/A
= 100/ [10×10-4] = 105 N
According to Pascal’s law
P= F/A
105 = F / [3000×10-4 ] = [ F×104 ] / 3000
104 × F = 105 × 3000
F = 3000 × 101
F = 3 × 104N
Student Activities
Activity 1
Fix a matrix of sharp pins on a wooden board in rows and columns. Take a big blown up balloon. Place it gently over the pins and place a small book on the top of the balloon. Will the balloon burst? Will the pins prick the balloon?
Answer:
Aim : To understand the effect of a force depends on the magnitude of the force and the area over which it acts.
Materials required : Sharp pins, wooden board, balloon.
Procedure :
(i) Fix a matrix of sharp pins on a wooden board in rows and columns.
(ii) Take a big blown up balloon.
(iii) Place it gently over the pins.
(iv) Place a small book on the top of the balloon.
(v) Observe what happens?
(vi) Will the balloon burst? Will the pins prick the balloon?
Inference :
(i) The balloon will not burst. If you prick the balloon with a single pin it will burst. But this did not happen even though many more pins were pricking the balloon.
(ii) A single pin produces a large pressure over a small area. But, when a large number of pins prick a body, each pin exerts very little pressure on the balloon, as the applied force gets distributed over a large surface of the body. So, the balloon will not burst.
Conclusion: We conclude that the effect of a force depends on the magnitude of the force and the area over which it acts.
More to know
Cooking in a place located at a higher altitude is difficult. Why? At a higher altitude, due to lack of atmospheric pressure the boiling point of a substance reduces. So, water boils even at 80°C. The thermal energy that is produced at this temperature is not sufficient enough for baking or cooking. So, cooking is difficult at higher altitude.
Activity 2
Take a conical flask and a well boiled egg, after removing its shell. Place the egg on the mouth of the flask. It will not enter the flask. Now take a piece of paper. Burn it and drop it inside the flask. Wait for a few seconds to burn fully. Now, keep the egg on the mouth of the flask. Wait for a few minutes. What do you observe?
When the paper is burning in the flask, the oxygen present in the air inside the conical flask is used up for its combustion. This reduces the pressure of the air in the flask. The air in the atmosphere tends to occupy the low pressure region in the flask. So, it rushes through the mouth of the flask, thus pushing the egg into the flask.
Answer:
Aim : To realise the atmospheric pressure.
Materials required: Conical flask, boiled egg, piece of paper.
Procedure:
(i) Take a conical flask.
(ii) Take a well boiled egg, after removing its shell.
(iii) Place the egg on the mouth of the flask.
(iv) It will not enter the flask.
(v) Take a piece of paper.
(vi) Burn it and drop it inside the flask.
(vii) Wait for a few seconds, let it burnt fully.
(viii) Now keep the egg on the mouth of the flask.
(ix) Wait for a few minutes.
Observation: The egg placed at the mouth of the flask gets compressed and it falls into the flask, due to the atmospheric pressure.
Inference:
(i) When the paper is burning in the flask, the oxygen present in the air inside the conical flask is used up for its combustion. This reduces the pressure of the air in the flask. The air in the atmosphere tends to occupy the low pressure region near the flask.
(ii) So, it rushes through the mouth of the flask, thus pushing the egg into the flask. Eventually, the egg falls down to the bottom of the flask.
Activity 3
Take a plastic bottle. Punch three holes on its side in the same direction, but at different heights. Now pour some water into it and let it flow through the holes. Observe the flow of water. Water from the lowest hole comes out with the greatest force and the water from the topmost hole comes out with the least force.
Answer:
Aim : To understand that the pressure in a liquid varies with the depth of the point of observation in it.
Materials required : Plastic bottle, Water
(i) Plastic bottle
(ii) Water
Procedure :
(i) Take a plastic bottle.
(ii) Punch three holes on its sides in the same direction but at different heights.
(iii) Now pour some water into it and let it flow through the holes.
(iv) Observe the flow of water.
Observation:
i) The water comes out from all the holes with a different force and falls on the table at points that are at variable distances from the bottle.
(ii) Water from the lowest hole comes out with the greater force and falls at a point that is at the maximum distance from the bottle.
(iii) Water from the top most hole comes but with the least force and falls at the point that is at the minimum distance from the bottle.
Inference: This shows that the pressure is a liquid varies with the depth of the point of observation on it.
Answer 2 : Take a plastic bottle. Punch three holes on its side in the same direction, but at different heights. Now pour some water into it and let it flow through the holes. Observe the flow of water.
Inference : The water comes out from all the holes with a different force and falls on the table at points that are at variable distances from the bottle. Water from the lowest hole comes out with the greatest force and falls at a point that is at the maximum distance from the bottle. Water from the topmost hole comes out with the least force and falls at the point that is at the minimum distance from the bottle.
Reason: This activity confirms that the pressure in a liquid varies with the depth of the point of observation in it.
This activity confirms that the pressure in a liquid varies with the depth of the point of observation in it.
Activity 4
Take a glass tube that is open at both ends. Fix a rubber balloon at the lower end of the tube. Pour some water into the tube and observe the balloon. Now, pour some more water into the balloon and again observe the balloon. The balloon starts bulging outwards.
Answer:
Aim : To demonstrate that liquid exerts pressure at the bottom of its container depends on the height of the liquid column in it.
Materials required ; Transparent glass tube, balloon, water.
Procedure :
(i) Take a glass tube that is open at both ends.
(ii) Fix a rubber balloon at the lower end of the tube.
(iii) Put some water into the tube and observe the balloon.
(iv) Now, pour some more water into the balloon and again observe the balloon.
Observation : The balloon starts bulging outwards. The bulge increase with an increase in the height of the water column.
Inference: The pressure exerted by a liquid at the bottom of a container depends on the height of the liquid column in it.
Activity 5
Take a plastic bottle. Punch three holes on its sides at the same height from its base. Now, pour some water into it and let it flow through the holes. Observe the flow of the water. Water comes out from all the holes with the same force and falls on the ground/ table, at the same distance from the bottle.
Answer:
Aim : To demonstrate that liquid exerts equal pressure at same depth.
Material required : Plastic bottle, water.
Procedure :
(i) Take a plastic bottle.
(ii) Punch three holes on its sides at the same height from its base.
(iii) Now, pour some water into it and let it flow through the holes.
(iv) Observe the flow of the water.
Observation : The water comes out from all the holes with the same force and falls on the ground / table, at the same distance from the bottle.
Inference : This activity confirms that liquids exert the same pressure in all directions at a given depth in their container.
Why dams are made stronger and broader at the bottom than at the top? Why do scuba divers wear a special suit while they go into deep sea levels?
Activity 6
Take a rubber ball and fill it with water. Make tiny holes on its surface with a pin at different points. Press anywhere on the ball. What do you observe?
Answer:
Aim : To demonstrate pressure applied on one point of liquid transmits equally in all directions.
Materials required : Rubber ball, water, pin.
Procedure :
(i) Take a rubber ball. Fill it with water.
(ii) Then take tiny holes on it with a pin at different points on its surface.
(iii) Press anywhere on the ball.
(iv) What do you observe?
Observation : There are identical streams of water flowing in all directions from the holes.
Inference : This is due to the phenomenon that the pressure, which is applied on the liquid, is equally transmitted in all directions.
Activity 7
Take some water in a beaker and spread a tissue paper on the surface of the water. Gently place the paper clip on the tissue paper. Observe what happens to the paper pin after some time.
After a few moments the tissue paper will submerge and the paper clip will make a small depression on the surface of the water. It will instantly begin to float on the surface, even though it is denser than water.
Answer:
Aim : To understand about surface tension property of liquid.
Materials required : Glass beaker, water, paper clip, tissue paper.
Procedure :
(i) Take a paper clip. Take a beaker of water.
(ii) Take a tissue paper and spread it on the surface of the water.
(iii) Gently, place the paper clip on the tissue paper.
(iv) Observe what happens to the paper pin after some time.
Observation : After a few moments the tissue paper will submerge and the paper clip will make a small depression on the surface of the water. It will instantly begin to float on the surface, even though it is denser than water.
Inference : This is due to the water molecules on the surface, which tend to contract themselves like the molecules of an elastic membrane. A force exists on them, which tends to minimize the surface area of water. The paper clip is balanced by the molecules on the water surface that is now behaving like a stretched elastic membrane. So, it does not submerge.
Activity 8
Take a small quantity of different kinds of liquid like coconut oil, honey, water and ghee etc. , Place one drop of each liquid on a separate glass plate. Now gently raise one end of the glass plate, so as to allow the liquid to slide down the smooth surface of the plate. Observe the speed of each liquid.
Answer:
Aim : To understand about the frictional force between the layers of liquid in motion.
Materials required : Different kinds of liquid (coconut oil, honey, water, ghee), glass plates – 4 nos.
Procedure :
(i) Take a small quantity of different kinds of liquid like coconut oil, honey, water and ghee etc., in a cup.
(ii) Place one drop of each liquid on a separate glass plate.
(iii) Next, gently raise one end of the glass plate, one by one, so as to allow the liquid to slide down the smooth surface of the plate.
(iv) Observe the speed of each liquid.
Observation : Each liquid moves with a different speed. Water flows faster than other liquids. Coconut oil flows with a moderate speed. Ghee flows very slowly.
Inference : Between the layers of each liquid, in motion, there is a frictional force parallel to the layers of the liquid. This frictional force opposes the motion of the liquid layers while they are in motion.
ACTIVITY
Make two groups of students. Let them stand along a straight line, one behind the other, on a playground. Start the game of “tug of war” with a rope. Observe the movement of the students.
Who are the winners?
Answer:
Aim: To understand the net effect of two forces acting on a body in opposite directions.
Procedure:
(iv) Make two group of students.
(v) Let them stand along a straight line, one behind the other, on a playground.
(vi) Start the game of ‘tug of war’ with a rope.
(vii) Observe the movement of the students.
Inference:
(i) The rope will move in the direction of the team that applies more force.
(ii) If the two teams apply the same force, the rope will not move at all.
ACTIVITY
Fill two identical syringes with water. Connect them with a plastic tube. Press gently on one end of a piston. What do you observe?
Answer:
Aim: To understand that the pressure exerted on a liquid at rest is transmitted equally to other portions of the liquid.
Materials required : Two identical syringes, water.
Procedure :
(i) Fill two identical syringes with water. Connect them with a plastic tube.
(ii) Press gently on one end of a piston.
(iii) What do you observe?
Observation: If one piston is pressed downward then the other piston will move up slightly, depending on the pressure given on the first piston.
Inference: The pressure exerted on a liquid at rest is transmitted equally to other portions of the liquid.
ACTIVITY
Answer:
Aim: To understand that the rolling friction is less than the sliding friction.
Materials required: Book, table (rough surface), cylindrical pencils.
Procedure :
(i) Push or slide a book on a rough surface. It is difficult to push it. Isn’t it?
(ii) Now, keep some cylindrical pencils underneath the book.
(iii) Again, push it. It is easy to move the book. Why?
Observation: When you push the book, the pencils roll in the direction of the applied force. They prevent the contact of the book with the rough surface. Rolling pencils offer the least amount of friction.
Inference: So, it is easy to displace the book in comparison with sliding it on the table. This method is often used in moving heavy wood from one place to another.
ACTIVITY
Answer:
Aim To understand the effect of the force of friction, which increases as the roughness the surface increases.
Materials required : Paper, glass, cotton cloth, wood, table, books, newspaper, writing pad, glass marbles in a bouch.
Procedure :
(i) Arrange some notebooks one over the other to form a platform, on a table.
(ii) Keep a wide scale, as a slide, such that one of its ends rests on the pile of books.
(iii) Take different kinds of materials like cotton cloth, plastic paper, newspaper, writing pad etc.
(iv) Place some glass marbles in a bowl placed on the table.
Experiment : First, keep a rectangular piece of paper near the end of the scale, which is in contact with the table. Now, release a glass marble from the top end of the scale such that it rolls down the scale. Allow the marble to roll over the piece of paper and finally, come to rest. Replace the rolling surface by placing the plastic sheet, wooden plank, cotton cloth.
Observation: Measure the distance travelled by the marble over the different objects using the meter scale.
Inference: In different objects each trial measure the distance travelled by the glass marble is tabulated. Tabulate the distance covered by the marble over different surfaces is different.