Force of Rolling Friction on a Horizontal Plane

From our experience, we know that it takes very different amounts of a push/ pull to move each cubical block and a plastic ball initially kept on a table. This force is lesser in the case of a ball than for the box for a jerk. This is because for a ball, the resistive force offered by the surface, known as the rolling friction, is far lesser than the one for the box, known as the sliding friction. The following simple experiment is aimed to provide a measure of the strength of rolling friction. 

Table of Contents

• Aim

• Apparatus required

• Theory

• Procedure

• Observations

• Result

Aim

To measure the force of limiting rolling friction for a roller on a horizontal surface.

Apparatus Required

• Wooden block with hook on one side

• Set of small masses

• An elevated horizontal surface with a pulley at one end

• A lightweight pan with a hook on top

• Spring balance

• Light and strong thread (about 150 cm)

• Spirit level

• Lead rollers

Theory

Rolling friction is the resistive force coming into play to an object with a circular surface when it tends to roll over a given horizontal plane. Limiting rolling friction is the maximum value of rolling friction for the initial rolling of the object from rest.

This experiment aims to determine the limiting rolling friction for a given wooden block using different weights and consequent applied forces required to overcome the limiting value of friction between the block and the horizontal plane.

Procedure

1. Check if the pulley is frictionless. Apply oil if required.

2. Check the level of the horizontal surface using the spirit level. Spread lead rollers on the surface.

3. Weight the wooden block. Place it over the lead rollers.

4. Weight the pan. Tie one end of the thread with a pan and the other end with the hook of the wooden block kept on rollers. Pass the thread over the pulley so that the pan is hanging down the horizontal surface.

5. Put a small weight in the pan. Observe if the block starts rolling due to tension in the string.

A Simple Apparatus Consisting of a Block on Lead Rollers for Demonstration of the Friction.

6. If the block does not start rolling, put some more weight in the pan till the block begins to roll.

7. Note the total weight of the pan.

8. Put some weight over the block till the block stops. 

9. Increase the weight in the pan again till the block begins to roll. Note the observations.

10.  Repeat this step one more time.

Observations

Mass of the wooden block, W = ....... g = ....... kg

Mass of empty pan, P = ....... g = .......kg

Acceleration due to gravity, g =\[9.8\;m{s^{ - 2}}\]

Observation Table

Result

As the total weight to be pulled increases, the total force needed to pull also increases. Hence, the limiting rolling friction increases.

Precautions

1. The pulley should be smooth and frictionless.

2. The string should be taut and parallel to the surface.

3. Weight of the empty pan should be very less than that of the block.

Lab Manual Questions

1. How will you determine the coefficient of rolling friction \[{\mu _r}\] from the graph between rolling friction F and normal reaction N?

Ans: The limiting rolling friction F is the total force applied on the body to begin pulling it, while the normal reaction N is the total weight to be pulled. Mathematically, they are related as:

\[F = {\mu _r}N\]

With a graph drawn with F along the y-axis and N along the x-axis, the slope of the graph gives the coefficient of rolling friction \[{\mu _r}\].

Graph Between the Force of Limiting Friction and Normal Reaction.

2. Why do we grease the lead rollers and the surface on which they are placed?

Ans: Greasing the rollers and surface reduces the additional unwanted friction between the two so that we do not get errors in the measurement of the quantities of rolling friction and normal reaction. That’s why it becomes essential to grease the lead rollers and horizontal surfaces.

3. What will happen if the string does not happen to be taut?

Ans: If the pulling string is not taut, the body will not be pulled. So, the observer would not be able to measure the frictional force.

4. Is the rolling friction experienced by an object a constant of motion?

Ans: Yes, for a given object, the values of rolling and kinetic friction are constant.

Viva Questions

1. What is tension in the string? What is its SI unit?

Ans: Tension is the transmitted force through the string when pulled by forces from opposite ends. The SI unit of tension is Newton (N).

2. Why doesn’t the block move if we pull it with very less force?

Ans: The block doesn’t move if pulled with very less force, as a small force is insufficient to overcome the frictional force acting between the block and the surface.

3. How does friction come into play in rolling motion?

Ans: In rolling motion, the interlocking between the surfaces of the object and the surface gives rise to frictional force. Rolling friction comes into play when an object is freely rolling over a surface and not simply sliding. The resistive forces opposing this kind of motion of an object due to the surface and medium is known as rolling friction. The value of rolling friction is less than the sliding friction.

4. What is the limiting force of friction?

Ans: The maximum resistive force offered by the surface to the object just before the object begins to move is known as the limiting force of friction.

5. How will the result change if the string is heavy?

Ans: If the string happens to be heavy, the tension in the string varies from point to point and the observer will not obtain the correct reading of the frictional force.

6. Why does the block start moving after putting some weight in the pan?

Ans: After putting some weight in the pan, the force applied to pull the block increases. This overcomes the limiting force of friction offered by the surface on the block at one point of time and the block begins to move.

7. How does the area of contact affect the result?

Ans: The area of contact has no effect on the result, as the frictional force is independent of the contact area.

8. Specify the direction in which the frictional force acts.

Ans: The frictional force acts in a direction opposite to that of the motion of the object.

9. How does a lubricant between surfaces affect the value of friction between them?

Ans: A lubricant such as oil or grease reduces the frictional force between the surfaces.

10. How does the presence of dust affect the readings?

Ans: The presence of dust tends to increase the friction between surfaces. Hence, more weight in the pan will be required to move the block in a dusty environment slightly.

Practical-Based Questions

1. How are the mass and weight of a body related?

1. Same quantities with same units

2. Same quantities with different units

3. Differ by a factor of 9.8

4. None of the above

Ans: (C) 

\[Weight = Mass \times 9.8\]

2. Which among the following is the best choice of the type of string used in the above experiment?

1. Rubber band

2. Woollen thread

3. Cotton thread

4. Independent of the choice

Ans: (C) 

A cotton thread is most appropriate due to its strength, taut and less weight.

3. Streamlined bodies of marine creatures help them to:

1. Swim faster

2. Reduce viscous drag

3. Increase friction

4. Increase contact area

Ans: (B)

Streamlined bodies of marine creatures help them to reduce viscous drag.

4. The normal reaction acting on a block kept on a table is due to:

1. Ground on block

2. Table on block

3. Ground on table

4. Block on ground

Ans: (B)

The normal reaction acting on a block kept on a table is due to the table on the block.

5. Action and reaction forces do not cancel each other because

1. Action force overcomes reaction force

2. Reaction force overcomes action force

3. They act in same direction

4. They act on two different bodies

Ans: (D)

Action and reaction forces act on two different bodies.

6. If the forces acting on an object are balanced, then

1. There is no motion of the object

2. There is a constant motion of the object

3. The object starts accelerating

4. None of the above

Ans: (A)

There is no motion of the object if the forces acting are balanced because the net force will be zero.

7. If an applied force of 5N causes a body to move with constant velocity, then:

1. Frictional force is less than 5N

2. Frictional force is equal to 5N

3. Frictional force is greater than 5N

4. There is no frictional force

Ans: (A)

Frictional force is less than 5N.

8. The friction that comes into play between the surfaces of two objects when they are at rest is:

1. Static friction

2. Kinetic friction

3. Rolling friction

4. Viscous friction

Ans: (A)

The friction that comes into play between the surfaces of two objects when they are at rest is static friction.

9. The force of friction always acts ................ the direction of motion of the body.

1. Along

2. Opposite to

3. Perpendicular to

4. At an angle of\[60^\circ \]

Ans: (B)

The force of friction always acts opposite to the direction of motion of the body.

10. We tend to slip on a wet floor because:

1. Inertia of motion is small

2. Friction is very large

3. Friction is very small

4. Friction is zero

Ans: (C)

Due to small but non-zero friction, we tend to lose balance on a wet floor and slip.

Conclusion

From this experiment, we can say that friction plays an integral part of our day-to-day lives. The force of friction acts between every pair of bodies in contact and proves to us in both advantageous and disadvantageous ways as we have discussed. 

We also saw how this force is different from the fundamental forces in Physics and is mostly associated with indirect actions.