CBSE Physics Experiment To Study the Relationship Between the Normal Reaction and the Limiting Friction

Physics Experiment – To Study the Relationship Between the Normal Reaction and the Limiting Friction and to Find the Coefficient of Friction Between a Block and a Horizontal Surface

A force known as the force of friction develops as a reaction to the applied force when two surfaces come in contact. When bodies are at rest, static friction takes place and the experimental observations manifest that the coefficient of static friction is the limiting frictional force, which is a dimensionless constant, but depends on the nature of surfaces in contact.

In daily life static friction helps us to stand on a surface and rest the objects on a table.

Table of Content

• Aim

• Theory

• Observations

• Results

Aim

1. To find the relationship between the normal reaction and the limiting friction force.

2. To find the frictional coefficient between a block and a horizontal surface.

Theory 

When two objects touch each other while at rest, static friction takes place. The least amount of force needed to move the body from rest is the limiting value of limiting friction force. There is a stage where the body is almost ready to move when the external force F is increased. Limiting friction force is the force of friction at this point, when it is at its greatest.

The force resisting motion is referred to as the kinetic or sliding friction when the applied force F, is raised further (beyond the limiting frictional force). Limiting friction is greater than kinetic friction. The coefficient of kinetic friction is the measurement of the force of kinetic friction.

Experimentally it is found that limiting frictional force is

${f_s} = {\mu _s}Mg$

Where the coefficient of friction is being taken as $\mu _{s}$ .Although it has no dimensions, it depends on the kind of surfaces coming into touch. Here, the force which is being applied on the kinetic friction${f_k} = {\mu _k}R$, where the coefficient of kinetic friction is being taken as${\mu _k}$

Therefore, ${\mu _k} < \;\;{\mu _s}$

Three laws of static frictions states that:

1. The nature and state of the polished two surfaces in contact determine the amount of limiting friction.

2. As long as the usual reaction doesn't change, the amount of limiting friction is independent of the area and shape of the surfaces in contact.

3. The amount of normal reaction (R) between the two surfaces in contact is directly proportional to the amount of limiting friction (F).

Therefore, we can say that

$F \propto R$

$F = R$

So, coefficient of friction will be, $\mu  = \dfrac{F}{R}$

Apparatus Required

• A wooden block

• 30 g or 60 g of slotted weight

• Tabletop or other horizontal surface with a frictionless pulley attached to one end of it

• Thread

• Spring balance

• Weight box

Procedure

1. Least count and range of the spring balance should be determined first.

2. Place the wooden block on the table with the frictionless pulley at one end and use a spring balance to determine its mass (see the below figure for reference).

3. Pass a thread over the pulley and tie one end to the hook on the wooden block.

4. Allow the thread's other free end to hang vertically by tying it to the weight hanger.

5. Increase the mass until the block barely begins to slide when the table is tapped.

6. Record the total weights added in the observation table after adding up the total mass added to the weight hanger and multiplying it by the acceleration caused by gravity, g. These results should be in the frictional limiting force, F.

7. The mass of the wooden block, M, along with the acceleration brought on by gravity, g, are multiplied to produce the normal reaction, R.

8. The experiment should be repeated with various masses on the block, and in each instance, the estimated value is confirmed to be a constant for the specified pair of surfaces.

9. Draw a graph with the limiting friction, F, along the Y axis and the normal reaction, R, along the X axis. The graph will be an upward-sloping straight line.

10. By measuring the slope of the graph, the coefficient of friction can also be determined. $\mu  = \dfrac{F}{R}$

Relationship Between the Force of Limiting Friction and Normal Reaction

Observation

Weight of the wooden block, W=_________N

Results

1. The value of limiting friction is precisely related to normal reaction since $\dfrac{F}{R}$  is a constant.

2. The coefficient of friction between two specified surfaces, $\mu$ =_________

Precautions

1. The surface on which the wooden block will be placed must be dust free and plane.

2. Between the block and the pulley, the thread portion should be horizontal.

3. The pulley should have less friction.

Lab Manual Questions

1. Why is the thread between the block and the pulley kept horizontal?

Ans: By pulling the block horizontally, the combined weight of the pan and weights becomes effective and therefore the thread remains horizontal.

2. What would happen if the pulley had friction?

Ans: In a frictionless pulley the pan's weight and the effectiveness of the weights is maintained without any loss, however, for Pulley's effective value will be decreased by friction.

3. In what way does frictional force follow?

Ans: In the way opposite to the direction of motion of the object.

4. What kind of friction occurs when a body rolls over another body?

Ans: Rolling friction

Viva Questions

1. What is limiting friction's alternate name?

Ans: The amount of friction is at its highest. The term "limiting friction" also refers to the friction's maximum value.

2. What differentiates limiting friction from static friction?

Ans: Due to the fact that static friction acts when a body is lying over the surface of another body without moving and it is a self-adjusting force. The greatest amount of static friction known as limiting friction, is reached when that body exceeds the force of static friction.

3. Find the normal force acting on a body whose limiting friction is 60 N and whose coefficient of friction is 12 N.

Ans: We know that, $F = \mu N$

Where F and $\mu$ are the limiting friction and coefficient of limiting friction respectively,

Taken, N as the normal force,$N = \dfrac{F}{\mu } = \dfrac{{60}}{{12}} = 5\;N$

4. Why do roads get slippery after it rains?

Ans: Between our feet and the road, a thin layer of rainwater accumulates. It functions as a lubricant to stop imperfections on the surface of the wheel and the road from interlocking. Slipping occurs as the amount of friction decreases.

5. Why are brake surfaces kept flat when the area of the surface doesn't increase friction?

Ans: More surface area under the same force results in less pressure. It doesn't leave scratches on the surface being braked.

6. What happens to the coefficient of friction when the weight of the body is being doubled?

Ans: The coefficient of friction remains the same when the weight of the body is being doubled.

7. What role does lubricant serve in reducing the friction?

Ans: A thin layer of lubricant is created between the two surfaces that are in contact. Consequently, liquid friction, which is significantly smaller, plays the role of sliding friction.

8. What makes ball bearings so effective in reducing friction?

Ans: The reason for this is that ball bearings converts sliding friction into rolling friction.

9. Why does there come a limit after which further polishing of a surface increases the frictional resistance rather than decreasing it?

Ans: Normally, friction reduces as surface smoothness improves; however, when a surface is made excessively smooth, binding force of adhesion increases and friction increases. Therefore, the term given for this phenomena is cold welding.

10. Why in hilly areas, sand is thrown on a track covered in snow?

Ans: It is done so to prevent the tires of a vehicle from slipping on the snow as sand increases the force of friction when it is being thrown on the snow.

Practical Based Questions

1. What surface will have the least friction out of the following?

1. Plywood

2. Plastic

3. Ice

4. Vinyl floor

Ans: C) Ice

2. Out of which factor from the following causes bicycle tyres to wear out?

1. Electrostatic force

2. Muscular force

3. Frictional force

4. None of the above

Ans: C) Frictional force

3. There is a very thin layer of lubricant between the two surfaces in contact when this occurs, and this is referred to as,

1. Rolling friction

2. Greasy friction

3. Film friction

4. Solid friction

Ans: C) Film friction

4. How can friction be reduced?

1. Oiling

2. Polishing

3. Use of ball bearings

4. All of the above

Ans: D) All of the above

5. When a body is just about to slide over the surface of another body, the most significant amount of static friction that can be measured is known as

1. Sliding friction

2. Limiting friction

3. Rolling friction

4. None of the above

Ans: B) limiting friction

6. The reason why meteors burn up in the atmosphere is

1. Gravitational force

2. Electrostatic force

3. Magnetic force

4. Frictional force

Ans: D) Frictional force

7. What is the most common way to frequently to lessen friction?

1. Lubrication

2. Elimination of the motion

3. Warning signs

4. None of the above

Ans: A) Lubrication

8. When does the force of friction come into play, if the surfaces in contact tend to move or move with respect to each other?

1. If all the objects are solid

2. One of the two object is in gaseous form

3. One of the two object is in solid form

4. regardless of the objects' state as solids, liquids, or gases.

Ans: D) regardless of the objects' state as solids, liquids, or gases.

9. On what conditions, force of friction depends on,

1. Materials of objects in contact

2. Nature of the surface in contact

3. Both A and B

4. None of the above

Ans: C) Both A and B

10. From which of the following, increasing the friction is very necessary?

1. Handle bar of the bicycle

2. Ball bearings in the front wheel

3. Chain of a bicycle

4. None of the above

Ans: A) Handle bar of the bicycle

Conclusion

This experiment tells the coefficient of friction between a block and a horizontal surface in order to analyse the relationship between the force of limiting friction and normal reaction. A force opposing the object originates in response to the applied force and acts in the opposite direction when a body tries to slide over another body. Between two bodies in touch with one another, there occurs static friction. The limiting amount of static friction is referred to as "limiting friction."