Friction Physics - Friction

The contact force exerted by a body A on another body B is equal to the normal force between the bodies. We conclude that:

1. The surfaces must be frictionless.
2. The force of friction between the bodies is zero.
3. The magnitude of normal force equals that of friction.
4. The bodies may be rough but they don't slip on each other.

Consider a vehicle going on a horizontal road towards east. Neglect any force by the air. The frictional forces on the vehicle by the road:

1. Is towards east if the vehicle is accelerating.
2. Is zero if the vehicle is moving with a uniform velocity.
3. Must be towards east.
4. Must be towards west.

Let F, F_N and f denote the magnitudes of the contact force, normal force and the friction exerted by one surface on the other kept in contact. If none of these is zero:

1. F > F_N
2. F > f
3. F_N > f
4. F_N - f < F < F_N + f

For most of the surfaces used in daily life, the friction coefficient is less than 1. Is it always necessary that the friction coefficient is less than 1?

What is the average friction force when a person has a usual 1km walk?

Why is it easier to push a heavy block from behind than to press it on the top and push?

Can a tug of war be ever won on a frictionless surface?

Can you accelerate a car on a frictionless horizontal road by putting more petrol in the engine? Can you stop a car going on a frictionless horizontal road by applying brakes?

A classroom demonstration of Newton's first law is as follows: A glass is covered with a plastic card and a coin is placed on the card. The card is given a quick strike and the coin falls in the glass.

1. Should the friction coefficient between the card and the coin be small or large?
2. Should the coin be light or heavy?
3. Why does the experiment fail if the card is gently pushed?

A body slipping on a rough horizontal plane moves with a deceleration of 4.0m/s². What is the coefficient of kinetic friction between the block and the plane?

When two surfaces are polished, the friction coefficient between them decreases. But the friction coefficient increases and becomes very large if the surfaces are made highly smooth. Explain.

Spring fitted doors close by themselves when released. You want to keep the door open for a long time, say for an hour. If you put a half kg stone in front of the open door, it does not help. The stone slides with the door and the door gets closed. However, if you sandwitch a 20g piece of wood in the small gap between the door and the floor, the door stays open. Explain why a much lighter piece of wood is able to keep the door open while the heavy stone fails.

You are standing with your bag in your hands, on the ice in the middle of a pond. The ice is so slippery that it can offer no friction. How can you come out of the ice?

A person (40kg) is managing to be at rest between two vertical walls by pressing one wall A by his hands and feet and the other wall B by his back. Assume that the friction coefficient between his body and the walls is 0.8 and that limiting friction acts at all the contacts.

1. Show that the person pushes the two walls with equal force.
2. Find the normal force exerted by either wall on the person. Take g = 10m/s².

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A block is projected along a rough horizontal road with a speed of 10m/s. If the coefficient of kinetic friction is 0.10, how far will it travel before coming to rest?

A car starts from rest on a half kilometer long bridge. The coefficient of friction between the tyre and the road is 1.0. Show that one cannot drive through the bridge in less than 10s.

The friction coefficient between an athelete's shoes and the ground is 0.90. Suppose a superman wears these shoes and races for 50m. There is no upper limit on his capacity of running at high speeds.

1. Find the minimum time that he will have to take in completing the 50m starting from rest.
2. Suppose he takes exactly this minimum time to complete the 50m, what minimum time will he take to stop?

In a children-park an inclined plane is constructed with an angle of incline 45° in the middle part. Find the acceleration of a boy sliding on it if the friction coefficient between the cloth of the boy and the incline is 0.6 and g = 10m/s².

Consider the situation shown in figure. Suppose a small electric field E exists in the space in the vertically upward direction and the upper block carries a positive charge Q on its top surface. The friction coefficient between the two blocks is g but the floor is smooth. What maximum horizontal force F can be applied without disturbing the equilibrium?
[Hint: The force on a charge Q by the electric field E is F = QE in the direction of E.]