Friction Physics - Friction

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

While walking on ice, one should take small steps to avoid slipping. This is because smaller steps ensure:

1. Larger friction.
2. Smaller friction.
3. Larger normal force.
4. Smaller normal force.

Two cars of unequal masses use similar tyres. If they are moving at the same initial speed, the minimum stopping distance:

1. Is smaller for the heavier car.
2. Is smaller for the lighter car.
3. Is same for both cars.
4. Depends on the volume of the car.

In a situation the contact force by a rough horizontal surface on a body placed on it has constant magnitude. If the angle between this force and the vertical is decreased, the frictional force between the surface and the body will:

1. Increase.
2. Decrease.
3. Remain the saint.
4. May increase or decrease.

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

Why is it difficult to walk on solid ice?

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

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?

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

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.

A block of mass 2kg is pushed against a rough vertical wall with a force of 40N, coefficient of static friction being 0.5. Another horizontal force of 15N, is applied on the block in a direction parallel to the wall. Will the block move? If yes, in which direction? If no, find the frictional force exerted by the wall on the block.

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.

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?

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.

​​​​​​​

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?

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.

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?

Consider the situation shown in figure. Calculate:

1. The acceleration of the 1.0kg blocks.
2. The tension in the string connecting the 1.0kg blocks.
3. The tension in the string attached to 0.50kg.

A 2kg block is placed over a 4kg block and both are placed on a smooth horizontal surface. The coefficient of friction between the blocks is 0.20. Find the acceleration of the two blocks if a horizontal force of 12N is applied to:

1. The upper block.
2. The lower block. Take g = 10m/s².

Repeat part (a) of problem 6 if the push is applied horizontally and not parallel to the incline.