Laws of Friction

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

A block is placed on a rough floor and a horizontal force $F$ is applied on it. The force of friction $f$ by the floor on the block is measured for different values of $F$ and a graph is plotted between them.

Mark the correct statements about the friction between two bodies.

In order to stop a car in shortest distance on a horizontal road, one should

Consider the situation shown in the figure. Suppose all the surfaces are rough. The direction of friction on $B$ due to $A$

Consider the situation shown in the figure. The wall is smooth but the surfaces of $A$ and $B$ in contact are rough. The friction on $B$ due to $A$, in equilibrium

A scooter, starting from rest, moves with a constant acceleration for a time $\Delta t_1,$ then with a constant velocity for the next $\Delta t_2$ and finally with a constant deceleration for the next $∆t_3$ to come to rest. A $500 \mathrm{N}$ man, sitting on the scooter behind the driver, manages to stay at rest with respect to the scooter without touching any other part. The force exerted by the seat on the man is

Why do tyres have a better grip of the road while going on a level road than going on an incline?

The 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 $\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$ $\mathrm{kg}$ 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 sandwich a $20 \mathrm{g}$ 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 a heavy stone fails.

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?

What is the average frictional force when a person has a usual $1 \mathrm{km}$ 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 should be less than $1$?