# NCERT Physics Class 11 Exemplar Ch 5 Laws Of Motion Part 4

Q. 19. A person driving a car suddenly applies the brakes on seeing a child on the road ahead. If he is not wearing seat belt, he falls forward and hits his head against the steering wheel. Why?

Answer: The only retarding force that acts on him, if he is not using a seat belt comes from the friction exerted by the seat. This is not enough to prevent him from moving forward when the vehicle is brought to a sudden halt.

Q.20. The velocity of a body of mass 2 kg as a function of t is given by . Find the momentum and the force acting on it, at time

Q.21. A block placed on a rough horizontal surface is pulled by a horizontal force . Let f be the force applied by the rough surface on the block. Plot a graph of .

Answer: f = F until the block is stationary.

f remains constant if F increases beyond this point and the block starts moving.

Q.22. Why are porcelain objects wrapped in paper or straw before packing for transportation?

Answer: In transportation, the vehicle say a truck, may need to halt suddenly. To bring a fragile material, like porcelain object to a sudden halt means applying a large force and this is likely to damage the object. If it is wrapped up in say, straw, the object can travel some distance as the straw is soft before coming to a halt. The force needed to achieve this is less, thus reducing the possibility of damage.

Q.23. Why does a child feel more pain when she falls down on a hard cement floor, than when she falls on the soft muddy ground in the garden?

Answer: The body of the child is brought to a sudden halt when she/he falls on a cement floor. The mud floor yields and the body travels some distance before it comes to rest , which takes some time. This means the force which brings the child to rest is less for the fall on a mud floor, as the change in momentum is brought about over a longer period.

Q.24. A woman throws an object of mass 500 g with a speed of 25 m s1.

(a) What is the impulse imparted to the object?