Conceptual Questions - Chapter 3 Laws of Motion 11th Science Guide Samacheer Kalvi Solutions - SaraNextGen [2024-2025]

Updated On May 15, 2024

By SaraNextGen

Conceptual Questions
Question 1.
Why it is not possible to push a car from inside?
Answer:
While trying to push a car from outside, he pushes the ground backwards at an angle. The ground offers an equal reaction in the opposite direction, so car can be moved. But the person sits inside means car and the person becomes a single system, and the force given will be a internal force. According to Newton's third law, total internal force acting on the system is zero and it cannot accelerate the system.
Question 2.
There is a limit beyond which the polishing of a surface increases frictional resistance rather than decreasing it why?
Answer:

Friction is a contact force. Friction is directly proportional to area of contact. In the normal surfaces there are bumps to interlock the surfaces in contact. But the surfaces are polished beyond certain limit. The area of contact will be increased and the molecules come closer to
each other. It increases electrostatic force between the molecules. As a result it increases friction.
Question 3.
Can a single isolated force exist in nature? Explain your answer.
Answer:
No. According to Newton's third law, for every action, there is an equal and opposite reaction. So, whatever case we consider, if there is an action there is always a reaction. So it is impossible.
Question 4.
Why does a parachute descend slowly?
Answer:
A parachute descends slowly because the surface area of parachute is large so that air gives more resistance when it descends down.
Question 5.
When walking on ice one should take short steps. Why?
Answer:
Let $R$ represent the reaction offered by the ground. The vertical component $R \cos \theta$ will balance the weight of the person and the horizontal component $R \sin \theta$ will help the person to walk forward.
Now, normal reaction $=\mathrm{R} \cos \theta$
Friction force $=\mathrm{R} \sin \theta$
Coefficient of friction, $\mu=\frac{R \sin \theta}{R \cos \theta}=\tan \theta$
In a long step, $\theta$ is more. So $\tan \theta$ is more. But $\mathrm{p}$ has a fixed value. So, there is danger of slipping in a long step.

Question 6 .
When a person walks on a surface, the frictional force exerted by the surface on the person is opposite to the direction of motion. True or false?
Answer:
False. In frictional force exerted by the surface on the person is in the direction of his motion. Frictional force acts as an external force to move the person. When the person trying to move, he gives a push to ground on the backward direction and by Newton's third law he is pushed by the ground in the forward direction. Hence frictional force acts along the direction of motion.
Question 7.
Can the coefficient of friction be more than one?
Answer:
Yes. The coefficient of friction can be more than one in some cases such as silicone rubber. Coefficient of friction is the ratio of frictional force to normal force, i.e. $F=\mu \mathrm{R}$. If $\mathrm{p}$ is greater than one means frictional force is greater than normal force. But in general case the value is usually between 0 and 1 .
Question 8.
Can we predict the direction of motion of a body from the direction of force on it?
Answer:
Yes. The direction of motion is always opposite to the force of kinetic friction. By using the principle of equilibrium, the direction of force of static friction can be determined. When the object is in equilibrium, the frictional force must point in the direction which results as a net force is zero.
Question 9.
The momentum of a system of particles is always conserved. True or false?
Answer:
True. The total momentum of a system of particles is always constant i.e. conserved. When no external force acts on it.