LAWS OF MOTION
Very Short Question Answers
Q1. What is inertia? What gives the measure of inertia?
Ans. Inertia: The inability of a body to change its state by itself is known as inertia. Mass is the measure of inertia.
Q2. According to Newton’s third law, every force is accompanied by an equal and opposite force. How can a movement ever take place?
Ans. According to newton’s third law of motion action and reaction act on two different bodies. Hence motion is possible
Q3. When a bullet is fired from a gun, the gun gives a kick in the backward direction.Explain?
Ans. According to the law of conservation of linear momentum.
$m_{1}u_{1}+m_{2}u_{2}=m_{1}v_{1}+m_{2}v_{2}$
$0=m_{1}v_{1}+m_{2}v_{2}$
$ m_{1}v_{1}=\,-\,m_{2}v_{2}$
minus sign indicates gun and bullet move in opposite direction after fired. Hence, gun will
kick backwards.
Q4. Why does a heavy rifle not recoil as strongly as a light rifle using the same cartridges?
Ans. According to law of conservation of linear momentum
Hence heavy rifle recoil less as compare to light rifle.
Q5. If a bomb at rest explodes into two pieces, the pieces must travel in opposite directions. Explain?
Ans. According to the law of conservation of linear momentum.
$m_{1}u_{1}+m_{2}u_{2}=m_{1}v_{1}+m_{2}v_{2}$
$0=m_{1}v_{1}+m_{2}v_{2}$
$ m_{1}v_{1}=\,-\,m_{2}v_{2}$
minus sign indicates two pieces move in opposite direction after explosion.
Q6. Define force. What are the basic forces in nature?
Ans. Force: The physical quantity that changes or tries to change the state of rest or of uniform
motion along a straight line is called force.
The basic forces in nature are :
- Gravitational force
- Electromagnetic force
- Strong nuclear force
- Weak nuclear force.
Q7. Can the coefficient of friction be greater than one?
Ans. Yes. The coefficient of friction is greater than one.
Q8. Why does the car with flattened tyre stop sooner than the one with inflated tyres?
Ans. A flattened tyre has got more surface area of contact due to deformation than an inflated tyre. Hence friction is more in the case of a flattened tyre.
Q9. A horse has to pull harder during the start of the motion than later. Explain?
Ans. Static friction is more than kinetic friction. To overcome static friction initially horse
should apply more force than later.
Q10. What happen to the coefficient of friction, if the weight of the body is doubled?
Ans. Coefficient of friction (𝜇) is independent of mass of the body. Therefore, coefficient of
friction does not change.
Short Question Answers
Q1. A stone of mass 0.1kg is thrown vertically upwards. Give the magnitude and direction of the net force on the stone a) During Its Upward Motion. b) During Its Downward Motion. c) At the highest point, where it momentarily comes to rest?
Ans.
stone mass $m=0.1$ kg acceleration due to gravity $g=9.8$ $m/s^2$
Force acting on stone in three conditions is same
That is $F=ma=0.1×9.8=0.98 N$
Q2. Define the terms momentum and impulse. State and explain the law of conservation of linear momentum. Give its exampls?
Ans.- Considering the system of gun and bullet, both are at rest before the gun fires the bullet. After firing the bullet, their momentum is equal in magnitude and opposite in direction and thus the total momentum after firing is also zero.
- Similarly when a bomb at rest explodes into two unequal pieces, their momentum is equal and opposite.
Q3. Why are shock absorbers used in motor cycles and cars?
Ans. Shock absorbers are used in motor cycles and cars to increase time interval of jerk. As a result the momentum rate slows down thus less force act during the jerk.
Q4. Explain the terms limiting friction, dynamic friction and rolling friction?
Ans. Limiting friction: The maximum value of static friction is known as Limiting friction.
$f_{L}=μ_L N$
Dynamic Friction: The force which is resisting the motion of sliding between the two surfaces is called as Dynamic friction (O r Kinetic friction).
$f_k=μ_k N$
Rolling friction: The force which is resist the motion of rolling of a object on surface is called as Rolling friction.
$f_r=μ_r N$
Q5. Explain advantages and disadvantages of Friction?
Ans.
| Advantages | Disadvantages |
| Walking is not possible without friction | Due to friction wear and tear of machine parts increases |
| Breaks of vehicles working without friction | Due to friction life time of machine decrease |
| Nuts and Bolts holdings the surface due to friction | Due to friction energy is lost and hence reduces its efficiency |
Q6. Mention the methods used to decrease friction?
Ans. Polishing: polishing will reduce the static friction.
Lubricants: lubricating the surfaces can reduce kinetic friction.
Ball bearings: Use of ball bearing will reduce the rolling friction
Streamlining: streamlining design of vehicles will reduce air resistance.
Q7. Sate the laws of rolling Friction?
Ans.- Rolling friction is directly proportional to the Normal. $ f_r∝N$
- Rolling friction is directly proportional to area of contact. $f_r∝A$
- Rolling friction is inversely proportional to the radius of wheel.$ f_r∝1/r$
Q8. Why is pulling the lawn roller is preferred to pushing it?
Ans. As shown in the diagram a lawn roller is pulling by a force F making an angle θ with respect to horizontal. Then the normal reaction is
$N+F sinθ=mg$
$N=mg-F sinθ$
Rolling friction $ f_r=μ_r N$
$f_r=μ_r (mg-F sinθ )$
Pushing: As shown in the diagram a lawn roller is pushing by a force F, making an angle θ with respect to horizontal. Then the normal reaction is
$ N=F sinθ+mg$
Rolling friction $ f_r^{'}=μ_r N $
$ f_r^{'}=μ_r (mg+F sinθ ) $
Therefore, $ f_r<f_r^{'} $ hence pulling is preferred than pushing.
Q9. State Newton’s second law of motion. Hence derive the equation of motion F = ma from it.
Ans.