Laws of motion

Assertion reason
1. A: On a banked curved track, horizontal component of normal reaction provides
the necessary centripetal force
R: centripetal force is not always required for turning.
2. A: Two bodies having mass m1 and m 2 collide elastically the net momentum of the
system conserves whereas the individual momentums of the particles changes.
R: During elastic collision, by Newton’s third law of motion force on m1 is exactly equal
and opposite to force on m2.
3. A: Angle of repose is 300. Coefficient of friction between object and a surface is 1 1/√3.
R: Coefficient of friction is equal to tan (angle of repose).
4. A: In uniform circular motion the speed of the object doesn’t change. 1 R: Centripetal
force is responsible for changing the direction and magnitude of the speed.
5. A: Friction between the tyres and the road produces considerable wear and tear. To avoid
friction the curved road is given an inclination
R: When the road is banked the horizontal component of the normal reaction provides the
necessary centripetal force.
6. A: when we jump on to a Cushion we get less hurt. 1 R: when change in momentum of an
object takes place in more time it experiences less force.
7. A: when an object is placed on a surface the frictional force between the surface and the
object is more if the mass of the object is heavy.
R: static friction is directly proportional to normal reaction.
8. A: In a perfectly inelastic collision kinetic energy of the system is not 1 conserved.
R: In a perfectly in elastic collision momentum of the system is conserved.

1. If two objects of masses 2.5 kg and 100 kg experience the same force 5 N, what is the
acceleration experienced by each of them?
2. A block of mass m slides down the plane inclined at an angle 60° with an acceleration
g/2. Find the coefficient of kinetic friction?
3. To what angle must a racing track of radius of curvature 600 m be banked so as to be

4. The body of mass 10 𝑘𝑔 is placed on a wedge (fixed to the ground) inclined at 60o to the
suitable for a maximum speed of 180 km/h?

horizontal and released. What is the magnitude of velocity of the block after 1 sec?
5. A block of mass 2 kg is placed on a plane surface. The coefficient of static friction is .4.
When a 2.8 N force is applied on the block parallel to the surface, calculate the force of
friction between the block and surface. (7.84 N)
6. A body of mass 3kg travels according to the law x=at+bt2+ct3
where a=3 m/s , b=4 m/s2 , c= 5 m/s3. Calculate the force acting on the body at t = 2
seconds. (204 N)
7. The coefficient of friction between the two contact plane is √ 3 ,what is the angle of
friction between those two planes? (60o).
8. A block of mass 10 kg is sliding downwards on a inclined plane of angle 30° with
horizontal. The coefficient of kinetic friction between the block and surface is .5. Find the
accleration of the block (take g=9.8 m/s2)
9. A circular race track of radius 300 m is banked at an angle of 15°. If the coefficient of
friction between the wheels of the race case and road is .2 ,find the below optimum speed
of the race car to avoid wear and tear on its tyres (28.1m/s)
10. What is the maximum permissible speed to avoid slipping(38.1m/s)
11. A block of mass 2 kg rests on a inclined plane at an angle 30° with the horizontal. The
coefficient of friction between the block and surface is .7. What will be the frictional
force acting on the block. (11.9 N)
12. A 75 Kg man stands in a lift.what force does the floor exert on him when the elevator
starts moving upwards with an acceleration of 2.0 m/s2 . Take g=10 m/s2(900N)
13. A person of mass 50 kg stands on a weighing scale on a lift. If the lift is descending with
a downward acceleration of 9 m/s2, what would be the reading of the weighing scale?
Take g=10 m/s2(5kg)
14. A block placed on a rough horizontal surface is pulled by a horizontal force F. Let f be the
force applied by the rough surface on the block. Plot a graph of f versus F.
15. A car of mass 1000kg travelling at 32m/s dashes into the rear of the truck of mass 8000kg
moving in the same direction with a velocity of 4m/s. After the collision the car bounces
with a velocity of 8 m/s. What is the velocity of the truck after the impact. (9m/s)
16. A body having mass of .4 kg is whirled in a vertical circle making 2 revolutions per
second. if the radius of the circle is 1.2m . Find the tension in the string when body is
a. At the top of the circle (71.88N)
b. At the bottom of the circle (79.32N)
17. Two masses of 5 kg and 3 kg are suspended with help of massless inextensible strings as
shown in below figure. Calculate T1 and T2 when whole system is going upwards with
acceleration = 2 m/s2 (use g =9.8 m/s2).
 18. A rocket of initial mass 6000kg ejects mass at a constant rate of 16kg/s with the constant
relative speed of 11 km/sec. What is the acceleration of the rocket a minute after the blast.
Neglect gravity
19. The velocity of a body of mass 2 kg as a function of t is given by
v=2ti+t2j. Find the momentum and the force acting on it, at time t= 2s. (8(i+j), 4(i+2j).
20. A block of mass 10kg is placed on a smooth wedge of inclination 30o. The whole system
is accelerated horizontally so that the block does not slip on the wedge. Calculate the
force exerted by the wedge on the block has a magnitude.
21. Two masses 10kg and 15 kg are connected to the ends of a light inextensible string that
goes over a frictionless pulley. Find the acceleration of the masses and the tension in the
string when the masses are released.
22. A block of mass 10 kg is sliding on a surface inclined at an angle of 30° with the
horizontal. If the coefficient of friction between the block and the surface is 0.5, find the
acceleration produced in the block. (Ans. 0.657 ms-2)
23. A block of mass 2 kg rests on a plane inclined at an angle of 30° with the horizontal. The
coefficient of friction between the block and the surface is 0.7. What will be the frictional
force acting on the block? (Ans. 11.9 N)

1.What is the unit of coefficient of friction? Ans: It has no unit.

2.Name the factor on which coefficient of friction depends?

Ans:Coefficient of friction depends on the nature of surfaces in contact and

nature of motion.

3.What provides the centripetal force to a car taking a turn on a level road?
Ans: Centripetal force is provided by the force of friction between the tyres and the road.

4.Why is it desired to hold a gun tight to one’s shoulder when it is being fired?
Ans: Since the gun recoils after firing so it must be held lightly against the shoulder because

gun and the shoulder constitute one system of greater mass so the back kick will be less.

5.Why does a swimmer push the water backwards?

Ans: A swimmer pushes the water backwards because due to reaction of water he is able to

swim in the forward direction

6.Friction is a self adjusting force. Justify.

Ans: Friction is a self adjusting force as its value varies from zero to the maximum value to
limiting friction.

7.A thief jumps from the roof of a house with a box of weight W on his head. What will be the
weight of the box as experienced by the thief during jump?

Ans: Weight of the box W = m (g – a) = m (g – g) = 0.

8.Which of the following is scalar quantity? Inertia, force and linear momentum.

Ans:Inertia and linear momentum is measured by mass of the body and is a vector quantity and
mass is a scalar quantity.

9.Action and reaction forces do not balance each other. Why?

Ans:Action and reaction do not balance each other because a force of action and reaction

acts always on two different bodies.

10.If force is acting on a moving body perpendicular to the direction of motion, then what will be
its effect on the speed and direction of the body?

Ans:No change in speed, but there can be change in the direction of motion.
11.The two ends of spring – balance are pulled each by a force of 10kg.wt. What will be

the reading of the balance?

Ans:The reading of the balance will be 10kgwt.

12.A lift is accelerated upward. Will the apparent weight of a person inside the lift increase,
decrease or remain the same relative to its real weight? If the lift is going with uniform speed,
then?

Ans:The apparent weight will increase. If the lift is going with uniform speed, then the apparent
weight will remain the same as the real weight.

Ans. (i) When a particle connected to a string revolves in a circular path around a centre, the
centripetal force is provided by the tension produced in the string. Hence, in the given case, the
net force on the particle is the tension T, i.e.,

F=T=
Where F is the net force acting on the particle.
14. If, in Exercise 5.21, the speed of the stone is increased beyond the maximum permissible
value, and the string breaks suddenly, which of the following correctly describes the trajectory of
the stone after the string breaks:

(a) the stone moves radially outwards,

(b) the stone flies off tangentially from the instant the string breaks,

(c) the stone flies off at an angle with the tangent whose magnitude depends on the speed of the
particle ?

Ans.(b)When the string breaks, the stone will move in the direction of the velocity at that instant.
According to the first law of motion, the direction of velocity vector is tangential to the path of
the stone at that instant. Hence, the stone will fly off tangentially from the instant the string
breaks.