Laws of Motion
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5.21 A stone of mass 0.25 kg tied to the end of a string is whirled round in a circle of radius 1.5 m with a speed of 40 rev./min in a horizontal plane. What is the tension in the string ? What is the maximum speed with which the stone can be whirled around if the string can withstand a maximum tension of 200 N ?
5.21 A stone of mass 0.25 kg tied to the end of a string is whirled round in a circle of radius 1.5 m with a speed of 40 rev./min in a horizontal plane. What is the tension in the string ? What is the maximum speed with which the stone can be whirled around if the string can withstand a maximum tension of 200 N ?
Mass of the stone, m = 0.25 kg
Radius of the circular path = 1.5 m
Speed = 40 rev/min = 0.67 rev/s
Angular velocity, = 2 = 2 = 4.19 rev/s
The tension of the string, T = m r = 0.25 = 6.59 N
Maximum tension of the string, = 200 N
From the e
5.20 A batsman deflects a ball by an angle of 45° without changing its initial speed which is equal to 54 km/h. What is the impulse imparted to the ball? (Mass of the ball is 0.15 kg.)
5.20 A batsman deflects a ball by an angle of 45° without changing its initial speed which is equal to 54 km/h. What is the impulse imparted to the ball? (Mass of the ball is 0.15 kg.)

Velocity of the ball, u = 54 km/h = 15 m/s, mass of the ball, m = 0.15 kg
The ball is deflected back such that the included angle is 45
The initial momentum of the ball, along the direction of NO is = mu = 0.15 15 cos 22.5 = 2.0787 kg-m/s
The final mome
5.19 A shell of mass 0.020 kg is fired by a gun of mass 100 kg. If the muzzle speed of the shell is 80 m s-1, what is the recoil speed of the gun?
5.19 A shell of mass 0.020 kg is fired by a gun of mass 100 kg. If the muzzle speed of the shell is 80 m s-1, what is the recoil speed of the gun?
The mass of the shell, m = 0.020 kg
The mass of the gun, M = 100 kg
Speed of the shell, v = 80 m/s
The initial velocity of the shell and the gun = 0, so the initial momentum of the system = 0
Applying the law of conservation of momentum, the initial momentum = final momentum
0 = mv – MV, where V is the r
5.18 Two billiard balls each of mass 0.05 kg moving in opposite directions with speed 6 m s-1collide and rebound with the same speed. What is the impulse imparted to each ball due to the other?
5.18 Two billiard balls each of mass 0.05 kg moving in opposite directions with speed 6 m s-1collide and rebound with the same speed. What is the impulse imparted to each ball due to the other?
Mass of each ball, m = 0.05 kg
Initial velocity of each ball, u = 6 m/s
The initial momentum of each ball before collision = mu = 0.05 x 6 = 0.3 kg-m/s
After the collision, the ball changes the direction of motion without the change in magnitude of the velocity, so
The final momentum after collision of
5.17 A nucleus is at rest in the laboratory frame of reference. Show that if it disintegrates into two smaller nuclei the products must move in opposite directions.
5.17 A nucleus is at rest in the laboratory frame of reference. Show that if it disintegrates into two smaller nuclei the products must move in opposite directions.
Let m1 & m2 be the masses of two nuclei and m be the mass of the main nuclei.
m = m1 + m2
If and be the corresponding velocities of two nuclei then total linear momentum after disintegration = m1 + m2
Since at the initial stage, mass nuclei was at rest, so the initial linear momentum
5.16 Two masses 8 kg and 12 kg are connected at the two 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.
5.16 Two masses 8 kg and 12 kg are connected at the two 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.

Given, small mass, m1 = 8 kg. bigger mass, m2 = 12 kg
Let T be the tension in the string
The heavier mass will go downward and the lighter mass will go upward.
Applying Newton's 2nd law,
For mass m1, T –m1g = m1a …… (1)
For mass m2, m2g – T = m2a……. (2)
Adding (1) & (2), we get
(m2 - m1) g = (m1 + m2) a
a
5.15 Two bodies of masses 10 kg and 20 kg respectively kept on a smooth, horizontal surface are tied to the ends of a light string. A horizontal force F = 600 N is applied to (i) A, (ii) B along the direction of string. What is the tension in the string in each case?
5.15 Two bodies of masses 10 kg and 20 kg respectively kept on a smooth, horizontal surface are tied to the ends of a light string. A horizontal force F = 600 N is applied to (i) A, (ii) B along the direction of string. What is the tension in the string in each case?


Mass of the body A, m1 = 10 kg, mass of the body B, m2 = 20 kg, Horizontal force = 600 N
Total mass of the system, m =m1 + m2 = 30 kg
From the relation F = ma, we get a = 600 / 30 = 20 m/s2
(i) When the force is applied on A
F – T = m1a or T = F - m1a = 600 – 10 = 400 N
(ii) When the force is app
5.14 Figure 5.16 shows the position-time graph of a particle of mass 4 kg. What is the
(a) Force on the particle for t < 0, t > 4 s, 0 < t < 4 s?
(b) Impulse at t = 0 and t = 4 s? (Consider one-dimensional motion only)

5.14 Figure 5.16 shows the position-time graph of a particle of mass 4 kg. What is the
(a) Force on the particle for t < 0, t > 4 s, 0 < t < 4 s?
(b) Impulse at t = 0 and t = 4 s? (Consider one-dimensional motion only)

(a) For t<0, the distance covered by the particle x is zero. Hence force on the particle is zero. For t>4s, the particle is moving at a constant distance, so the force will be zero. For 0
(b) Impulse is given by the equation, Impulse = total change of momentum. At t = 0, u = 0, v = distance / time = ¾ = 0.75 m/s
Impulse = 4 (0.75-0) kg-m/s = 3 kg-m/s
Impulse at t=4, u = 0.75,
5.13 A man of mass 70 kg stands on a weighing scale in a lift which is moving
(a) Upwards with a uniform speed of 10 m s-1
(b) Downwards with a uniform acceleration of 5 m s-2
(c) Upwards with a uniform acceleration of 5 m s-2. What would be the readings on the scale in each case?
(d) What would be the reading if the lift mechanism failed and it hurtled down freely under gravity?
5.13 A man of mass 70 kg stands on a weighing scale in a lift which is moving
(a) Upwards with a uniform speed of 10 m s-1
(b) Downwards with a uniform acceleration of 5 m s-2
(c) Upwards with a uniform acceleration of 5 m s-2. What would be the readings on the scale in each case?
(d) What would be the reading if the lift mechanism failed and it hurtled down freely under gravity?
The mass of the man, m = 70 kg, g = 10 m/s2, Let W be the reading on the weighing scale
(a) When the lift is moving upward with uniform speed of 10 m/s, the acceleration = 0
The value of W is given by W = mg = 70 = 700 N
(b) The lift is moving downward with acceleration, a = 5m/s2 Us
5.12 A bob of mass 0.1 kg hung from the ceiling of a room by a string 2 m long is set into oscillation. The speed of the bob at its mean position is 1 m s-1. What is the trajectory of the bob if the string is cut when the bob is (a) At one of its extreme positions (b) At its mean position?
5.12 A bob of mass 0.1 kg hung from the ceiling of a room by a string 2 m long is set into oscillation. The speed of the bob at its mean position is 1 m s-1. What is the trajectory of the bob if the string is cut when the bob is (a) At one of its extreme positions (b) At its mean position?
(a) When the bob is at extreme position, the velocity at that point is zero. If the string is cut at that location, the bob will fall because of its own weight
(b) At the mean position, if the string is cut, due to horizontal velocity, the bob will act like a projectile and will fall on the ground,