1. Internal forces have no role to play in acceleration.
a) True
b) False
Explanation: The cause of every accelerated motion is an external force. Internal forces have no role to play.
2. A bullet of mass 0.04kg moving with a speed of 90m/s enters a wooden block and is stopped after a distance of 60cm. What is the average resistive force exerted by the block on the bullet?
a) 15N
b) 54N
c) 22.50N
d) 270N
Explanation: m = 0.04kg, u = 90m/s
v = 0, s = 60cm = 0.60m
v2-u2 = 2as
a = -6750m/s2
Therefore, retardation force = 0.04×6750 = 270N.
3. A scooterist moving with a speed of 36km/h sees a child standing in the middle of the road. He applies the brakes and brings the scooter to rest in 5s just in time to save the child. Calculate the average retarding force on the vehicle, if mass of the vehicle and the driver is 300kg.
a) 270N
b) 600N
c) 150N
d) 125N
Explanation: u = 36km/h = 10m/s, v = 0, t = 5s
a = (v-u)/t = (0-10)/5 = -2m/s2
Retardation = 2m/s2
Average retarding force = 300×2 = 600N.
4. Which of the following is an example for impulsive force?
a) A person in a moving bus
b) A horse suddenly stopping
c) Force exerted by a bat hitting a ball
d) Sharpening of knife
Explanation: A large force acting for a short time to produce a finite charge in momentum is called an impulsive force. Thus a force exerted by a ball hitting a bat is an example for an impulsive force.
5. A person is falling from a certain height. Which of the following is likely to happen?
a) If he falls on cement floor, he gets more injuries
b) If he falls on a heap of sand, he receives less injuries
c) He gets the same injuries in both the cases
d) He is unaffected
Explanation: In both cases, the impulse is the same. On the cemented floor, the person is topped abruptly. So the cemented floor exerts a large force of reaction causing him severe injuries. When he falls on a heap of sand, the sand yields under his weight. This decreases the force exerted by the floor on the person.
6. A batsman hits back a ball straight in the direction of the bowler without changing its initial speed of 12m/s. If the mass of the ball is 0.15kg, determine the impulse imparted to the ball. (Assume linear motion of the ball)
a) -45Ns
b) -3.6Ns
c) 3.78Ns
d) 60Ns
Explanation: m = 0.15Kg, u = 12m/s, v = -12m/s
Impulse = m(v-u) = 0.15(-12-12) = -3.6Ns
The negative sign indicates that the direction of the impulse is from the batsman to the bowler.
7. While launching a rocket of mass 2×104 kg, a force of 5×105 N is applied for 20s. Calculate the velocity attained by the rockets at the end of 20s.
a) 3.6m/s
b) 300m/s
c) 120m/s
d) 500m/s
Explanation: m = 2×104kg F = 5×105N t = 20s u = 0 v = ?
Impulse of force = F×t = m(v-u)
5×105×20 = 2×104(v-0)
v = 500m/s.
8. Swimming is an example of which of the following?
a) Newton’s first law of motion
b) Newton’s second law of motion
c) Newton’s third law of motion
d) Law of inertia
Explanation: While swimming, a man pushes water backward and in turn he is pushed forward. Thus for the backward action there is an equal and opposite forward reaction.
9. Single force can exist.
a) True
b) False
Explanation: The force always exists in pairs. The two forces act simultaneously. Anyone of them may be called the action and the other will be called reaction.
10. The coefficient of kinetic friction is less than the coefficient of static friction.
a) True
b) False
Explanation: The value of kinetic energy is directly proportional to the normal reaction between the two surfaces. Thus the coefficient of kinetic friction is less than the coefficient of static friction.