FORCE AND LAWS OF MOTION

MULTIPLE CHOICE QUESTIONS

Choose the correct option:1. Which of the following statement is not correct for an object moving alonga straight path in an accelerated motion?(a) Its speed keeps changing(b) Its velocity always changes(c) It always goes away from the earth (c) (d) A force is always acting on it

2. According to the third law of motion, action and reaction(a) always act on the same body(b) always act on different bodies in opposite directions(c) have same magnitude and directions(d) act on either body at normal to each other (b)

3. A goalkeeper in a game of football pulls his hands backwards after holdingthe ball shot at the goal. This enables the goal keeper to(a) exert larger force on the ball(b) reduce the force exerted by the ball on hands(c) increase the rate of change of momentum(d) decrease the rate of change of momentum (b) & (d)

4. The inertia of an object tends to cause the object(a) to increase its speed(b) to decrease its speed(c) to resist any change in its state of motion(d) to decelerate due to friction (c)

5. A passenger in a moving train tosses a coin which falls behind him. Itmeans that motion of the train is(a) accelerated(b) uniform(c) retarded(d) along circular tracks (a)

6. An object of mass 2 kg is sliding with a constant velocity of 4 m s–1 on africtionless horizontal table. The force required to keep the object movingwith the same velocity is(a) 32 N (b) 0 N (c) 2 N (d) 8 N (b)

7. Rocket works on the principle of conservation of(a) mass (c) momentum(b) energy (d) velocity (c)

8. A water tanker filled up to 2/3 of its height is moving with a uniform speed.On sudden application of the brake, the water in the tank would(a) move backward(b) move forward(c) be unaffected(d) rise upwards (b)

9. An unbalanced force acts on a body. The body musta) remain at restb) move with uniform velocityc) accelerated) move along a circle (c)

10. A gun recoils after firing to conserve a) velocityb) energyc) speed d) momentum (d)

11. Action and reaction are equal and opposite, they act a) on the same bodyb) on the different bodiesc) such that their resultant effect is zero d) such that their resultant effect is added (b)

VERY SHORT ANSWER QUESTIONS

1. What is the S.I. unit of force?Ans.__________Newton_______________________________________

2. If a ball is moving on a frictionless horizontal surface and no forces are applied on it, will its speed decrease, increase or remain constant?Ans._____________remains constant____________________________________

3. How much force acts on a body whose momentum is constant (i.e. ¯p¯ = constant)?Ans._____________no force____________________________________

4. 1 N = …105…. dyne.

5. State whether the following statement is true or false. If false, correct it. “Action and reaction are equal in magnitude and act in the same direction.”

Ans. Action and reaction are equal in magnitude and act in the opposite direction.”

NUMERICAL QUESTIONS

1. A constant force of 20 N acts on a body for 5 s. Find the change in momentum of body.Ans. change in momentum of body =Fxt =20x5= 100 kgm/s

2. How much force is needed to produce an acceleration of 10 m/s² in a body of mass 250 g?Ans. F=mxa, =0.250x10=2.5N

3. A body of mass ‘m’ strikes against a wall with a velocity ‘v’ and rebounds with the same velocity. Find its change in momentum.Ans. Initial momentum=mv, Final momentum= -mv change in momentum= -mv-mv= -2mv

4. Observe the following velocity-time graph for the motion of a body of mass 8 kg and answer the following questions.

a) What is the force acting on the body at t = 2 s?

F=m(v-u)/t, =8x(10-0) =40N

b) What is the force acting on the body at t = 4 s?

F= m(v-u)/t =8x0=0N

c) What is the force acting on the body at t = 6 s?

F=m(v-u)/t =8x(10-15)/(6-5)= -40N

d) Write the time interval during which the body shows retardation. 5 to 8 second

Down:1. inertia. 2. Resultant force. 3. Momentum.Across:4. unbalanced forces. 5. Force 6. Mass. 7. Impulsive force. 8. Balance forces.