CBSE Class 10 Physics Force and Laws of Motion MCQs Set 06

Answer: (c) \( t^2 \)

Explanation:
1. Starting from rest with constant force, the acceleration \( a = F/m \) is constant.
2. \( S = \tfrac12 at^2 \), so S is proportional to \( t^2 \).

Teacher's Note:
1. Distance from rest grows as the square of time under constant acceleration.
2. Doubling the time makes the distance four times larger.

Answer: (b) force

Explanation:
1. This is the definition of force: a push or a pull that changes the state of motion or shape of a body.

Teacher's Note:
1. Force is a vector quantity measured in newton.
2. It can change speed, direction or shape.

Answer: (c) the gravitational force of the earth

Explanation:
1. The only significant force on a freely falling body is the pull of the earth, a non-contact force.
2. This unbalanced force produces a constant downward acceleration.

Teacher's Note:
1. Gravity acts without contact.
2. Absence of air resistance alone cannot cause acceleration.

Answer: (c) It always goes away from the earth

Explanation:
1. In accelerated motion the velocity changes, so some net force acts on the object.
2. Nothing requires the object to move away from the earth: a falling stone moves towards it.

Teacher's Note:
1. Acceleration needs a net force by the second law.
2. The direction of motion is independent of the earth.

Answer: (d) have zero net force on it

Explanation:
1. Constant velocity means zero acceleration.
2. By Newton's second law the net force is then zero.

Teacher's Note:
1. Forces may act, but they must cancel.
2. An object may still feel gravity, balanced by a support force.

Answer: (b) 100 N

Explanation:
1. Constant velocity requires zero net force.
2. Friction must therefore be equal and opposite to the applied 100 N.

Teacher's Note:
1. Net force = 100 - f = 0 gives f = 100 N.

Answer: (b) moves with acceleration

Explanation:
1. A non-zero net force produces an acceleration \( a = F/m \).

Teacher's Note:
1. Unbalanced force changes the velocity in magnitude or direction.

Answer: (c) to resist any change in its state of motion

Explanation:
1. Inertia is the tendency to keep the present state of rest or uniform motion.

Teacher's Note:
1. Mass is the measure of inertia.
2. Inertia is not a force.

Answer: (a) accelerated

Explanation:
1. The coin keeps the horizontal velocity the train had at the moment of tossing.
2. It falls behind only if the train speeds up while the coin is in the air.

Teacher's Note:
1. In a train moving uniformly the coin would drop back into the hand.
2. Falling behind means the train gained speed.

Answer: (b) move forward

Explanation:
1. The water tends to keep moving with the earlier velocity (inertia of motion).
2. When the tanker stops suddenly, the water surges forward.

Teacher's Note:
1. The same effect throws passengers forward when a bus brakes suddenly.

Answer: (c) in front of him

Explanation:
1. The coin keeps the horizontal velocity of the train at the instant of toss.
2. The train slows down, so the coin gets ahead of it and lands in front of the passenger.

Teacher's Note:
1. Compare with a uniformly moving train, where the coin returns to the hand.
2. Speeding up gives the opposite result.

Answer: (a) mass

Explanation:
1. Inertia is measured quantitatively by mass.

Teacher's Note:
1. A heavier mass resists a change of motion more.

Answer: (a) inertia of rest

Explanation:
1. The leaves tend to stay at rest while the branch moves suddenly.
2. The loosely attached ones are left behind and fall off.

Teacher's Note:
1. This is the same idea as beating a carpet to remove dust.

Answer: (a) inertia of direction

Explanation:
1. Passengers tend to keep moving in the original straight direction.
2. The car turns, so they seem to be pushed outward.

Teacher's Note:
1. Inertia of direction means a body resists any change of the direction of its motion.

Answer: (d) The inertia is unchanged

Explanation:
1. Inertia is measured by mass.
2. Mass does not change when velocity changes.

Teacher's Note:
1. Momentum doubles when velocity doubles, but inertia does not change.

Answer: (a) below the stone

Explanation:
1. A sudden jerk acts for a very short time.
2. The stone does not have time to move because of its inertia of rest, so the extra tension is felt only by the lower part of the string, which breaks.

Teacher's Note:
1. A slow, steady pull would instead break the upper part, which also carries the weight of the stone.

Answer: (b) 38 m/s

Explanation:
1. Momentum of the truck = 38000 x 1.20 = 45600 kg m/s.
2. Car velocity = 45600 / 1200 = 38 m/s.

Teacher's Note:
1. Equal momentum means mass times velocity is equal.
2. The lighter body must move faster.

Answer: (b) 1 : 1

Explanation:
1. Momentum of P = 2 x 5v = 10v.
2. Momentum of Q = 8 x 1.25v = 10v.
3. Ratio = 1 : 1.

Teacher's Note:
1. Always multiply mass and velocity for each body before taking the ratio.

Answer: (d) \( \dfrac{p^2}{m\,r} \)

Explanation:
1. Centripetal force \( F = mv^2/r \).
2. With p = mv, \( v = p/m \).
3. \( F = m(p/m)^2/r = p^2/(mr) \).

Teacher's Note:
1. Express v in terms of p, then substitute.
2. Check the units: (kg m/s)\( ^2 \) / (kg m) = N.

Answer: (a) with uniform velocity

Explanation:
1. Constant momentum p = mv with constant mass means constant velocity.
2. Constant velocity is uniform motion in a straight line.

Teacher's Note:
1. Zero rate of change of momentum means zero net force.