CBSE Class 9 Physics Force and Laws of Motion MCQs Set 05

Answer: (b) friction there is small or the sand yields, so we get little forward push

Explanation:
1. We walk by pushing backward on the ground; the friction forward on us moves us on.
2. On smooth or loose surfaces friction is small, so walking is difficult.

Teacher's Note:
1. Friction is needed to walk.

Answer: (c) inertia of motion (Newton's first law)

Explanation:
1. The feet stop with the bus but the upper body continues to move forward.
2. This is the property of inertia.

Teacher's Note:
1. A body continues in its state of motion.

Answer: (d) increases the time to stop the ball and so reduces the force

Explanation:
1. \( F=\frac{\Delta p}{t} \): the change in momentum is fixed.
2. A larger \( t \) gives a smaller force on the hand.

Teacher's Note:
1. Impulse equals change of momentum.

Answer: (a) stops in a very short time, so the force is very large

Explanation:
1. \( F=\frac{\Delta p}{t} \); a very short contact time gives a very large force.

Teacher's Note:
1. Short time means large force.

Answer: (b) they need oxygen from the atmosphere to burn their fuel

Explanation:
1. A jet engine carries its own fuel but takes oxygen from the air.
2. Space has no air, so rockets which carry their own oxidiser are used.

Teacher's Note:
1. Rockets are self-contained.

Answer: (c) it has to accelerate repeatedly after stopping

Explanation:
1. Each acceleration needs a force and hence energy from fuel.
2. On a free road the speed is nearly uniform, so less fuel is used.

Teacher's Note:
1. Repeated acceleration costs fuel.

Answer: (d) mass is unchanged but its weight decreases

Explanation:
1. Mass is the quantity of matter and does not change.
2. Weight \( =mg \) and \( g \) on the moon is about one sixth of that on earth.

Teacher's Note:
1. Mass is constant; weight depends on g.

Answer: (a) not move because the forces are internal and cancel

Explanation:
1. The passengers push the carriage and the carriage pushes them back with equal and opposite force.
2. These internal forces cannot produce a net external force on the system.

Teacher's Note:
1. Internal forces do not move the system.

Answer: (b) gain the speed needed to get sufficient lift

Explanation:
1. The lift on the wings depends on the speed of the air flowing past them.
2. It becomes enough to carry the weight only after a certain speed.

Teacher's Note:
1. Lift increases with speed.

Answer: (c) lowers the centre of gravity and makes them stable at sharp turns

Explanation:
1. A low centre of gravity and large base make the car hard to topple.
2. This reduces the risk of overturning.

Teacher's Note:
1. Stability.

Answer: (d) the combined centre of gravity stays over the base of support

Explanation:
1. If the vertical line through the centre of gravity falls outside the base, the body topples.
2. Bending to the right brings the centre of gravity back over the feet.

Teacher's Note:
1. Centre of gravity must stay above the base.

Answer: (a) can lie outside the material of the body, for example in a ring

Explanation:
1. The centre of gravity of a uniform ring is at its centre.
2. That point is empty space, outside the metal.

Teacher's Note:
1. It is a point, not necessarily on the body.

Answer: (b) changes the direction of the applied force

Explanation:
1. With a fixed pulley we can pull downward to lift a load upward.
2. This is more convenient and lets us use our weight.

Teacher's Note:
1. Direction change is its use.

Answer: (c) reduces friction between the nail and the wood

Explanation:
1. A lubricant forms a thin film between surfaces.
2. Less friction makes the nail go in more easily.

Teacher's Note:
1. Lubricants reduce friction.

Answer: (d) increase the time of impact so that jolts are reduced

Explanation:
1. \( F=\frac{\Delta p}{t} \): springs lengthen the time over which the jolt is absorbed.
2. So the force transmitted to passengers is smaller.

Teacher's Note:
1. Longer time, smaller force.