Test: Force - Class 9 MCQ

Given,

initial velocity, u = 150 ms−1,

final velocity, v = 0 (since the bullet finally comes to rest),

mass of the bullet, m = 10, g = 0.01 kg and

time taken to come to rest, t=0.03 s.
Let the acceleration of a bullet be a.
We use first equation of motion, v=u+at,
⇒0 =150 + a×0.03
⇒a = −1500.03 = −5000 ms−2
(Negative sign indicates that the velocity of the bullet is decreasing.)
Now we use third equation of motion:
v²=u²+2as,
⇒0=150²+2×(−5000)s
⇒s=2250010000=2.25 m
Hence, the distance of penetration of the bullet into the block is 2.25 m.
From Newton's second law of motion:
Force, F= m×a =0.01× - 5000 = -50 N
Hence, the magnitude of force exerted by the wooden block on the bullet is -50 N.

• The Assertion is correct. When the forces acting on an object are balanced, it either remains at rest or continues to move at a constant velocity according to Newton's First Law of Motion.
• The Reason is also correct. If there is no net external force acting on an object, it will maintain its current state of motion due to the absence of acceleration.
• Therefore, in this case, Option A is the correct answer as both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

• The correct answer is Option B: Inertia. Inertia is the property of matter by which it retains its state of rest or its velocity along a straight line, unless acted upon by an external force.
• For example, a body at rest tends to remain at rest, and a body in motion tends to continue moving at a constant velocity unless acted upon by an external force.

The correctly matched pair is Option A: Momentum - Mass x Velocity.

• Option A: Momentum - Mass x Velocity is correct.
  By definition, momentum (p) is the product of the mass (m) of an object and its velocity (v), i.e., p = mv.
• Option B: Second Law of Motion - Force = Mass x Distance is incorrect.
  According to Newton's Second Law of Motion, the force (F) is equal to the mass (m) of an object multiplied by its acceleration (a), i.e., F = ma.
• Option C: Unit of Force - Kilogram per second is incorrect.
  The correct SI unit of force is the Newton (N), which is defined as kg·m/s² (kilogram meter per second squared).
• Option D: Acceleration - Rate of change of Momentum is incorrect.
  Acceleration is the rate of change of velocity, not momentum. The rate of change of momentum is related to force as per the Second Law of Motion.

The rate of change of momentum is directly proportional to the force applied according to Newton's second law of motion, which states that the force acting on an object is equal to the rate of change of its momentum.

Option A: A fielder catching a ball - Decreases time of impact:
Incorrect: When a fielder catches a ball, he pulls his hands backward to increase the time of impact, which decreases the force exerted on his hands.

Option B: High jump on cushioned bed - Increases time of fall
Correct: In a high jump event, athletes fall on a cushioned bed or sand bed to increase the time it takes for them to come to a stop, thereby decreasing the force of impact.

Option C: Karate player breaking ice - Decreases force applied
Incorrect: A karate player breaks a slab of ice with a single blow by applying a large force over a very short period of time.

Option D: First law of motion - F = ma
Incorrect: The first law of motion states that an object will remain at rest or in uniform motion unless acted upon by an external force. The equation F = ma is Newton's second law of motion.

Thus, the correctly matched pair is Option B: (b) High jump on cushioned bed - Increases time of fall.

To accelerate a 2 kg mass at 5 m/s², a force of 10 N is required as per the equation F = ma, where F is force, m is mass, and a is acceleration. This force is necessary to overcome the inertia and change the velocity of the object.

• The Assertion is correct. When a gun is fired, it does exert a forward force on the bullet. This action leads to the bullet moving in the direction the gun is aimed.
• The Reason is also correct as the bullet, in return, exerts an equal and opposite force on the gun, as per Newton's third law of motion. This reaction is what causes the gun to experience the recoil, moving in the opposite direction to the bullet's motion. 
  But Reason does not explain the assertion so option (a) is correct

To determine the correctly matched pair, we must understand the concepts of action and reaction forces as explained by Newton's third law of motion. This law states that for every action, there is an equal and opposite reaction. These forces act on different objects.

• Option A: Action force - Gun recoil This is incorrect.
  The gun recoil is a reaction force to the bullet being fired forward. The action force here is the force exerted by the gun on the bullet.
• Option B: Reaction force - Bullet acceleration This is incorrect.
  Bullet acceleration is a result of the action force applied by the gun. The reaction force is the force the bullet exerts on the gun, causing it to recoil.
• Option C: Action force - Pushing the ground backwards This is correct.
  When starting to walk, the action force is exerted by our feet pushing the ground backwards. The reaction force is the ground pushing our feet forward, allowing us to move.
• Option D: Reaction force - Muscular effort on the road This is incorrect.
  Muscular effort exerted on the road is the action force. The reaction force is the road pushing back on our feet.

Thus, the correctly matched pair is Option C: Action force - Pushing the ground backwards.

Inertia is the natural tendency of objects to resist a change in their state of rest or of uniform motion. It is a measure of an object's mass and is quantitatively described in kilograms (kg).