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Force and Law of Motion are an essential concept in Physics that deals with the study of motion and its relationship with force. It is a fundamental concept that helps us understand how objects move and why they move. A force is defined as a push or pull on an object that can cause a change in its motion. The unit of force is Newton (N).
- According to Newton's first law of motion, a body at rest remains at rest, and a body in motion continues to move with a constant velocity unless acted upon by an external force.
- In simple terms, this law implies that an object will continue to be in its state of motion until an external force acts on it.
- Newton's second law of motion states that the force acting on an object is directly proportional to its acceleration.
- The mathematical representation of this law is F = m × a, where F is the force, m is the mass of the object, and a is the acceleration produced by the force.
- Newton's third law of motion states that every action has an equal and opposite reaction.
- In other words, for every force applied to an object, there is an equal and opposite reaction force.
These laws of motion have various practical applications, ranging from the study of the motion of planets to designing rockets and spacecraft.
MCQ on Force and Laws of Motion
Ques 1. A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goalkeeper to -
- Increase the rate of change of momentum
- Maintain the same rate of change of momentum
- Decrease the rate of change of momentum
- None of the above
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Answer: C) Decrease the rate of change of momentum
Explanation: When a ball is shot towards the goal, it has a certain momentum. If the goalkeeper catches the ball and stops it immediately, the momentum of the ball will change to zero, which would result in a high force being exerted on the goalkeeper's hands. This sudden change in momentum can injure the goalkeeper's hands and make it difficult for them to hold onto the ball.
- To prevent this, the goalkeeper pulls their hands backwards after catching the ball.
- This allows the momentum of the ball to be gradually reduced over a longer period of time, thus reducing the force exerted on the goalkeeper's hands.
- By decreasing the rate of change of momentum, the goalkeeper is able to hold onto the ball more securely and reduce the risk of injury.
- Therefore, option C) Decrease the rate of change of momentum is the correct answer.
Read More: Difference between force and momentum
Ques 2. The unit of measuring the momentum of a moving body is -
- kg.m/s
- kg/s
- m/s2
- N/m2
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Answer: A: kg.m/s
Explanation: Momentum is defined as the product of the mass and velocity of an object. It is a vector quantity and has both magnitude and direction. The unit of mass is the kilogram (kg) and the unit of velocity is the meter per second (m/s). Therefore, the unit of momentum is kg.m/s.
Other options:
- B. kg/s - This is not the correct unit of momentum. It does not take into account the velocity of the object.
- C. m/s2 - This is the unit of acceleration, not momentum.
- D. N/m2 - This is the unit of pressure, not momentum.
Ques 3. What does the third law of motion state about action and reaction?
- They always act on the same body.
- They always act on different bodies in the same direction.
- They always act on different bodies in opposite directions.
- They only occur in certain situations.
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Answer: C. They always act on different bodies in opposite directions.
Explanation: The third law of motion, also known as the law of action and reaction, states that for every action, there is an equal and opposite reaction.
- This means that when one body exerts a force on another body, the second body exerts an equal and opposite force back on the first body.
- These forces always act on different bodies and in opposite directions.
- For example, when you push on a wall, the wall pushes back on you with the same amount of force, but in the opposite direction.
- This is why you don't fall through the wall.
- Another example is when a rocket expels gas out of its back end, it experiences a force pushing it forward in the opposite direction.
- Therefore, the correct answer is c. They always act on different bodies in opposite directions.
Ques 4. A man wearing a bulletproof vest stands on roller skates. The total mass is 80 kg. A bullet of mass 20 g is fired at 400 m/s. It is stopped by the vest and falls to the ground. What is then the velocity of the man?
- 1.0 m/s
- 0.1 m/s
- 10.0 m/s
- 0.01 m/s
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Answer: b) 0.1 m/s
Explanation: According to the law of conservation of momentum, the total momentum of a system remains constant unless an external force acts on it. In this case, the initial momentum of the bullet and the man is:
Initial momentum = mass x velocity
= (0.02 kg) x (400 m/s) + (80 kg) x (0 m/s)
= 8 kg m/s
After the bullet is stopped by the vest, the final momentum of the system is:
Final momentum = mass x velocity
= (0.02 kg) x (0 m/s) + (80 kg) x (v m/s)
= 80v kg m/s
Since the total momentum is conserved, we can equate the initial and final momenta:
Initial momentum = Final momentum
8 kg m/s = 80v kg m/s
Solving for v, we get:
v = 0.1 m/s
Therefore, the velocity of the man after the bullet is stopped is 0.1 m/s.
Ques 5. The speed of a car weighing 1500 kg increases from 36 km/h to 72 km/h uniformly. What will be the change in momentum of the car?
- 7500 kg m/s
- 15000 kg m/s
- 22500 kg m/s
- 30000 kg m/s
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Answer: B) 15000 kg m/s
Explanation: Momentum is the product of mass and velocity. It is a vector quantity, which means it has both magnitude and direction. The change in momentum is given by the difference between the final and initial momentum of the object.
The mass of the car is 1500 kg. The initial speed is 36 km/h, which is equivalent to 10 m/s. The final speed is 72 km/h, which is equivalent to 20 m/s. The change in velocity is 20 m/s - 10 m/s = 10 m/s.
Therefore, the change in momentum of the car is given by:
change in momentum = mass x change in velocity
change in momentum = 1500 kg x 10 m/s
change in momentum = 15000 kg m/s
Hence, the correct answer is b) 15000 kg m/s.
Read More:
Ques 6. Which law explains why a person falls in the forward direction upon the sudden stopping of the car?
- Newton's first law of motion
- Newton's second law of motion
- Newton's third law of motion
- Archimedes' principle
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Answer: A. Newton's first law of motion
Explanation: Newton's first law of motion, also known as the law of inertia, states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.
- In the case of a car suddenly stopping, the passengers sitting in the front seats continue to move forward at the same speed as the car until an external force (in this case, the dashboard or windshield) acts upon them and stops their motion.
- This is why they fall in the forward direction and can get injured.
- The seat belt acts as an external force that prevents the passengers from continuing to move forward and keeps them in place, thus saving them from getting injured.
Ques 7. The masses of the two bodies are in a ratio of 5:6 and their velocities are in a ratio of 1:2. Then their linear momentum will be in the ratio:
- 1:2
- 5:6
- 5:12
- 6:5
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Answer: C) 5:12
Explanation: The linear momentum of a body is given by the product of its mass and velocity. Therefore, the linear momentum of a body is directly proportional to its mass and velocity.
Let the masses of the two bodies be 5m and 6m, respectively, where m is a constant of proportionality. Let the velocities of the two bodies be v and 2v, respectively.
The linear momentum of the first body is given by:
P1 = 5m x v = 5mv
The linear momentum of the second body is given by:
P2 = 6m x 2v = 12mv
Therefore, the ratio of their linear momenta is:
P1 : P2 = 5mv : 12mv = 5 : 12
Hence, the correct answer is option C) 5:12.
Ques 8. A ball is thrown vertically upward in a train moving with uniform velocity. The ball will:
- Fall behind the thrower
- Move forward with the train
- Return back to the thrower
- Move in the opposite direction of the train
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Answer: C) Return back to the thrower
Explanation: When a ball is thrown vertically upward in a train that is moving with uniform velocity, it has two types of motion – vertical and horizontal. The vertical motion is affected by gravity, while the horizontal motion is unaffected by the train's velocity.
- According to the first law of motion, an object in motion will remain in motion with the same velocity unless acted upon by an external force.
- Therefore, the horizontal motion of the ball remains unaffected by the train's velocity, and it will move forward with the train.
- However, the vertical motion of the ball is affected by the force of gravity, which acts in the opposite direction to the ball's upward motion.
- As a result, the ball will slow down and eventually come to a stop before falling back to the ground.
- Since the train's velocity has no effect on the ball's vertical motion, it will return back to the thrower in the same manner as if it was thrown vertically upward on the ground.
- Hence, the correct answer is option C) Return back to the thrower.
Also Check Out:
| Important Topics Related to Force and Laws of Motion | ||
|---|---|---|
| Distance and Displacement | Vector and Scalar | Centripetal Acceleration |
| Type of Motion Measurement | Laws of Motion MCQs | Wave Motion |
| Inertia and Mass | Rectilinear Motion of Particles | Kinetic Friction |
Ques 9. If the mass of a body is doubled and its velocity becomes half, then the linear momentum of the body will:
- Double
- Half
- Remain the same
- Cannot be determined
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Answer: C) Remain the same
Explanation: Linear momentum is the product of the mass and velocity of a body. It is a vector quantity and has both magnitude and direction. The formula for linear momentum is:
Linear momentum = mass x velocity
According to the given scenario, the mass of the body is doubled, i.e., 2m, and its velocity is halved, i.e., v/2. Therefore, the new linear momentum can be calculated as -
New linear momentum = (2m) x (v/2)
New linear momentum = mv
- Comparing the new and old linear momentum, we can see that they are the same, i.e., mv = mv.
- This means that the linear momentum of the body remains the same even though the mass has doubled and the velocity has halved.
- Hence, the correct answer is C) Remain the same.
Ques 10. A rocket works on the principle of conservation of -
- Energy
- Momentum
- Force
- Power
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Answer: B) Momentum
Explanation: The rocket works on the principle of conservation of momentum. This means that the total momentum of the system (rocket and exhaust gases) remains constant.
- According to Newton's third law of motion, for every action, there is an equal and opposite reaction.
- In the case of a rocket, the force of the exhaust gases moving in one direction generates an equal and opposite force on the rocket in the opposite direction, propelling it forward.
- This exchange of momentum between the rocket and the exhaust gases ensures that the total momentum of the system remains constant.
- This principle of conservation of momentum is essential in the design and functioning of a rocket.
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