Question

Which of the following is/are correctly matched pairs concerning Newton's Law of Motion?

1. First Law of Motion: Every body continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise.

2. Second Law of Motion: To every action, there is always an equal and opposite reaction. 

3. Third Law of Motion: The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which the force acts.

1. 1 only
2. 2 and 3 only
3. 2 only
4. 1, 2 and 3

Answer 1

Correct Answer - Option 1 : 1 only

The correct answer is 1 only.

Newton's First Law of Motion

• Every body continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise. Hence, Statement 1 is correct.
• The state of rest or uniform linear motion both implies zero acceleration.
• The first law of motion can, therefore, be simply expressed as: If the net external force on a body is zero, its acceleration is zero. 
• Acceleration can be non-zero only if there is a net external force on the body.
• The first law refers to the simple case when the net external force on a body is zero.

Newton's Second Law of Motion

• The second law of motion refers to the general situation when there is a net external force acting on the body.
• The momentum of a body is defined to be the product of its mass m and velocity v and is denoted by p: [p = mv]        
• Momentum is clearly a vector quantity. 
• The same force for the same time causes the same change in momentum for different bodies.
• The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which the force acts. Hence, Statement 3 is incorrect.

Newton's Third Law of Motion

• To every action, there is always an equal and opposite reaction. Hence, Statement 2 is incorrect.
• There is no cause-effect relation implied in the third law. 
• Consider a pair of bodies A and B.  According to the third law, F_AB = – F_BA [(force on A by B) = – (force on B by A)]