Question

Explain which of Newton's laws of motion are appli­cable in the following cases.

1. A person swimming in water.
2. A person gets hurt when jumping on the hard cement surface.
3. A mango falls from the tree when the branch of the tree is suddenly shaken.
4. The recoil of the gun when the bullet is fired from it.
5. To compare accelerations produced in spheres of mass $m$and $2m$, when equal force is applied to them.

Answer 1

a. A person swimming in water:

1. Newton's third law of motion is appli­cable in the case of a person swimming in water.
2. According to the Newton's third law of motion, every action there is an equal and opposite direction.
3. When a person swims, he pushes the water backwards and the water in turn pushes the person with an equal force in the forward direction.

b. A person gets hurt when jumping on the hard cement surface.

1. Newton's third law of motion is appli­cable in the case of a person gets hurt when jumping on the hard cement surface.
2. When you jump onto the concrete surface, your feet are brought to rest almost instantaneously due to this, the rate of change of momentum is very high.
   $F=\frac{dp}{dt}$, where “dp” is the change in momentum and “dt” is the time taken
3. If time taken is less then the force will be more.
4. Hence, you get injured due to a large force on your body on account of the hard floor.

c. A mango falls from the tree when the branch of the tree is suddenly shaken:

1. Newton's first law of motion is appli­cable in the case of a mango falls from the tree when the branch of the tree is suddenly shaken.
2. When the branch of the tree is suddenly shaken, the inertia of mango makes it fall.
3. When we shake a mango tree, the mangoes fall, It is because when we shake the tree, the mangoes tend to be at rest due to inertia whereas the branches are in motion. That is why the mangoes fall from the branches.

d. The recoil of the gun when the bullet is fired from it.

1. Newton's third law of motion is appli­cable in the case of the recoil of the gun when the bullet is fired from it.
2. When a gun is fired, the gun exerts a force in the forward direction. This is the action force. The bullet exerts an equal force on the gun on the opposite direction. This is the reaction force.

e. To compare accelerations produced in spheres of mass $m$and $2m$, when equal force is applied to them.

1. Newton's second law of motion is appli­cable in the case of comparison of accelerations spheres of mass $m$and $2m$, when equal force is applied to them.