Force and Laws of Motion

What causes a body at rest to move?What causes a moving body to stop? What causes the change of direction of a moving body?What causes the change of shape of a body? It is the force, precisely the unbalanced force, that causes all these! Let us learn more about force, laws of motion and conservation of momentum . Let us also try to solve some problems related to force and laws of motion.

Force is a push or pull. It is an external influence that causes a body to move or a moving body to stop. It can also change the direction of motion of a body. Force can change the shape of a body as well.

Friction

Friction is the force resisting the motion of an object. When we try to push a wooden box kept on a table, the force of friction between the two surfaces in contact tries to stop it from moving.

Balanced forces

Two or more forces are said to be balanced if they either do not move a body at rest or do not change the velocity of a moving body.

Unbalanced forces

Unbalanced forces acting on a body at rest will move the body. Similarly unbalanced forces acting on a moving body will either change its velocity or direction or both.

Newton’s first law of motion

An object remains in its state of rest or of uniform motion in a straight line unless compelled to change that state by an unbalanced force.

Inertia

The tendency of undisturbed objects at rest to stay at rest or moving bodies to keep moving with the same velocity is called inertia.

There are three types of inertia namely, inertia of rest, inertia of motion and inertia of direction. Suppose we are standing in a bus and the driver starts the bus suddenly, we will be pushed backwards. Our feet moves along the bus whereas the upper part of our body resists the motion. This is an example of inertia of rest. Similarly when the driver applies breaks suddenly, we will be pushed forward. Our feet stops with the bus whereas the upper part of our body continues to move! This is an example of inertia of motion. Let us take another situation. When we negotiate a turn while travelling by a bike, we sway in the opposite direction. Upper part of our body resists the change of direction whereas its lower part changes its direction of the bike. This is an example of the inertia of direction.

Momentum

Momentum is a physical quantity, which is the product of its mass and velocity. All moving bodies have momentum. The more the mass of the moving body, the more its momentum. Similarly, the more the velocity, the more its momentum.

Mathematically we express momentum as

p = mv

where p is the momentum, m is its mass and v is its velocity. The direction of momentum will be in the direction of its velocity.

Newton’s second law of motion

The second law of motion states that the rate of change of momentum of an object is proportional to the unbalanced force applied in the direction of the force.

Let us derive an expression of force mathematically.

Consider an unbalanced force F applied on a body of mass m which changes its velocity from u to v in time t seconds.

Change in momentum = mv - mu

Rate of change of momentum = (mv-mu)/t

As per Newton’s second law of motion,

Rate of change of momentum is proportional to the unbalanced force.

Or the unbalanced force is directly proportional to the rate of change of momentum.

Newton’s third law of motion

The third law of motion states that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first. These two forces are always equal in magnitude but opposite in direction.

Law of conservation of momentum

The sum of momenta of the two objects before collision is equal to the sum of momenta after the collision provided there is no external unbalanced force acting on them.

Now it is time for a quiz.

Choose the right answer from the options given in brackets and fill in the blanks.

1. Recoil of a gun is an example of Newton’s _______ law of motion.(1st, 2nd, 3rd)

2. ___________ is the tendency of a body to resit the change of state.(friction/inertia).

3. ___________ friction is more.(sliding/ rolling).

4. We slip on a wet floor because of ________ friction (reduced/ increased)

5. Newton’s ___________ law of friction is also known as the law of inertia.(1st, 2nd, 3rd)

6. The unit of force is_______ (kg/N)

7. The unit of momentum is _______{(kg.m/s)/ N}

8. The momentum of a truck is ______ than that of a car moving with the same velocity. (less/more)

9. The momentum of a stationary object of mass 10 kg is _______ (0/ 10 kg)

10. The force acting on a body of mass 1 kg which produces an acceleration of 1 meter per square second is ____ (1 N/ 1 dyne)

Answers:

1. 3rd 2. inertia 3. sliding 4. reduced 5. 1st 6. N 7. kg.m/s 8. more 9. 0 10. 1 N