## Physics | Notes

Laws of Motion – First, Second and Third

For: Science Class 11

**First law of motion (Law of Inertia)**

According to first law of motion “Everybody in the universe continue its state of rest or of uniform motion in a straight line unless it is affected by some external unbalance force to change the state”. This law is also called the law of inertia.

**a) Inertia of rest**

When a bus suddenly moves, the passengers’ falls backward, it is because the lower part of the body of the passengers in contact to the bus is in motion but upper part tends to be at rest. So, the passenger tends to fall backward.

When we shake mango tree, the mangos fall down. It is because when we shake the tree, the mangos tend to be at rest due to inertia where as the branches are in the motion. That is why the mangos get unattached from the branches.

**Second law of Motion**

According to second law of motion “The rate of change in momentum of a body directly proportional to the net external force applied on it and the change in momentum takes place in the direction of force applied.

Let us consider body of mass (m) moving with initial velocity (v_{1}) then initial momentum (p1) = m.v_{1}

Now, a force (f) is applying on the body a small duration of time (d_{f}) due to which its velocity changes from v_{1} to v_{2} then final momentum (p_{2}) = m.v_{2}

Change in momentum (d_{p}) = m.v_{2} – m.v_{1}

d_{p} = p_{2} – p_{1}

= m (v_{2} – v_{1})

= m.d_{v}

Rate of change in momentum

d_{p} / d_{f} = m.d_{v} / d_{f}

According to definition

f α (m.d_{v}) / d_{f}

Or f = k.ma

Where, (d_{v} / d_{f}) = a (Acceleration)

K = proportionality constant.

If m = 1kg , f = 1N, a = 1 m/s^{2} then k = 1

f = ma

If m = 1 kg, a = 1 m/s^{2} then,

f = 1 kgm/s^{2} = 1 N

Therefore, One Newton is that much force which when applied on a body of 1 kg produced acceleration of 1 m/s^{2}.

**Third law of motion**

According to 3^{rd} law of motion “Every action, there is always an equal and opposite reaction.”

**Impulse**

Impulse of a force is measured by change in linear momentum produce by the force.

Impulse = Force × time of impact

= Change in momentum

Therefore, Impulse = F × d = mv – mu