# Physical meaning of momentum equation

1. Dec 3, 2008

### venky56

could anyone explain me the newton's second law of motion in detail?

It tells that rate of change of momentum is equal to force applied. what does it physically mean in real applications? is it like force is causing the change in momentum or is it like if there is a change in momentum, a force will be applied!

what is it really that makes the force be applied on one body?
Energy, Force, motion, momentum.....how r these concepts related?

Plz dont mind even if i am not thinking in correct direction! i want to learn

Last edited: Dec 3, 2008
2. Dec 6, 2008

### p.tryon

Another way of stating Newton's Second law is that the acceleration (rate of change of velocity) of a particular object is directly proportional to the force applied (providing the mass doesn't change). When I refer to force I mean overall Net force (remember there may be a number of different forces acting in different directions)

Force = Mass x Acceleration
(*I have shown how this is equivalent to rate of change of momentum at the bottom)

This means that if a net force is applied to a body it will accelerate. It also follows that if a body is accelerating a net force must be acting on it (**Note, here acceleration is just a change in velocity and could mean speeding up or slowing down) There are two consequences of this:

1. If you double the mass of a body and apply a particular force you will only get half the acceleration. Imagine loading up a supermarket trolly- you know from experience that the more goods (the greater the mass) the greater the force is needed to get it moving! On the other hand, a car that is loaded up with passangers will have a much greater stopping distance because the force of the breaks produces a smaller acceleration (deceleration) on the bigger mass.

2. If you double the applied net force (on a particular mass) the you will get double the acceleration. This is why it is better to have 3 people pushing your car when it breaks down, rather than one! Or if you increase the breaking force by getting new breaks, the car will accelerate (decelerate!) much more rapidly- greater force on a mass means greater acceleration (be it speeding up or slowing down).

I admire your willingness to learn. Does this help?

*This shows how the force = rate of change of momentum is mathematically equivalent to the equation force = mass x acceleration:
(1) Momentum (kgm/s) = mass(kg) x velocity(m/s)
For a body of fixed mass we can say
(2) Rate of change of momentum (kgm/s/s) = mass(kg) x change in velocity(m/s) / time(s)
However,
(3)Acceleration(m/s/s) = change velocity(m/s) / time(s)
Therefore
Force (or rate of change of momentum) = Mass (kg) x acceleration (m/s/s)

Last edited: Dec 6, 2008