# Why define and use Impulse?

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• rudransh verma

#### rudransh verma

Gold Member
Sometimes there are forces which act for very small time known as impulsive forces. We cannot measure such large forces acting in a very short time but we can ##\Delta p##. ##\Delta p=F\Delta t##.This quantity is defined as Impulse.
Why do we keep introducing new quantities? When is a new quantity defined or introduced in physics? What is the need for impulse?

There is the work - energy theorem and the impulse-momentum theorem. Impulse is to momentum what work is to energy.

Both theorems are consequences of Newton's 2nd law.

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• • russ_watters, rudransh verma and Lnewqban
When is a new quantity defined or introduced in physics?
When it's useful.
What is the need for impulse?
When you need the momentum change but don't know the details of the force. For example, consider a collision between non-rigid bodies. The force depends on the current deformation, so is strongly time dependent and it's difficult to work with. But if we just want to know their velocities after the collision we can just conserve momentum and energy, and the change in momentum is the impulse.

• russ_watters and Lnewqban
Sometimes there are forces which act for very small time known as impulsive forces. We cannot measure such large forces acting in a very short time but we can Δp. Δp=FΔt.This quantity is defined as Impulse.
Yours is an approximate definition, exactly true only for constant forces.
The actual definition of impulse is $$Impulse= \int F(t)\,dt$$ and by Newton's law it is equal to the change in momentum. It is sometimes useful: if you don't like it, don't use it.

• rudransh verma
When is a new quantity defined or introduced in physics?
Conserved quantities like momentum and energy are useful for predictions, because they restrict the possible outcomes.

What is the need for impulse?
Impulse is transfer of momentum just like work is transfer of mechanical energy.

• Lnewqban and rudransh verma
When you need the momentum change but don't know the details of the force. For example, consider a collision between non-rigid bodies. The force depends on the current deformation, so is strongly time dependent and it's difficult to work with. But if we just want to know their velocities after the collision we can just conserve momentum and energy, and the change in momentum is the impulse.
Its nothing but ##\Delta p## and that is used to find u,v,m. So, ##F\Delta t## is useless on its own.

• Lnewqban
• vanhees71
Why do we keep introducing new quantities?
Why are you so resistant in learning new things? For the same reason we use any quantity - it helps us solve problems. If you do everything the old hard way and don't learn new things, you will continue to do things the hard way.

• nasu, russ_watters, berkeman and 2 others
Its nothing but ##\Delta p## and that is used to find u,v,m. So, ##F\Delta t## is useless on its own.
So... ##\Delta p## is used for something but is useless, according to you?

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