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What is the Momentum of an Explosion?

The forces
involved in an explosion make objects

move away from each
other very quickly.

This is an example of Newton's
third law of motion.

The law of conservation
of momentum tells us that the

total momentum
is the same, before
and
after the explosion.

Before
an explosion the objects are not
moving

and so the total momentum
is zero.

After the explosion
the total momentum is also still zero.

The momentum of objects
moving in one direction

must be exactly balanced by the

momentum of objects
moving in the opposite
direction.

An example of an explosion
is a gun firing a bullet.

The same force that pushes
the bullet forwards

and gives it a fast velocity

also pushes the gun backwards
(called recoil velocity).

Q1. A gun
has a mass of 2·5
kg and a recoil velocity of
8 m/s.

What is the velocity of the bullet
if it has a mass of 0·02 kg?

A1. The momentum of
the gun going backwards

after the explosion, must be the

same as the momentum
of the bullet going forwards.

momentum = mass x velocity

For the gun

momentum = 2·5
x 8

= 20·0 kgm/s.

For the bullet

20·0 =
0·02
x v

v
= 20·0
÷ 0·02

= 1000 m/s.

The velocity of the bullet = 1000 m/s.

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