Momentum

An object that’s moving is said to have momentum. This refers to the object’s tendency to continue moving in the same direction. The more momentum an object has the harder it is to change its direction.

Momentum can be calculated using the following equation:

p = m x v

• – p is the momentum in kilograms metres per second (kg m/s)
• – m is the mass in kilograms (kg)
• – v is the velocity in metres per second (m/s)

The momentum of an object depends on an object’s:

• – mass
• – speed
• – direction

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Conservation of momentum

The total momentum of collisions and explosions stays constant as long as no external forces act on the objects involved. This means that the momentum is conserved. Using this idea you can figure out the momentum, velocity or mass of an object involved in a collision or explosion.

For example: two cars collide on a road then move off together. Car A has a mass of 1,500 kg and is moving at 5 m/s before it collides. Car B is stationary and has a mass of 1,000. After the collision, what is the velocity of the cars?

First you need to work out the total momentum before the collision took place:

p = m x v

• – momentum of car A = 1,500 x 5 = 7,500 kg m/s
• – momentum of car B = 1,000 x 0 = 0 kg m/s
• – total momentum before = 7,500 + 0 = 7,500 kg m/s

Now figure out the total momentum after the collision.

• – the momentum is conserved so the total momentum afterwards = 7,500 kg m/s

Now calculate the total mass after the collision:

• – total mass = mass of car A + mass of car B = 1,500 + 1,000 = 2,500 kg

You can now work out the new velocity:

• – velocity after the collision = 7,500 / 2,500 = 3 m/s