P2aL11 Off with a Bang

Key Words

Conservation of momentum - momentum before is the same as the momentum after.

Direction  - a line along which something moves.

Mass - the amount of material in an object.

Negative momentum - the product between mass and velocity, going from left to right.

Positive momentum - the product between mass and velocity, going from right to left.

Velocity - speed in a certain direction

Grade E

Momentum is useful for explaining collisions.  Now we will see how momentum explains what happens in explosions and rockets.  An explosion is not just a bang, it is any system which has zero initial momentum, then flies apart.  The key point is that the two bits have the same momentum:

Negative momentum is equal and opposite to the positive momentum

Two masses m1 and m2  are involved in an explosion.  The two bits go off at velocities -v1 and +v2 respectively.

So we can say:

-m1v1 + m2v2 = 0

Therefore:

m1v1 = m2v2

Grade C

There are a number of applications of this idea:

Look at this coin trick:

Momentum from Coin A is transferred to apply a force on the Coin B.  Since Work done = force × distance moved, work is done in compressing the metal of the coin.  This is transferred as elastic energy to Coin C, which in turn receives a force from B.  This results in acceleration. 

It's a rather complicated way of saying that momentum is transferred.

Aeroplanes use jet engines or propellers to throw huge amounts of air backwards.  The momentum of the aeroplane is equal and in the opposite direction to the momentum of the air.

When a gun fires a bullet, the total momentum of the gun and the bullet remains zero.  Therefore the momentum of the gun is the same as the momentum of the bullet.  Since the gun is much heavier than the bullet, the velocity of the gun is much less.

Grade A

When a propeller aeroplane takes off, the propeller is spinning at high speed, about 2500 revolutions per minute (r.p.m.).  This results in the propeller having a high angular momentum.  It results in a force acting on the aeroplane, turning it to one side (usually to the left).  The pilot needs to counteract this by applying some rudder to the right, to keep the aeroplane in a straight line.

In extreme cases, the areoplane can end up facing the wrong direction.  This is called a ground-loop.

In flight, the angular momentum from the propeller still turns the aeroplane to the left, but this can be counteracted by applying a certain amount of aileron.  In many aeroplanes, there is a certain amount of built in trim to help with the problem.

Angular momentum is essential to the operation of gyroscopes that keep aeroplanes and ships steady.  A helicopter mounted camera has a gyroscope that enables it to stay steady while the helicopter makes frequent adjustments to its course.

The artificial horizon on an aeroplane control panel also has a gyroscope.

It enables the pilot to see if he is flying level.  In this plane which has an electronic cockpit, the mechanical gyroscope is used as a back up in case the electronic panel fails.