# 3 Act Physics – Momentum

Basic Principles of the 3 Acts are shown here

Momentum – Demolition balls

You may want to start with the shortest question you can. In this case;

Should demolition balls be bouncy or not?

How do ‘dead blow’ hammers work ?

Stilleto Tools suggest that their titanium hammers have a 97% efficiency compared to 70% for a steel hammer. Could this be true and how would you test it?

Act 1:  Demolition Ball

Act 2: The explore

What questions does this make you think of?

Which one would be the most effective at knocking the glass over?

Which is the most efficient at transferring energy?

What could happen if ….? (what matters?)

Students could investigate

• Different material balls
• Different material ‘buildings’ most demolitions would be brick built
• Materials to dampen the ball
• The effect of the height
• Finding the best compromise between effectiveness and bounce (practically we don’t want our demolition ball to bounce too far or we will struggle to control it and may demolish many things we don’t want to!

Thinking

Two things we may want to consider are the law of conservation of momentum (The total momentum of the system is conserved)  and the law of conservation of energy (energy is neither lost, nor destroyed)

There would be  little change in the kinetic energy of the bouncy ball as it rebounds with a similar speed to the impact speed, however there is a large change in the velocity as although the magnitude of the velocity doesn’t change much, the direction has reversed. So there will be a large change in the momentum of the ball.

The plasticine ball is likely to stop on impact and so there will be a large change in the kinetic energy of the ball, but how efficient is this transfer? There will be less change in the momentum than for the bouncy ball as there is no rebound.

So what will happen?

Act 3: The reveal

The bouncy ball is clearly more effective at knocking the glass over.

Why?

If we ignore the energy aspects and simply look at the momentum it is very clear

Momentum (kgm/s) = Mass (kg) x velocity (m/s)

Total momentum before collision = total momentum after collision

Both balls have the same initial momentum before the collision. For the bouncy ball this momentum becomes negative after the collision and zero for the plasticene ball. Hence the glass must have more momentum after the bouncy ball hits it than the plasticene one

There are many other aspects that could be investigated

The efficiency of hammers

Why ‘dead blow’ hammers do less damage to soft surfaces

What real demolition balls are like