# Momentum and Energy

We invite any aspiring physicists to try their hand at these physics problems dealing with the concepts of momentum and energy:

1. Martial arts such as Judo, Aikido and Japanese Jiu Jitsu teach dynamic throws. These are explained to students as redirecting their attacker’s momentum. Can you explain this by separately considering the linear motion of the attacker’s centre of mass and their rotational motion about their centre of mass?
2. Particle annihilation occurs when a particle meets its anti-particle (same mass, opposite physical properties e.g. opposite charge) and the mass is converted into kinetic energy of two photons. The energy released is given by Einstein’s famous equation E = mc2. Estimate how much energy would be released if all electrons on Earth were annihilated.

Bonus Question:

When computer designers are programming a computer game they have to take into account the laws of physics – many gamers take these for granted and do not realise how much their gaming experience would change if these were not consistent with reality!

Consider the effects of the following changes on gameplay:

• The gravitational field strength changes.
• The law of reflection doesn’t apply.
• Any or all of Newton’s laws don’t apply.
• Electromagnetic waves don’t all travel at the speed of light in a vacuum.
• Hooke’s law doesn’t apply.
• The density of water changes.
• The speed of light gets much smaller (could think about special relativity or E=mc2).
• There is no air resistance.

You may have to do some research! How creative can you be? Can you think of any other scenarios?

Rosalind Jack – KS4 Physics co-ordinator and STEM / enrichment, Woodkirk Academy