Electricity
Particles
Space
Waves

#### Terminal velocity

To explain this concept, let's take the example of a sky diver.

• To begin with our sky diver will be accelerating because the weight is greater than the upward force.
• The upward force, air resistance, increases becoming increasingly closer to the force due to gravity. Until...
• The two forces balance and the diver falls at a constant speed called terminal velocity. He will not get any faster.
• When the parachute opens, speed will decrease until air resistance and gravitational pull balance and a new, lower terminal velocity is reached.
• So objects reach a terminal velocity because all forces are balanced. And this terminal velocity can only be changed if one of the forces change. The sky diver would get to an even faster terminal velocity if he took off the parachute pack for example.
www.scienceaid.co.uk does not advise sky diving without a parachute!

#### Elastic and Spring Extension

When a force is applied to a rubber band or spring it changes shape.

Spring. The extension is directly proportional to the force (straight line) until a point where more force is required to have equal extension. This point is known as the limit of proportionality or elastic limit. If the force is removed once the spring has reached this point, it will not return to the original shape, and has been deformed.

Rubber band. The force-extension relationship is not proportional. First less force is required to stretch it and then more but unlike the spring, the rubber band will return to its original shape and size.