# Principles of mechanical systems

Most products rely on movement to work, eg in a pair of scissors the blades need to move together to cut. This movement is called a motion, and the motion of a product may be hidden or visible. The scissors are an example of a visible motion. Alternatively, although the movement of a sewing machine needle may be seen, the that moves it is hidden inside its and is an example of hidden motion.

For products to move and give an output motion, a force or an input motion is required. Squeezing the scissor handles provides the input motion, which pushes the blades together, and the output motion is the blades cutting the material. Forces don’t just push - they can twist or pull as well. These forces are measured in units called Newtons (N).

The size of the force or motion affects the output.

In the example of a push bike:

• input - the force applied to the pedals by the cyclist’s feet
• process - the chain turns the gear system, converting the energy produced to cause …
• output - the wheels turn and the bike moves

### Force vs. distance

If a cyclist is pushing with 150 watts (W) of power when cycling uphill, it is the same power as 150 W when cycling on flat ground. Although 150 W of power is the same no matter what the gradient, the 150 W for uphill cycling feels harder and, as such, the speed of travel can change and less distance will be covered due to the force of gravity.