What do you feel when a car or a bus starts (i.e. when it is speeding up -- accelerating)? What does it feel like when it brakes? What happens if you let something hang from string, e.g. a yo-yo? Can you see how the the yo-yo is a bit behind when the bus starts. This is related to the concept of inertia -- a force is required to change the velocity. Similarly, the yo-yo moves ahead when the bus brakes again. When the string pulls the yo-yo backwards to slow it down, the yo-you pulls the string forward (Newton's third law). The photos show a keyring on chain during start and landing of airplane.

Not only starting  and braking make the yo-yo hang in a different way. When the bus turns left, a force to the left is required to change the direction of motion. The bus pushes  you to the left – and you push to the right on the bus (action and reaction!). When the string pulls to the left, the yo-yo pulls the string to the right. The yo-yo thus hangs slightly outward during the turn. Try walking with a yo-yo on a string and change the motion in different ways. Observe what happens.

The figure shows a team-skating wing. Everyone leans towards the centre, since they are moving around in a circle. The skater furthest away from the centre must move fastest. She must thus have the largest acceleration and lean the most. The angle provides a measure of the horizontal acceleration.

How large is the acceleration of an airplane during start and landing?