The shopping bag or rescue helicopter

A plastic shopping bag can carry a lot of heavy shopping but if you try to lift it up quickly the handles break - why is this?
This is because the bag has to do two things when you lift it quickly.
First support the shopping (mg) and secondly accelerate it upwards (ma).

As you know acceleration needs a force and so this extra force must be supplied through the handles. If they are not strong enough they snap. The greater the mass of the shopping the greater its inertia and the more force is needed to accelerate it.




Moral - always lift shopping slowly!

The same problem occurs when a rescue helicopter lifts a stretcher off a mountainside – the combined weight of the patient and the stretcher plus the accelerating force must be less than the breaking force in the lifting cable.






The same thing could happen in a lift that accelerated upwards quickly - people standing inside the lift holding their shopping could find the handles of their plastic shopping bags snapping!

As you get older it is more important than ever to "warm up" properly before running. Your muscles get less flexible with age and the sudden forces produced in sprinting can cause them to tear in extreme cases.

Free fall parachutist

A sky diver leaves a plane and starts to fall towards the ground. Initially their weight exceeds the drag on them due to the air and so they accelerate downwards. As the speed increases so does the drag until drag = weight. Then the parachutist falls at a constant terminal velocity