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Centripetal force theory
To make an object accelerate, there has to be an unbalanced force acting in the same direction as the
acceleration (Newton's first law). In the case of circular motion the unbalanced force is called the
When a train travels around a bend, the centripetal force comes from the push of the outer rail on
the wheel. For a road vehicle such as a car, bus of bicycle it is the push of the road on the tyres; if
there is insufficient friction due to ice or a slippery surface, the vehicle does not complete the turn
and may leave the road.
The size of the unbalanced force required to maintain circular motion can be calculated using F = ma
The theme park ride
Some theme park rides involve motion in a vertical circle. Part of the thrill of being on one of these
rides is the apparent change in weight as you travel around the circle. This is because the force you
feel as you sit or stand is not your weight, but the normal contact force pushing up. If the forces on
you are balanced, this force is equal in size to your weight, but it can become bigger or smaller if the
forces are unbalanced, causing you to feel heavier or lighter.
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At the bottom of the circle:
The person feels heavier than usual as the size of the normal reaction force is equal to the
person's weight plus the centripetal force required to maintain circular motion.
At the top of the circle:
The person may feel lighter than usual as the person's weight is providing part of the
centripetal force required to maintain circular motion.…read more