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Forces - Circular Motion

This GCSE Physics quiz about forces will challenge you on circular motion. The International Space Station (the ISS) is a temporary home to astronauts of many different nationalities. It orbits the Earth with a period of about one and a half hours, moving at a speed of almost 28,000 kilometres per hour. Its path across the sky is entirely predictable and it can be spotted easily crossing the night sky if you know where and when to look. Although its orbit is not entirely circular, it essentially follows the rules of circular motion.

Circular motion can be defined as a movement along a circular path. Newton's first law of motion states that a moving object will travel in a straight line at a constant velocity unless a force acts on it.

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This means that in circular motion, there must be a force constantly acting on the object as it is constantly changing direction. This force is given the name centripetal and is a resultant force of the motion and whatever is causing the circular motion. Centripetal force is a virtual force and is provided by the action of something else.

In the case of the ISS, the Earth's gravity is responsible for the centripetal force. Be careful not to confuse centripetal force with centrifugal force - they act in opposite directions. Fairground rides are very good examples of circular motion, the centripetal force is provided by the framework of the ride. It is the centrifugal force that feels like it is trying to throw you off the ride that adds the excitement. There are many other examples of circular motion that are used for the GCSE; vehicles going round a corner, a planet orbiting the Sun, a satellite orbiting a planet and throwing a discus or a hammer at an athletics event crop up quite often. For the GCSE exam, you need to be able to identify what is responsible for the centripetal force in specific examples and also what factors affect the magnitude of the force.

The magnitude of the centripetal force depends on the mass of the object that is moving in circular motion, the speed of the object and the radius of the circle in which it is travelling. Increasing the mass of the object will mean that the centripetal force required to keep it in circular motion will be greater. If all other things are equal, an object that is travelling with a greater speed will require a greater centripetal force to hold it in place. Unlike the other two factors, increasing the radius of the circle in which an object is travelling will actually decrease the centripetal force required to hold it on track. You may even be asked to explain why these factors affect the force in the way that they do.

The final thing to remember about circular motion is that it is an acceleration. Since the object is moving along a circular path, its direction of travel is constantly changing. This means that its velocity is also constantly changing (remember velocity is motion in a specific direction, it is a vector quantity). By definition, acceleration is a change in velocity so an object following a curved path must therefore be accelerating. Its speed remains the same.

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  1. Which of the following is an example of uniform circular motion?
    Whilst a racing car during a race will experience circular motion, it is not uniform circular motion. Strictly speaking, only satellites in geostationary orbits are circular, however, for the GCSE, artificial satellites are regarded as following circular motion rules
  2. Which force acts as the centripetal force responsible for the circular motion of planets orbiting the Sun?
    Gravity is always a pulling force
  3. Which force acts as the centripetal force responsible for a car travelling around a bend?
    If the centripetal force required to maintain the circular motion of the car is greater than the friction of the tyres against the road, the car skids. In wet conditions, friction is greatly reduced which is why you have to drive more slowly round corners when the road is wet
  4. What can increase the centripetal force required to keep an object in uniform circular motion about a point?
    An increase in mass and speed or a decrease in radius increases the centripetal force
  5. An object moving in a circle continuously accelerates towards what?
    It accelerates towards the point of origin of the centripetal force
  6. If the centripetal force acting on the Moon were to suddenly stop acting, what would happen to the Moon?
    It would behave in accordance with Newton's first law and continue on a straight path through space until influenced by another force. The direction of travel would be the directional component of the velocity at the instant that the force of gravity stopped
  7. What does increasing the acceleration of an object following uniform circular motion change?
    Acceleration is related to velocity so any answer that includes the word speed must be incorrect
  8. If a driver drives around a bend faster than normal, what happens to the size of the centripetal force needed to keep the car going around the bend?
    Make sure that you learn the factors that affect the magnitude of a centripetal force
  9. What name is given to the resultant force that acts towards the centre of a circle?
    You need to be able to identify what is responsible for the centripetal force, for example gravity for orbiting bodies
  10. If an object orbits around a point five metres away in three seconds, what will happen to the speed of the object if the object's orbit is increased to six metres?
    The speed will increase as the object will take the same time to complete one orbit. As the distance the object needs to travel increases, the speed must also increase to obey the laws of motion

Author: Martin Moore

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