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Forces - Forces and Motion

Forces and motion looks at movement and acceleration of objects. Considering how forces affect the motion of objects is an important part of GCSE Physics. Many of the concepts and formulae used in this module were worked out by Sir Isaac Newton, the famous and talented seventeenth century scientist whose name is now used for the unit of force.

Newton formulated three laws of motion based on his observations. His first law is written down slightly differently, according to what source of information you use. Essentially, it says that "an object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force". In other words, objects will tend to keep doing exactly what they are doing, unless a force is applied. It is sometimes called the 'law of inertia'.

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But the crucial point that Newton realised was that the force must be unbalanced. So, in practical terms, when a force is applied to a stationary object, it could move or, it could stay exactly where it was or it could change shape. It all depends on the balancing forces. If the applied force is insufficient to overcome friction, or if the object is fixed in place, it won't move. If the applied force is sufficient to overcome the reaction force of the fixing, the object will move. If the forces keeping the object in shape are less than the force applied, then the object will change shape and could break.

When a force acts on a moving object, several things could happen. It could change shape, direction or speed. What definitely won't happen is nothing! It is all about the resultant force.

When an unbalanced force is acting on something, it will accelerate, but as soon as the force is removed, the acceleration will stop and the object will continue with the same velocity. Remember, velocity is a vector and has both magnitude and direction. From this idea, Newton's second law links the resultant force to the mass and the acceleration of an object in the form of the equation F = m x a.

Where you don't have the value of the force causing the acceleration, or the mass of the object, you can calculate acceleration from the change in speed or velocity divided by the time taken for the change. If you plot a velocity-time (or speed-time) graph, the gradient at any given point is the acceleration. Acceleration is a measure of the change in speed in a given time so if you think about how you work out the gradient of any graph, you will see why that is the case.

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  1. What is the acceleration of an object dependent on?
    Changing either mass or resultant force will change the acceleration of an object
  2. If a boat of mass 500 kg is accelerating at 1 m/s2, what is the resultant force?
    Only unbalanced forces produce an acceleration
  3. What is the unit used to measure force?
    Named after Isaac Newton since he carried out the key work that lead to an understanding of forces, although Galileo had already worked out the first law many years before Newton was born
  4. What does the gradient of a velocity-time graph represent?
    The gradient of a velocity-time graph measures the change in velocity per second, in other words, exactly the definition of acceleration
  5. What is the acceleration of a ball whose initial velocity is 0 m/s and final velocity is 10 m/s if the ball takes 5 seconds to get to this speed?
    Remember that acceleration is the rate of change of velocity and its units are m/s2
  6. What does the gradient of a distance-time graph represent?
    The gradient of a graph is calculated by dividing the y value by the x value. In this case, you are dividing metres by seconds (or kilometres by hours and so on) so the units will be metres per second (or kilometres per hour etc) which you should recognise as being the units of speed
  7. What is velocity?
    Velocity is a vector quantity and has both magnitude (size) and direction. The magnitude represents how fast the object is moving and the direction represents in which direction it is going
  8. What is the equation to find the acceleration of an object, knowing its initial and final velocities and the time taken?
    It is always the final velocity minus the initial velocity divided by time. A negative value from the calculation means there was a deceleration
  9. What is the correct formula which links force, mass and acceleration?
    This is the equation of Newton's second law of motion
  10. What is the mass of a person who is accelerated at a rate of 2 m/s2 when a resultant force of 130 N is applied?
    Using the equation F = m x a, you can calculate the force required to accelerate an object, the mass of it or the acceleration of it when given two of the three values

Author: Martin Moore

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