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Forces - Resultant Forces 01

This GCSE Physics quiz takes a look at resultant forces. Resultant forces are the results of two or more forces acting at the same time. If the resultant force is zero, the effect depends on the initial state of the object. If it is already moving, it will continue to move at the same velocity, in other words, at the same speed and in the same direction as when the resultant forces became zero. If it was stationary to begin with, then it will remain stationary. Resultant forces occur in our daily lives, every day we encounter them but we tend not to consider them. Every time something moves, it is because the resultant forces are not zero.

Recognising the forces acting on an object is the key to working out the resultant force.

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If you find the resultant of several forces is not zero, then there will be an acceleration in the direction of the resultant force. If the object is already moving, the effect of this acceleration can make the object speed up, slow down or change direction. Where a resultant force opposes the motion, the acceleration is negative (a deceleration) and the object slows down. Where the force is in the same direction as the motion, the acceleration is positive and the object speeds up. If the resultant force is at an angle to the direction of motion, the object changes direction.

The GCSE examiners like to use a car as an example of resultant forces but they could use anything. For a car, the forces of interest to the motion are the driving force, air resistance and friction. Let's start with the car at rest. Air resistance and friction are zero, the driving force is zero so the resultant force is zero. The driver then starts the engine and engages the gears. The engine provides a driving force. The driving force is now larger than the air resistance and friction therefore the resultant force is no longer zero and the car accelerates.

But as it does so, the air resistance and friction both increase and so the resultant force decreases. The acceleration of the car becomes less and when the resultant force is zero again, the car moves at a constant velocity. When the driver turns the steering wheel, that introduces a sideways force into the system. The resultant force is no longer zero. Because the new force is at an angle to the other forces, the resultant force is also at an angle, therefore the car changes direction. When the driver removes this sideways force by straightening the steering wheel, the resultant force drops back to zero in all directions and the car continues on, in the new direction.

One of the things that you may be asked to do in the exam is to predict what happens when resultant forces change magnitude. Firstly, you need to consider in what direction each force is acting. Next you should consider the initial state of the object in the question - was it already moving or stationary? Finally, you can perform your calculation to work out the magnitude and direction of the new resultant force. Once you have done all that, you should be able to predict the effect. Don't be afraid to use diagrams to help you.

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  1. The overall effect of the forces acting on an object will NOT ...
    Easy start, making sure that you understand the effect that forces have on objects
  2. What is a resultant force?
    Forces are vector quantities and you need to take into account both their magnitude and direction
  3. What could the effect of a resultant force be?
    If a resultant force is zero, the state of rest or motion will remain unchanged
  4. If an object is stationary and the resultant force acting upon it is zero, what will happen to the object?
    In other words, it remains at rest
  5. If an object is stationary and the resultant force acting upon it is non-zero, what will happen to the object?
    Whilst the resultant force is non-zero, not only will the object move, it will be accelerating
  6. If an object is moving west at a speed of 3 m/s and the resultant force acting upon it is zero, what will happen to the object?
    If an object is moving and the resultant force on the object is zero it will continue to travel in its current path at a constant speed
  7. The driving force exerted by a motorbike is 2000 N. The resultant force acting in a forward direction is 200 N. What is the size of the opposing force (air resistance and friction)?
    Working out the magnitude of resultant forces acting in direct opposition to each other is simply a matter of subtraction
  8. If a ball is travelling at a speed of 3 m/s west and encounters a force which slows the ball at a rate of 0.25 m/s, how long does the ball take to stop?
    Divide the velocity by the deceleration
  9. A person pushes on a box with a force of 10 N which does not move. Assuming no other forces are present, what is the size of the force the box exerts on the person?
    Testing that you understand Newton's third law and resultant forces in the same question
  10. Two forces act on a small ball. One force exerts a force of 10 N east, whilst the other force exerts a force of 15 N west. What is the size and direction of the resultant force?
    East and west indicate that the forces are pushing in directly opposing directions therefore the resultant will be the difference between them, in the direction of the larger one

Author: Martin Moore

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