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GCSE: Forces and Motion

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Balanced and unbalanced forces

  1. 1 There are many words which mean force. E.g. push, pull, friction, weight, air resistance, tension, thrust. All are measured in newtons (N).
  2. 2 When a body is acted on by more than one force at the same time, the overall force is called the resultant force. E.g. if a car is pushed to the right with a force of 500 N and to the left with a force of 200 N, the resultant force is 300 N.
  3. 3 When the resultant force is greater than zero, the forces are unbalanced and this will cause a change in speed or direction, or both. For the example of the car, the 200 N resultant force would cause the car’s speed to increase so the car is accelerating.
  4. 4 What if the brakes are applied to the car? The braking force acts in the opposite way to the direction in which the car is moving. This time the speed decreases and the car is decelerating.
  5. 5 When the resultant force is zero, the forces are balanced. The body will continue to move with a constant speed in the same direction. This is true for a skydiver falling with a constant speed called the terminal speed. The air resistance is equal to the weight.


  1. 1 When the forces on a body are unbalanced, the resultant force, F causes an acceleration, a. We can calculate the acceleration using an equation F = ma.
  2. 2 In this equation m is the mass of the body measured in kilograms (kg). F is the force measured in newtons (N) and a is the acceleration measured in m/s2.
  3. 3 You should practice how to write the equation in three different ways by rearranging it:

    1) F = ma
    2) m = F/a
    3) a = F/m
  4. 4 Suppose a resultant force of 20 N acts on a body giving it an acceleration of 4 m/s2. What is the mass of the body? Choose an equation for m, so we use m=F/a = 20/4 = 5N.
  5. 5 A car of mass 2000 kg is acted on by a force of 500 N. What is the acceleration? Choose the equation for a, so we use a = F/m = 500/2000 = 0.25 m/s2.

Motion under gravity

  1. 1 The weight of a body, W is a force and it can be calculated from the equation W=mg. g is the gravitational field strength. On Earth, g has a value of 9.81 N/kg.
  2. 2 What is the weight of a mass of 20 kg? W = mg = 20 x 9.81 = 196.2 N
  3. 3 On the Moon, the value of g is much smaller than on Earth , so the same body will have a smaller weight . The value of g on the Moon is about one sixth of g on Earth so the weight will be ⅙ of the weight on Earth. So the mass of a body doesn’t change when the body is moved from the Earth to the Moon but its weight changes.
  4. 4 If weight is the only force acting on a body, then we can use the weight to calculate the acceleration when a body is released. What is the acceleration of an apple of mass 0.1 kg which falls from a tree? W = mg = 0.1 x 9.81 = 0.981 N. Now we can calculate the acceleration using a = F/m. (Remember that F=W) so a = 0.981/0.1 = 9.81 m/s2.
  5. 5 Even if we had changed the mass of the apple to 0.2 kg, the acceleration would still be the same! The apples would hit the ground at the same time.

  • Marked by Teachers essays 28
  • Peer Reviewed essays 14
  1. Marked by a teacher

    The aim of this experiment was to compare the elasticity of arteries and vein tissue and to identify how the structure of blood vessels relates to their functions.

    4 star(s)

    Based on the above scientific knowledge it was believed that the arteries may well have more elastic fibres than the veins, as arteries have a high pressure, which needs to be kept constant for blood to reach the extremes of the body. Veins will therefore have less elastic fibres due to their lower pressure environment. This leads to the establishment of a hypothesis that arteries have more elastic fibres than veins. Also due to the artery having a smaller lumen (relative to its diameter)

    • Word count: 1703
  2. Marked by a teacher

    Investigating a Cantilever.

    4 star(s)

    Also a perfect length so that the cantilever is safe and isn't likely to break. Length (mm) Mass (g) Start Height (mm) Finish Height (mm) Deflection (mm) 100 500 831 831 0 200 500 831 828 3 300 500 831 823 8 400 500 831 813 18 500 500 831 789 42 600 500 828 759 75 700 500 825 716 109 These are results of our preliminary test From these results we decided that 500mm was the optimum length of the cantilever.

    • Word count: 1032
  3. Peer reviewed

    wind power

    4 star(s)

    Evidently, wind power is dependant on the weather. To make the most of the available wind, wind turbines need to be situated in areas with high and regular wind speeds which tend to be mountainous or near the coast. Transmitters need similar sites and this limits the locations available for turbines. In 1982, the UK's first turbine was built onshore in South Wales by the Central Electricity Generating Board. From the late 80's plans started emerging to build an offshore turbine of the coast of Norfolk in the North Sea.

    • Word count: 1499

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