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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
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    Parachute Coursework

    4 star(s)

    So using a larger weight will mean it will reach its terminal velocity slower. Plan Apparatus Bin liner 4 equal lengths of string 10 one gram weights of plastercine Stopwatch- to measure the time it takes for the parachute to reach the ground Meter ruler- to measure the distance the parachute has fallen To make it a fair test I repeated the test 3 times and made an average of my results. Pre-test As we had no idea on what size of parachute area, weight of plastercine or length of string to use we had to do a pre-test, we tested each variable.

    • Word count: 796
  2. Peer reviewed

    Physics Practical

    4 star(s)

    Lay the line over the pulley, and align the trolley so it will run straight 6. Place the accelerometer behind the trolley so it will record the acceleration by measuring the distance 7. Have the trolley pulled back far enough to allow an accurate reading for the accelerometer and a person at the end to stop it falling from the table 8. Run 5 trials per mass and record the values for acceleration 9. Repeat steps 7-8 for 700g, 800g, 900g, 1000g, 1100g, 1200g, 1300g, 1400g, 1500g Diagram of Investigation trolley fishing line accelerometer pulley bob mass table GLX Data Collection and Processing Table 1: Collection of Data for the acceleration of different masses Mass of trolley (g)

    • Word count: 823

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