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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.

  1. Marked by a teacher

    Physics of Rollercoasters

    4 star(s)

    Newton put forth a variety of laws that explain why objects move (or don't move) as they do. Newton's first law is a restatement of what Galileo had already described that force acting on a body determines acceleration, not velocity. This insight lead to Newton's First Law- an object at rest will stay at rest and an object in motion will stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

    • Word count: 2587
  2. Marked by a teacher

    Physics Investigation: The effect of speed on braking distance

    4 star(s)

    E.g., ice is more 'slippery' than a rough surface because it is very smooth, and therefore exerts less friction on the object travelling on it. The friction force gradually converts the kinetic energy of the moving object into different forms of energy such as sound and heat, until the object's kinetic energy is reduced to zero - hence why it always stops eventually. The longer it takes for the object to have its kinetic energy reduced to zero, the longer the braking distance.

    • Word count: 2704
  3. Marked by a teacher

    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
  4. Marked by a teacher

    The purpose of this experiment is to see what factors affect the period of one complete oscillation of a simple pendulum.

    4 star(s)

    is the only force making the mass move and not gravitational potential energy (GPE). I will test the extremes of these factors as I can assume that if they have any effect on the period of oscillation it will become obvious. To make sure my results are accurate enough to allow for any anomalies I will repeat the experiment 2 times for each test. To keep the results as accurate as possible I will measure the period of 10 oscillations and only use one decimal place to allow for my reaction time. Prediction I predict that the factor that effects the swing time in the pendulum experiment will be the length.

    • Word count: 2115
  5. 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
  6. 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
  7. Marked by a teacher

    To investigate the factors that affect the stopping distance of a catapulted margarine tub. In this experiment, I will be concentrating on the effect that varying the mass of the catapulted tub has on its stopping distance (sd.).

    4 star(s)

    When the elastic band is pulled back then it has Elastic Potential Energy (EPE) which is changed to kinetic energy (KE) when the band is released. If the elastic band is pulled back further then it has a higher level of EPE to start off with, so when it is released, it will have more KE and the margarine tub will go further before stopping. If the margarine tub was on a sloped surface, going down, then the same amount of EPE and KE would take it further than it would on a flat surface because the KE would turn into GPE (Gravitational Potential Energy)

    • Word count: 3693
  8. Marked by a teacher

    The Simple Pendulum Experiment

    4 star(s)

    After gaining the results of this experiment, I will be able to plan my main experiment more thoroughly. Preliminary Experiment The diagram below shows a brief set-up of the simple pendulum experiment The pendulum (3) will be held by a clamp stand (1) which will be placed on a work bench table (2) Fig. 1: Simple Pendulum Experiment Set-up Figure 2 I will use a digital chronometer (4), which is accurate to within �0.005s to measure the time of the oscillation, and a ruler (accurate to within �0.0005m)

    • Word count: 2780
  9. Peer reviewed

    Investigating factors that affect the bounce height of a squash ball

    5 star(s)

    This means that once the squash ball reaches a certain speed (its constant, or terminal velocity), it will cease to accelerate. Therefore, no additional speed will be acquired, and so the impact force will be the same. This means that once the terminal velocity can be reached, the drop height should no longer affect the bounce height. There are also a number of variables other that could affect the bounce height of a squash ball. In order to make this experiment fair, I need to eliminate any other variables that might affect the results.

    • Word count: 3683
  10. 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
  11. 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
  12. Peer reviewed

    length of a simple pendulum affects the time

    4 star(s)

    As the pendulum falls the potential energy is transferred to kinetic energy. The speed increases as the pendulum falls and reaches a maximum at the bottom of the swing. Here the speed and kinetic energy are a maximum, and the potential energy is a minimum. As the pendulum rises the kinetic energy is transferred back to potential energy. The speed of the pendulum decreases and falls to zero as it reaches the top of its swing, with the potential energy a maximum again. A small amount of energy is lost due to air resistance as the pendulum swings.

    • Word count: 2341

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