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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. Marked by a teacher

    Investigate the relationship between GPE (Gravitational Potential Energy) and KE (Kinetic Energy) for a trolley or ball rolling down the slope.

    3 star(s)

    makes it more likely for my results to be more accurate. So the question I investigation is, Is there a relationship between Velocity (V) and Height (H). To investigate this I used the following equipment: Ramp- To roll the trolley off of, vary the height of the surface. Two light gates- To pick up the readings from point A to B Data Logger- Connected to the two light gates, to show readings Metre Ruler- To measure the height on the slope/ramp Standing Clamps-To holding the two light gates and slope in place.

    • Word count: 1341
  4. Marked by a teacher

    Physics Pendulum Experiment

    3 star(s)

    a simple pendulum swinging back and forth. A pendulum is able to operate when the mass is lifted to an angle and then released, and by doing this, the pendulum will be using Gravitational Potential Energy. In my experiment, I am going to find out if there is a relationship between the mass of the weights, and the time taken for a full oscillation. I predict, that as the mass increases, the time taken for a full oscillation will increase, as there will be more weight. Method The experiment was setup on the pendulum at a fixed length.

    • Word count: 1427
  5. Marked by a teacher

    Biology Investigation What affects the crawling speed of maggots?

    3 star(s)

    This kind of natural phenomenon is apparent especially in evolution of species. For example, for a cheetah to be able to survive in the wild, it must be fast so it can catch it's pray and outrun predators. But this is not the only thing the cheetah needs for survival; it must also have strength (mass) for it to be able to fight other cheetahs for mates or territory. If the cheetah was not large enough it would lose fights and may never produce any cubs and would not be able to fight for territory, if the cheetah did not have sufficient mass or strength, it would not survive and its chain would be broken.

    • Word count: 1643
  6. Marked by a teacher

    Stopping distances of toy cars travelling down a ramp

    3 star(s)

    In the first experiment I will keep the height of the ramp at 10cm so that height does not affect the stopping distance. Then the car will be placed at the top of the ramp and released, not pushed. The weight of the car will increase from 30g, the weight of the car, to 80g in 10g intervals. Six results will therefore be obtained and the car will be released from the top of the ramp three times on each weight to gain an average and hence fairer and more accurate results.

    • Word count: 1092
  7. Marked by a teacher

    Experiment Report: Studying a simple harmonic oscillator.

    3 star(s)

    During the oscillation, the pendulum bob accelerated and then decelerated when it was approaching its highest position. The acceleration of the bob was always pointing towards its equilibrium position. 3. The two most widely spaced dots were marked on the tape - the zero position of the pendulum bob was found. 4. Every dot on the tape was marked from the zero position. The displacements of these points from the zero position were measured and the corresponding time was worked out. Time interval between two successive dots = 0.02 s. 5. These data was plotted on a displacement-time (x-t)

    • Word count: 1482
  8. Marked by a teacher

    Investigating a factor which can affect the period of a pendulum.

    3 star(s)

    When the bob is above the point of natural suspension, it is going against the natural gravitation force and thus, gains Gravitation Potential Energy. When the bob is drawn and released at whatever angle, the gravitational force acts upon it by moving the pendulum down, towards the point where it is vertically suspended at rest. From this, we can say that as the pendulum is released, its Gravitational Potential Energy is converted to Kinetic energy in order for the pendulum to move.

    • Word count: 1991
  9. Marked by a teacher

    How does the height of a ball drop affect the bounce?

    3 star(s)

    * Temperature: The temperature of the ball will affect the speed that the air molecules move, and therefore changing the pressure within the ball and the bounce upon impact with the surface. * Colour: The colour may affect the bounce of ball as certain colours attract more sunlight and the more light there is, the more heat there is and this would consequently change the pressure within the ball. For my preliminary work I conducted a series of tests to work out the best variables to change for my experiment.

    • Word count: 1064
  10. Marked by a teacher

    Friction Between two surfaces

    3 star(s)

    There are two types of friction static and sliding. Sliding friction is the frictional force, which exists between two adjacent surfaces, which are in relative motion, and is usually slightly less than the limiting frictional force between the surfaces. Static friction is the force of friction present when there is no motion between the two surfaces in contact. After looking up experiments to show that there are two types of friction in A-level physics - Roger Muncaster not only did I find and experiment but it show another factor the coefficient of limiting friction.

    • Word count: 1760
  11. Marked by a teacher

    I aim to find out if the mass of an object affects the speed at which it falls.

    3 star(s)

    According to Vincenzo Viviani Galileo demonstrated his conclusions by dropping weights from the leaning tower of Pisa." I will drop the object, which will be a small container, from a height of 30cm. I will measure the speed of the object using a light gate, which will make my results accurate to two decimal places. I will use sand to increase the mass of my object because this will be very accurate and it will not alter the shape of the object. After my preliminary test I concluded that 30cm would be the best height to drop the container from.

    • Word count: 1122
  12. Marked by a teacher

    Investigate one factor which affects the time taken for a paper cone to fall a known distance.

    3 star(s)

    Apparatus * Two 1 metre rules * Card cone * Stop watch * Angle to surface area conversion sheet The stopwatch is to time how long the card cone takes to fall a set distance. I will find the best distance in my preliminary experiments. Card will be used to drop because that is the only available material and because it is more rigid than paper but not too heavy. In my preliminary experiments I will also find then best position to place the paper clips to hold the card cone in place.

    • Word count: 1245
  13. 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
  14. Peer reviewed

    Physics Pendulum

    3 star(s)

    Measure the time for 20 complete swings when the bob is given an amplitude of 4 cm, 6 cm, 8 cm, and 10 cm respectively. Enter your results into table B. 3. Determine the effect on the period of the pendulum when the length of string is altered. Keep the mass of the bob constant, and give the bob the same amplitude of 6 cm in each case, but change the length of the string so that results are obtained for lengths of 10 cm, 20 cm, 30 cm, and 40 cm.

    • Word count: 1283
  15. Peer reviewed

    road accidents

    3 star(s)

    Graph showing road deaths Source: Portugal: light blue line 2. Scientific Aspects 2.1 Cars Each year cars are getting better due to new technologies. Now many cars are starting to incorporate technologies like ABS which do not let cars slip, preventing many accidents. Air Bags also protect people from getting hurt, protecting the body from hitting in a hard surface. Many other scientific aspects are used in cars construction, which contribute to make them more comfortable and safer, reducing road accidents.

    • Word count: 1391
  16. Peer reviewed

    Hookes lab

    3 star(s)

    This is defined as the 'elastic limit' of the spring. The force constant 'k' of a spring is the force needed to cause unit extension, i.e. 5cm. If a force 'F' produces extension 'e' then, k = F e HYPOTHESIS: As seen the Hooke's law states that the extension of a spring is directly proportional to the tension supplied to it. Therefore I predict that as more weights will be added the extension will increase almost proportionately. Therefore I also predict that the graph will be as shown below: The x axis shows the extension produced by the spring, whereas the y axis shows us the stretching force applied to the spring.

    • Word count: 1716
  17. Peer reviewed

    investigation of a simple pendulum

    3 star(s)

    We can use the equation, T = 2 ? V L / G Where (T) is Time, (L) is length of string and (G) is the gravity, to calculate the acceleration due to gravity. I predict that the shortest length will produce the smallest time period. This would take place because the torque (force of metal bob (weight) X distance of string) produced would be relatively less, as the length of string (distance) would be smaller. This will also happen because the metal bob will have a lesser distance to travel, to complete an oscillation, due to the shorter length of the string.

    • Word count: 1941

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