Module P3: Forces for Transport P3b: Changing Speed In this item the idea of acceleration is developed. The concept of velocity is introduced here, and is developed further in P5. Accelerations (involving the change in speed) of cars can be used and graphically illustrated and studied. GRADE G - D C GRADE B – A* Targets for Improvement Describe the trends in speed and time from a simple graph: • horizontal line – constant • straight line positive gradient – increasing speed • straight line negative gradient – decreasing speed. Describe, draw and interpret qualitatively, graphs of speed against time for uniform acceleration to include: • greater acceleration shown by a higher gradient • the significance of a positive or negative gradient • calculations of distance travelled from a simple graph for uniform acceleration. Describe, draw and interpret graphs of speed against time including: • quantitatively for uniform acceleration • calculations of distance travelled from a speed-time graph for uniform acceleration • calculations of acceleration from a speed-time graph for uniform acceleration • qualitative interpretation of graphs for acceleration. Recognise that acceleration involves a change in speed (limited to motion in a straight line): • speeding up involves an acceleration Describe acceleration as change in speed per unit time and that: • increase in speed results from a positive acceleration Explain how acceleration can involve either a change: • in speed • in direction • in both and direction.
Describe thinking distance as: • the distance travelled between the need for braking occurring and the brakes starting to act. Describe braking distance as: • the distance taken to stop once the brakes have been applied. Describe stopping distance as: • thinking distance + braking distance. Calculate stopping distance given values for thinking distance and braking distance. Explain why thinking, braking and stopping distances are significant for road safety. Explain how certain factors may increase thinking distance: • driver tiredness • influence of alcohol or other drugs • greater speed • distractions or lack of concentration. Explain how certain factors may increase braking distance: • road conditions • car conditions • greater speed. Interpret data about thinking distances and braking distances. Explain the implications of stopping distances in road safety: • driving too close to the car in front (i.e. inside thinking distance) • speed limits • road conditions. Explain qualitatively everyday situations where braking distance is changed including: • friction • mass • speed • braking force. Draw and interpret the shapes of graphs for thinking and braking distance against speed. Explain the effects of increased on: • thinking distance – increases linearly • braking distance – increases as a squared relationship e.g. if speed doubles braking distance increases by a factor of four, if speed trebles braking distance increases by a factor of nine.
Recall that a sudden change in momentum in a collision, results in a large force that can cause injury. Explain how spreading the change in momentum over a longer time reduces the likelihood of injury. Explain, using the ideas about momentum, the use of crumple zones, seatbelts and airbags in cars. Explain why forces can be reduced when stopping (e.g. crumple zones, braking distances, escape lanes, crash barriers, seatbelts and airbags) by: • increasing stopping or collision time • increasing stopping or collision distance • decreasing acceleration. Describe the typical safety features of modern cars that require energy to be absorbed when vehicles stop: • heating in brakes, crumple zones, seatbelts, airbags. Explain why seatbelts have to be replaced after a crash. Recognise the risks and benefits arising from the use of seatbelts. Recall and distinguish between typical safety features of cars which: • are intended to prevent accidents, or • are intended to protect occupants in the event of an accident. Describe how seatbelts, crumple zones and airbags are useful in a crash because they: • change shape • absorb energy • reduce injuries. Describe how test data may be gathered and used to identify and develop safety features for cars. Evaluate the effectiveness of given safety features in terms of saving lives and reducing injuries. Describe how ABS brakes: • make it possible to keep control of the steering of a vehicle in hazardous situations (e.g. when braking hard or going into a skid) • work by the brakes automatically pumping on and off to avoid skidding • sometimes reduce braking distances. Analyse personal and social choices in terms of risk and benefits of wearing seatbelts.
Module P3: Forces for Transport P3g: Falling Safely Falling objects are usually subject to at least two forces - weight and drag. Some cars have similar engines to others yet have very different top speeds. This is to do with pairs of forces which may or may not balance. These ideas are of vital importance to the parachutist and drag-racer who want to slow down in time - safely! Investigating falling whirligig, parachutes or plasticine shapes provides the opportunity to explain phenomena by developing and using scientific theories. Work on the balance of forces illustrates the use of modelling in developing scientific understanding. GRADE G - D GRADE C GRADE B – A* Targets for Improvement Recognise that frictional forces (drag, friction, air resistance): • act against the movement • lead to energy loss and inefficiency • can be reduced (shape, lubricant). Explain how objects falling through the Earth’s atmosphere reach a terminal speed. Explain in terms of the balance of forces how moving objects: • increase speed • decrease speed • maintain steady speed. Explain, in of balance of forces, why objects reach a terminal speed: • higher speed = more drag • larger area = more drag • weight (falling object) or driving force (e.g. a car) = drag when travelling at terminal speed. Understand why falling objects do not experience drag when is no atmosphere. Recognise that acceleration due to gravity (g) is the same for any object at a given point on the Earth’s surface. Understand that gravitational field strength or acceleration due to gravity: • is unaffected by atmospheric changes • varies slightly at different points on the surface • will be slightly different on the top of a mountain or down a mineshaft.
Module P3: Forces for Transport P3h: Energy of Games and Theme Rides Rides at theme parks are designed to thrill and frighten you in a safe way. We pay good money to have our ‘gravity’ distorted. Theme ride designers are experts on energy and forces. Their simple trick is to use gravity and potential energy as the source of movement. This item will help you understand the science of theme rides and how scientific understanding can be applied by society. GRADE G - D C GRADE B – A* Targets for Improvement Recognise that objects have gravitational potential energy (GPE) because of mass and position in Earth’s gravitational field. Describe everyday examples in which objects have gravitational potential energy (GPE). Use the equation: GPE = mgh Recognise and interpret examples of energy transfer between gravitational potential energy (GPE) and kinetic energy (KE). Understand that for a body falling through the atmosphere at terminal speed: • kinetic energy (KE) does not increase • gravitational potential energy (GPE) is transferred to increased internal or thermal energy of the surrounding air particles through the mechanism of friction. Use and apply the equation, including a change of subject: GPE = mgh
Recognise everyday examples in which objects use gravitational potential energy (GPE). Interpret a gravity ride in terms of: • kinetic energy (KE) • gravitational potential energy (GPE) • energy transfer. Describe the effect of changing mass and speed on kinetic energy (KE): • doubling mass doubles KE • doubling speed quadruples KE. Use and the relationship Show that for a given object falling to Earth, this relationship can be expressed as h = v2 ÷ 2g and give an example of how this formula could be used.