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Published byBryan Linam Modified about 1 year ago

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Explaining Motion Revision

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Forces Forces arise from an interaction between 2 objects. Forces arise from an interaction between 2 objects. In an interaction pair the forces are equal in size but opposite in direction. In an interaction pair the forces are equal in size but opposite in direction. These forces act on different objects. These forces act on different objects. They explain how rockets work etc. They explain how rockets work etc.

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Forces (cont.) Draw diagrams showing forces with arrows. Draw diagrams showing forces with arrows. Some forces are a response to other forces eg friction, reaction. Some forces are a response to other forces eg friction, reaction.

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Forces (cont.) The resultant force on an object is the sum of all the individual forces acting on it All forces are in pairs

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Speed Average speed = distance/time Average speed = distance/time Distinguish between average speed and instantaneous speed. Distinguish between average speed and instantaneous speed. Remember that movement in one direction is positive and the other direction is negative. Remember that movement in one direction is positive and the other direction is negative.

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Distance-Time Graphs Draw and interpret graphs that show objects that are Draw and interpret graphs that show objects that are –Stationary –Moving at constant speed –Increasing or decreasing speed Recall that a steeper gradient means a faster speed. Recall that a steeper gradient means a faster speed. Make calculations from a distance- speed graph. Make calculations from a distance- speed graph.

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Velocity-Time Graphs Recall that velocity is speed with an indication of direction. Recall that velocity is speed with an indication of direction. Interpret velocity-time graphs for objects that are Interpret velocity-time graphs for objects that are –Stationary. –Moving in a straight line at steady speed. –Moving in a straight line with changing speed. Be able to read a tachograph. Be able to read a tachograph.

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Momentum Momentum = mass x velocity Momentum = mass x velocity When a force acts on an object it causes a change in momentum in the direction of the force. When a force acts on an object it causes a change in momentum in the direction of the force. The size of the change in momentum is related to the size of the force and the time it was applied. The size of the change in momentum is related to the size of the force and the time it was applied. Change in momentum= force x time Change in momentum= force x time

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Changing momentum safely In a collision, if the time taken is increased that size of the force will be smaller – causing less damage. In a collision, if the time taken is increased that size of the force will be smaller – causing less damage. This is how air bags, seat belts, crumple zones etc work This is how air bags, seat belts, crumple zones etc work

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Forces and Motion Motion can be described in terms of a driving force and a counter force. Motion can be described in terms of a driving force and a counter force. The driving force is the engine or pedals. The driving force is the engine or pedals. The counter force is air resistance and the brakes. The counter force is air resistance and the brakes. If the driving force is If the driving force is –Greater than the counter force the object speeds up –Equal to the counter force the object moves at constant speed (or remains stationary). –Less than the counter force the object slows down.

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Energy Changes Moving objects have kinetic energy Moving objects have kinetic energy KE= ½ mv 2. KE= ½ mv 2. Objects that are lifted have gravitational potential energy. Objects that are lifted have gravitational potential energy. GPE=weight x change in height. GPE=weight x change in height. When on object does work it loses energy. When on object does work it loses energy. When work is done on an object it gains energy. When work is done on an object it gains energy. Change in energy = work done. Change in energy = work done.

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Energy Changes When a force causes an object to move, work is done. When a force causes an object to move, work is done. Work done = force x distance. Work done = force x distance. Lifting an object gives it GPE, moving an object gives it KE. Lifting an object gives it GPE, moving an object gives it KE. The bigger or faster an object the greater the KE. The bigger or faster an object the greater the KE. The greater or higher an object the greater the GPE The greater or higher an object the greater the GPE

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Energy Changes A falling object loses GPE but gains KE. A falling object loses GPE but gains KE. The rise in KE is equal to the fall in GPE (assuming no air resistance or friction). The rise in KE is equal to the fall in GPE (assuming no air resistance or friction). Air resistance and friction will stop the gain in KE being equal to the work done. Air resistance and friction will stop the gain in KE being equal to the work done.

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