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P4 – Explaining Motion

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Average Speed Average Speed = Total Distance/Total Time

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Instantaneous Speed The speed at a particular point in the journey

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Distance-Time Graph

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Positive and Negative Distance Positive gradients are moving one direction. Negative gradients moving opposite direction.

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Velocity Speed + Direction of movement EG: 5m/s North Positive Velocity is moving in the opposite direction to Negative Velocity.

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Tachograph An entire rotation encompasses 24 hours. The marker moves further from the center the faster the vehicle is moving. These are used for buses and trucks for road safety.

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Force Force arises from an interaction between two objects One object exerts a force on another, it always experiences a force in return Two forces in an interaction pair are equal in size and opposite in direction and they act on different objects

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Identifying Forces 1.Identify forces arising from an interaction between two objects 2.Identify the ‘partner’ of a given force (i.e. the other force of the interaction pair) 3.Specify, for each force, the object which exerts it, and the object on which it acts 4.Use arrows to show the sizes and directions of forces acting

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Forces in Response to another Force Some forces (such as friction, reaction of a surface) arise in response to the action of an applied force

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Reaction of the Surface Describe the interaction between an object and a surface it is resting on. The object pushes down on the surface. The surface pushes up on the object with an equal force

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Friction Describe the interaction between two surfaces which slide (or tend to slide) relative to each other. Each surface experiences a force in the direction which prevents (or tends to prevent) relative movement.

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Rockets Be able to use the idea of a pair of equal and opposite forces to explain in outline how rockets and jet engines work.

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Resultant Force Be able to interpret situations in which several forces act on an object The resultant force on an object is the sum of all the individual forces acting on it, taking their directions into account

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Momentum Momentum (kg.m/s) = Mass (kg) x Velocity (m/s)

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Change in Momentum If a resultant force acts on an object, it causes a change of momentum in the direction of the force The size of the change of momentum is related to the size of the resultant force and the time for which it acts in the following way Change of Momentum (kg.m/s) = resultant force (N) x time for which it acts (s)

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Change in Momentum In situations involving a change in momentum (such as a collision), the longer the duration of the impact, the smaller the average force for a given change in momentum Discuss and explain the action of road safety measures, such as car seat-belts, crumple zones, air bags, cycle and motorcycle helmets

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Balanced Forces If the resultant force on an object is zero, its momentum does not change (if it is stationary, it stays at rest; if it is already moving, it continues at a steady speed in a straight line).

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Horizontal Forces The horizontal motion of objects (like cars and bicycles) can be analysed in terms of a driving force (produced by the engine or the cyclist), and a counter force (due to friction and air resistance)

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Unbalanced Forces For a moving object, IF the driving force is: – greater than the counter force - the vehicle will speed up – equal to the counter force - the vehicle will move at constant speed in a straight line – smaller than the counter force - the vehicle will slow down

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Kinetic Energy (KE) KE = Movement Energy Anything that is moving has KE The greater the mass of an object and the faster it is moving, the more kinetic energy it has

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Gravitational Potential Energy (GPE) Anything that is lifted a height has GPE GPE = Weight (kg) x Height (m)

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Work Work (J) = Force (N) x Distance (m) understand that when work is done on an object, the energy of the object increases and when work is done by an object, the energy of the object decreases according to the relationship

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Changing GPE When an object is lifted to a higher position above the ground, work is done by the lifting force against the gravitational force acting on the object (its weight); this increases the object’s gravitational potential energy (GPE);

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Work and Kinetic Energy If friction and air resistance can be ignored, an object’s kinetic energy increases by an amount equal to the work done on it by an applied force Air resistance or friction will cause the gain in an object’s kinetic energy to be less than the work done on it by an applied force, because some energy is dissipated through heating

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Energy Conservation Energy is always conserved Calculate the gain in kinetic energy, and the speed, of an object that has fallen through a given height

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