# P4- Explaining Motion

OCR 21st Century

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• Created by: Abi_xx
• Created on: 11-08-14 20:06
• P4- Explaining Motion
• Forces
• A FORCE occurs when 2 objects INTERACT
• The forces in an INTERACTION PAIR are        1. equal in size                2. Opposite in direction and act on different objects
• AIR RESISTANCE : Air tries to slow down a sky diver, the skydiver pushes the air out of their way (equal and opposite force)
• GRAVITY : 2 masses are attracted to each other eg You and Earth with an equal and opposite force
• ROCKET AND JET ENGINES : Engine pushes gas backwards and gas pushes rocket forward
• Forces and Motion
• ARROWS on diagrams are used to represent 1.The size of the force 2.Direction the force is acting in
• Arrows are always draw with the tail touching the object
• If MORE THAN 1 force acts on an object they will : 1.Add up if they are in the same direction 2.Subtract if different directions
• The overall effect is called RESULTANT FORCE
• There is always a DRIVING FORCE and a COUNTER FORCE
• If the driving force is 1.< counter force = speed increase 2.Equal to the counter force = constant speed in a straight line 3.>counter force=decrease in speed
• Kinetic Energy
• The amount of K.E an object has depends on 1.Mass and 2.Velocity
• K.E (Joules, J) = 1/2 x Mass (kg) x Velocity squared (m/s(squared)
• Work and Energy
• When work is done :     1.On an object, it gains energy 2.By an object it loses energy
• Work done by a force (j) = Force (n) x Distance moved in direction of force (m)
• When a force makes an objects velocity increase 1.Work is done on the object by the force      2.The object gains K.E
• Some of the energy will be dissipated (lost) as heat
• Gravitational Potential Energy
• When an object is lifted above the ground 1.Work is done by the lifting force against gravity    2.The object has the potential to do work when it falls
• Change in G.P.E (J) = Weight (N) x Vertical Height difference (M)
• To Find G.P.E you use WEIGHT not mass