P4: Explaining motion

Speed - the distance traveled in a certain time
1. Distance / Time
2. The speed of an object is unlikely to be the same
3. It is often more useful to work out the average speed of a journey
Velocity has direction
1. E.g. 30 mph due north
2. If two objects move in opposite directions, one has positive velocity and the other has negative velocity
  1. E.g. one would be 20 m/s and the other -20 m/s
Acceleration - how quick you're speeding up
1. Change in velocity / time taken
2. The change in velocity (whether it be +ve or -ve) within a certain amount of time
3. Unit is m/s2
4. Is a result of unbalanced forces (thrust is greater than drag)
  1. E.g. a rocket taking off has a greater amount of thrust than it does gravity or drag, otherwise it would fail to lift off
  2. The greater the resultant force, the greater the acceleration.
Forces of motion
1. Friction and drag
  1. Friction is the force that opposes movement
  2. They are reaction forces - they happen as a result of an applied force
  3. The frictional force will be equal to the force of movement up to the maximum point where the friction will be overcome and the object will move
  4. There is friction between solid forces which are gripping, this allows you to stand and walk e.t.c
  5. There is also friction between two solid surfaces moving past each other, this can be reduced by adding a lubricant like oil
  6. Drag, sometimes called resistance, is the equivalent to friction in gases and liquids instead of solids, so a streamline object would find it easier to move as it would be less effected by drag
2. Thrust
  1. Thrust is the force that opposes drag and friction
  2. For an object to increase its speed, thrust must be greater than drag
3. Reaction force/ lift and weight
  1. An object on a surface pushes down because of its weight
  2. The reaction force comes from the surface and equals the weight, if it cannot do this, the object will fall through the surface
  3. Lift is the equivalent of reaction force in the air for example with planes - it helps to keep the plane at a constant altitude
4. For an object to travel at a constant speed, the forces must be balenced
5. Resultant force is the overall force acting on an object
Momentum - how hard it is to stop an object from moving
1. Mass x Velocity
2. Change in momentum
  1. Dependent on the force
  2. When a resultant force acts on an object, it causes a change in momentum
  3. The change in momentum is proportional to the size of the force and the time it acts for
  4. Resultant force x time for which the force acts
Work - "work done" = "energy transferred"
1. When a force moves an object it does work and energy is transferred to the object
2. Amount of energy transferred (J) = Work done (J)
  1. The object doing the work transfers the energy therefore losing energy
  2. Not all the energy is transferred to the object having the work done, some is lost through other means, e.g. sound and heat
3. Work done by a force (J) = force (N) x distance moved in the direction of force (m)
Gravitational potential energy - 'Height Energy'
1. Change in G.P.E. = Weight x Vertical height difference
2. K.E. gained = G.P.E. lost
Kinetic energy - the energy of movement
1. 0.5 x mass x velocity^2

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P4: Explaining motion

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