P4: Explaining motion

P4: Explaining motion

1. Speed - the distance traveled in a certain time

   • distance x time
   • The speed of an object is unlikely to be the same
   • it is often more useful to work out the average speed of a journey

2. Velocity has direction

   • it is often more useful measurement of motion
   • e.g. 30 mph due north
   • if two objects move in opposite directions, one has positive velocity and the other has negative velocity
     • e.g. one would be 20 m/s and the other -20 m/s

3. Acceleration - how quick you're speeding up

   • change in velocity / time taken
   • the change in velocity (whether it be +ve or -ve) within a certain amount of time
   • unit is m/s2
   • is a result of unbalanced forces (thrust is greater than drag)
     • for example, a rocket taking off has a greater amount of thrust than it does gravity or drag, otherwise it would fail to lift off
     • the greater the resultant force, the greater the acceleration.

4. forces of motion

   • friction and drag
     • friction is the force that opposes movement
     • they are reaction forces - they happen as a result of an applied force
     • 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
     • there is friction between solid forces which are gripping, this allows you to stand and walk e.t.c
     • there is also friction between two solid surfaces moving past each other, this can be reduced by adding a lubricant like oil
     • 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
   • thrust
     • thrust is the force that opposes drag and friction
     • for an object to increase its speed, thrust must be greater than drag
   • reaction force/ lift and weight
     • an object on a surface pushes down because of its weight
     • the reaction force comes from the surface and equals the weight, if it cannot do this, the object will fall through the surface
     • lift is the equivalent of reaction force in the air for example with planes - it helps to keep the plane at a constant altitude
   • for an object to travel at a constant speed, the forces must be balenced
   • Resultant force is the overall force acting on an object

5. Momentum - how hard it is to stop an object from moving

   • mass x velocity
   • Change in m0mentum
     • dependent on the force
     • when a resultant force acts on an object, it causes a change in momentum
     • the change in momentum is proportional to the size of the force and the time it acts for
     • resultant force x time for which the force acts

6. Work - "work done" = "energy transferred"

   • when a force moves an object it does work and energy is transferred to the object
   • amount of energy transferred (J) = Work done (J)
     • the object doing the work transfers the energy therefore losing energy
     • not all the energy is transferred to the object having the work done, some is lost through other means, e.g. sound and heat
   • work done by a force (J) = force (N) x distance moved in the direction of force (m)

7. Gravitational potential energy - 'Height Energy'

8. Kinetic energy - the energy of movement