P4 explaining motion - speed
· Speed is the distance travelled in a certain time.
· To find the speed of an object you need to know the distance travelled and the time it takes.
· Speed is rarely constant, you accelerate and decelerate.
· So speed is normally average speed, even instantaneous speed is average speed, but over a really short period of time.
Speed x Time
P4 Speed and Velocity - Distance-time graphs
- gradient = speed
- flat sections = no speed
- steeper graph = faster
- ‘downhill’ means coming back to starting point
- curves show acceleration or deceleration
- steepening curve = speeding up
- levelling out curve = slowing down
Speed is the gradient of a graph, so you divide the vertical by the horizontal (so travel 300m in 20s, 300/20 = 15 m/s)
P4 - differences between speed and velocity
Distances can be positive or negative; it just means that they are travelling in opposite directions.
· Speed is just a number, velocity has a direction too.
· velocity can be more useful than speed.
· Scalar quantities – Mass, speed, time length ect
· Vector quantities – Force, velocity, acceleration, momentum ect
· Velocity can be positive or negative which shows direction.
· Velocity time graphs –
o Flat = steady speed
o Straight increasing = Acceleration or deceleration
o A curve shows increasing or decreasing acceleration.
· Tachographs plot speed against time so you can see how far someone travels. This is used in lorries so they don’t drive too far without a break. They are those circle things.
P4 Forces and friction
· Forces occur when two forces interact (interaction forces).
· If you push against something it pushes equally hard back.
· So you push down on a table with 10N of force, it pushed back with 10N or it breaks, and dies, and sets on fire.
· Moving objects experience friction;
o Friction between two gripping forces which stops them from moving.
o Friction between two sliding surfaces.
Resistance from liquid or gas. You push the molecules out the way causing friction, which is why you get friction, think aerodynamics
P4 Forces and motion
· Arrows show the size and direction of force, a larger force = longer arrow.
· If you have one force bigger than another the object moves, if they are equal then it doesn’t move.
· A resultant force shows the speed and direction the object will go.
· Acceleration – unbalanced forces.
· If the thrust on a rocket is greater than the gravity and drag, it’ll fly up and speed up till they are equal, at which point it’ll go at a constant speed.
P4 Forces and motion
· In the triangle,
o p = momentum
o m = mass
o v = velocity
Mass (m) x Velocity (V)
· Momentum is how hard an object is to stop; heavier and faster objects have a larger momentum.
P4 Forces and Motion
Change in momentum = resultant force (N) x Time for which force acts (s)
When a resultant force acts on an object you get a change in momentum.
· If a large force acts on you it hurts, so car safety features increase the time it takes to slow down, reducing forces.
o Crumple zones crumple on impact increasing time to stop.
o Helmet padding increases the time for your head to stop.
o Air bags slow you down more gradually.
o Seat belts stretch and slow you down slowly.
When a force moves an object, energy is transferred and work is done.
Work done = energy transferred
When you put ‘effort’ into moving something, you need an energy supply. The energy from your energy supply that you use up is work done.
· Change in energy (J) = Work done (J)
· In the triangle:
o W = Work done (J)
o F = Force (N)
o Distance = (M)
· Work done (J) = Force (N) x Distance (M)
It only works in the force is in exactly the same direction as the movement.
· Kinetic energy is movement.
· Anything moving has kinetic energy, the greater the mass and speed the bigger the kinetic energy (J).
Kinetic energy = ½ x mass x velocity²
· To increase something KE you need to increase its speed. The only way to do this is to apply a force.
· Increase in KE = Work done (Just about)
· Work done = energy transferred, but energy is wasted in heat, so like ‘the increase in an objects kinetic energy is normally a bit less than the amount of work done on it.