OCR Physics Module P3 FORCES FOR TRANSPORT
P3a Speed
Speed speed = distance ÷ time; unit is m/s (metres per second)
Increasing speed increases the distance travelled in the same time/reduces the time needed to cover the same distance
Speed cameras take two photographs: certain time apart; near marked lines on the road
Distance-time graphs straight line gradient (steady speed); steeper gradient (higher speed); horizontal line - stationary (zero speed)
Gradient calculate speed from gradient of graph [vertical axis change ÷ horizontal axis change]
P3b Changing Speed
Speed-time graphs horizontal line (constant speed); straight line positive [up] gradient (increasing speed); straight line negative
[down] gradient (decreasing speed); steeper gradient (more acceleration)
Area under graph area under the line of a speed-time graph is the distance travelled
Acceleration change in speed per unit time; acceleration = change in speed ÷ time taken
Acceleration unit m/s2 (metres per second squared)
Acceleration – speed-time graph straight line gradient (constant acceleration); curved line (changing acceleration)
Acceleration – either change in speed; change in direction
P3c Forces and Motion
Force and acceleration forces cause things to speed up or slow down
Force, mass, acceleration force (N) = mass (kg) X acceleration (m/s2); F = m x a
F, m and a (constant mass) more force = more acceleration; (constant force) more mass = less acceleration; (constant
Acceleration) more mass = more force
Forces – equal and opposite when body A exerts a force on body B, body B exerts an equal but opposite force on body A
(Total) stopping distance stopping distance = thinking distance [human reactions] + braking distance [car/road]
Thinking distance distance travelled between the need for braking occurring and the brakes starting to act
Thinking distance – factors driver tiredness; drugs/alcohol; higher speed; distractions/lack of concentration
Braking distance distance taken to stop once the brakes have been applied
Braking distance – factors road conditions (slippery/icy/wet); car conditions (bald tyres/poor brakes); more speed
Stopping distance – factors friction; mass (load); speed; braking force
Stopping distance – road safety driving too close to car in front; speed limits; road conditions
P3d Work and Power
Work energy is needed to do work; work is done when a force moves an object; unit of work/energy is Joule (J)
Work – equation work (J) = force (N) x distance (m); depends on size of the force (Newtons); distance (metres)
Power measurement of how quickly work is being done; measured in watts (W)
Power – equation power (W) = work done (J) ÷ time (s)
Cars power rating; fuel consumption; environmental issues; costs
P3e Energy on the move
Kinetic energy – KE Joules (J); kinetic energy is greater for objects with: higher speed; greater mass
KE – equation KE = ½ x mv2 [mass, kg; velocity, m/s]
KE equation – application relationship between braking distances and speed; everyday situations involving objects moving
Transport – fossil fuels petrol; diesel; cars pollute at the point of use
Transport – battery driven batteries need recharging; cars do not pollute at the point of use but power stations cause pollution
Transport – solar powered no pollution
P3f Crumple Zones
Safety features change shape; reduce injuries; absorb energy
Safety features – absorb energy heating in brakes; crumple zones; seat-belts; air bags
Safety features – reduce forces increasing stopping or collision time; increasing stopping or collision distance; decreasing acceleration
Safety features – active help reduce injuries; reduce chance of crash: ABS brakes; traction control
Safety features – passive help reduce injuries; help avoid distractions; electric windows; cruise control; paddle shift
Crumple zones – force crumple in collision; changes shape; increase collision time; reduce acceleration; reduce force
Crumple zones – energy crumple in collision; changes shape; absorbs KE
Seatbelts – force seatbelt stretches/increases in length; increase stopping distance; reduce acceleration; reduce force
Seatbelts – energy seatbelt stretches/increases in length; absorbs KE
Anti-lock brakes stops wheels locking; stops skidding; increases friction; keeps driver in control; reduces braking distance
P3g Falling Safely
Falling objects accelerate; weight pulls them towards centre of earth
Free-fall constant acceleration due to gravity (g);
Change forces increase/decrease speed
Terminal speed objects falling through Earth’s atmosphere reach a terminal (constant) speed; balanced forces
Terminal speed – forces higher speed = more drag; larger area = more drag; weight (driving force) = drag gives terminal speed
Frictional forces drag; friction; air resistance: act against movement; can be reduced (shape, lubricant)
Air resistance drag; slows falling objects; parachutes; badminton shuttle-cock; not in outer space/Moon
Streamlining wedge shape of sports car; deflectors on lorries and caravans; roof boxes on cars
P3h The energy of games and theme rides
Gravitational potential energy objects have GPE because of mass and position in Earth’s gravitational field
GPE, mass and height more mass = greater GPE; more height = greater GPE
GPE – equation GPE (J) = mgh [mass (kg); g (usually 10 m/s2; h (m)]
GPE – different planets GPE is also greater when the gravitational field strength (g) is higher – larger mass plant à higher value for g
Weigh weight (N) = mass (kg) x gravitational field strength
Energy transfer energy transfer between gravitational potential energy and kinetic energy – as objects fall/rise
At terminal speed KE does not increase; GPE works against friction
Roller coaster – gravity ride highest point = maximum GPE; lowest point = minimum GPE; changes of GPE à KE
Mass, speed and KE KE = ½ x mv2; doubling mass doubles KE; doubling speed quadruples KE