P3 - Forces for transport


P3a: Speed

Speed = distance / time

A higher speed means a steeper gradient on a graph:

  • a staright line indicates that the speed is constant
  • a curved line indicates that the speed is changing

Gradient = y / x

1 of 9

P3b: Changing speed

Speed-time graphs:

  • acceleration - change of speed per unit time
  • if speed increases, object is accelerating and if speed decreases the object is decelerating
  • area under a speed-time graph represents the distance travelled

Acceleration (m/s2) = change in speed (or velocity) / time taken


  • vector - it has both size / speed and direction
  • when two cars move past each other:
    • relative velocity is sum of individual velocities if they're going in opposite direction
    • relative velocity is the difference in individual velocites if they're going in the same direction

Circular motion:

A vehicle accelerates around a roundabout because its nto going in a straight line.

2 of 9

P3c: Forces and motion

Force = mass x acceleration

Reaction time and thinking distance increase if the driver is:

  • tired, under the influence of alcohol or other drugs, travelling faster, is distracted 

Braking distance increases if:

  • poor road conditions, car hasn't been properly maintained and speed has increased

Factors affecting braking distance:

  • greater mass of vehicle = greater braking distance
  • greater speed = greater braking distance 
  • worn brakes reduce friction force and increase breaking distance
  • worn tyres with little tread reduce grip of wheels leading to an increase in braking distance

Thinking distance increases linearly with speed and breaking distance increases as a squared relationship

3 of 9

P3d: Work and power

Work is done when a force moves an object in the direction of the force:

  • work done = force x distance travelled (in direction of force)

Weight is a measure of the gravitational attraction on a body acting towards the centre of  Earth:

  • weight = mass x gravitational field strength

Power is the rate at which work is done:

  • power = work done / time taken

Power is also related to force and speed:

4 of 9

P3e: Energy on the move

Kinetic energy

When a car stops, its kinetic energy changes into heat in the brakes, tyres and road and this is shown by the formula:

  • work done by brakes = loss in kinetic energy

Change in kinetic energy --> braking force x braking distance = loss in kinetic energy

Factors that affect fuel consumption:

  • amount of energy required to increase its kinetic energy
  • amount of energy required to do work against friction
  • speed and road conditions
  • how its driven, such as excessive acceleration and decelration and constant speed changes

Biofuels may reduce carbon dioxide emissions but this is not certain because deforestation leads to an increase in carbon dioxide levels.

5 of 9

P3f: Crumple zones:

6 of 9

P3f: Crumple zones:

Momentum = mass x velocity

Force = change in momentum / time

Force = mass x acceleration

  • force can be reduced by reducing acceleration by increasing stopping distance and time

Primary safety feature - ABS brakes:

  • wheel speed sensors send information to a computer about the rptational speed of wheel
  • computer controls pressure to the brakes via a pump to prevent wheels locking
  • this increases braking froce just before wheels start to skid

Other primary safety features:

  • cruise control that stops driver from becoming tired on long journey
  • electric windows and paddle shift controls that allow driver to concentrate on road
7 of 9

P3g: Falling safely

Size of air resistance force on a falling object depends on:

  • cross-sectional area - larger area = greater air resistance
  • speed - greater speed = greater air resistance

Parachutist and terminal speed:

  • 1) parachutist accelerates, air resistance force increases, reduces acceleration
  • 2) parachutist's weight is equal to air resistance, forces on him are balanced = terminal speed
  • 3) parachute opens and upward force suddenly increases as there is larger surface area, parachutist decelerates and air resistance force decreases
  • 4) parachutist reaches a new slower terminal speed when weight = air resistance so lands safely

Gravitational field strenght:

  • approx. 10N/kg on Earth and is unaffected by atmospheric conditions
  • varies with position on Earth's surface and height above or depth below Earth's surface
8 of 9

P3h: The energy of games and theme rides

GPE (J) = mass x gravitational field strength x height

A bouncing ball converts gravitational potential energy to kinetic energy and back to gravitational potential energy. It does not return to its origianl height because energy is transferred.

When skydivers reach terminal speed, their kinetic energy has a max value and remains constant. The GPE lost as they fall is used to do work against friction.

  • change in GPE = work done against friction

Roller coasters:

  • uses a motor to haul train up in air, riders at top of ride have a lot of GPE
  • when train released, it converts GPE into kinetic energy
    • loss of GPE = gain in KE
  • each peak is lower than the one before because some energy is transferred to heat and sound due to friction and air resistance
  • if speed doubles, KE quadruples, if mass doubles, KE doubles
9 of 9


No comments have yet been made

Similar Physics resources:

See all Physics resources »See all Forces resources »