Free Body Diagrams (OCR Gateway GCSE Physics: Combined Science)

• When forces act on an isolated body or system they can be represented as vectors
• These vectors can be added together to work out resultant forces to determine the motion of the body or system
• For example: 
  • The forces acting on a body in freefall are weight (downwards) and drag (upwards). This body will reach terminal velocity when these two forces are balanced
  • The forces acting on an aircraft are thrust (forwards) drag (backwards) lift (upwards) and weight (downwards). The engine can control the amount of thrust, and the tilt of the wings can control the lift

• Free body diagrams are useful for modelling the forces that are acting on an object
• Each force is represented as a vector arrow, where each arrow:
  • Is scaled to the magnitude of the force it represents
  • Points in the direction that the force acts
  • Is labelled with the name of the force it represents

• Free body diagrams can be used:
  • To identify which forces act in which plane
  • To resolve the net force in a particular direction

Free body diagrams can be used to show the various forces acting on objects

Part (a)

• • The size of the arrows should be such that adding the three forces (head-to-tail) would make a closed triangle as they are in equilibrium

Part (b)

• • There are three forces acting on the box
  • The normal contact force, R, acts perpendicular to the slope
  • Friction, F, acts parallel to the slope and in the opposite direction to the direction of motion
  • Weight, W, acts down towards the Earth

Part (c)

• • As the boat is not moving, the size of both arrows must be the same

Part (d)

• • As the car is accelerating, the size of the thrust must be larger than the size of the friction force
  • As in part (c), the upwards and downwards forces must be equal