What is a free body diagram? It is a simple picture of one object and every force acting on it. The object is drawn as a box. Each force is an arrow that starts at the object’s centre of mass (the dot in the middle of the box) and points in the direction the force pushes or pulls. A longer arrow means a bigger force, so forces of the same size get arrows of the same length.

The forces in this activity

• Weight: the pull of gravity on the object. It always points straight down.
• Normal force (support force): the push of a surface on the object that is resting on it. It points up, at right angles to the surface (the road or the launch pad). It only exists while the object is touching the surface.
• Driving force (car) and thrust (rocket): the push that makes the object go. For a car the engine turns the wheels and the road pushes the car forwards. For a rocket the hot gas pushed downwards pushes the rocket upwards.
• Friction: opposes the motion between surfaces that are touching (the tyres and the road, or the brakes). It points against the way the car is moving.
• Air resistance: the push of the air on an object that is moving through it. It points against the way the object is moving, and gets bigger as the object goes faster.

Balanced and unbalanced forces. If the forces in a direction are the same size and opposite, they are balanced and the net force is zero. An object with zero net force stays at rest or keeps moving at a constant speed in a straight line (Newton’s first law). If one force is bigger, the forces are unbalanced, there is a net force in that direction, and the object accelerates that way: it speeds up if the net force is in the direction of motion, and slows down if it is against it (Newton’s second law, net force = mass × acceleration).

Tips: (1) Only draw the forces that act on the object. (2) Ask whether it is moving up and down, and whether it is speeding up, slowing down or staying the same. (3) There is no “force of motion”: an object does not need a force to keep moving, it needs a force to change its motion.

Each square on the grid is one unit of force (the real forces are much bigger and are not drawn to scale).