Forces and the net force

A force is a push or a pull. It can:

• make something start moving (kicking a ball);
• speed it up, slow it down or stop it;
• turn it (a bat striking the ball);
• change its shape (squeezing a lemon).

"Is the size of a force enough?" No. Think of a tug of war: you need to know which way each team pulls. A force has a size (magnitude) and a direction:

• friction acts against the motion;
• magnets attract or repel;
• gravity pulls down; buoyancy pushes up.

Change the size or the direction and the effect changes. Force is measured in newtons (N).

A spring balance measures the size of any force, not just weight. (Weight is the Earth's gravitational pull on an object.) You can feel a force as tiny as about a millinewton (10⁻³ N); scientists can measure forces as small as yoctonewtons (10⁻²⁴ N).

Usually several forces act together. In a tug of war:

• if both teams pull equally, the rope stays still: the forces are balanced;
• if one team pulls harder, the rope moves towards it: the forces are unbalanced.

Adding forces along a line gives the net force:

• Same direction: add. Two people pushing a stalled car, 10 N and 6 N, give 16 N.
• Opposite directions: subtract. 10 N one way and 6 N the other give 4 N, in the direction of the bigger force.

An object's motion depends only on the net force. When a weightlifter holds a barbell still, its weight pulling down is balanced by the lifter pushing up, so the net force is zero.