Forces and Their Effects

Force — a push or a pull on an object, measured in newtons (N). A force is a vector: it has a size and a direction, and the direction matters.

What a force can do — change an object's speed, change its direction, or change its shape. Any of those three counts.

Types of force you are expected to name:

Weight — the force of gravity on a mass, acting downwards toward the centre of the Earth.

Friction — a force opposing motion between surfaces in contact, acting along the surface.

Air resistance (drag) — friction from a fluid, opposing motion through it. It increases with speed.

Normal contact force — the push of a surface on an object resting on it, at right angles to the surface.

Upthrust — the upward force from a fluid on an object in it.

Tension — the pull along a stretched rope, string or spring.

Balanced and unbalanced forces — when the forces on an object cancel, the resultant force is zero and the object either stays still or keeps moving at constant velocity. When they do not cancel, the resultant is non-zero and the object accelerates in the direction of the resultant.

Finding a resultant in one dimension — add forces in the same direction, subtract forces in opposite directions. A 30 N push right and a 12 N friction force left give a resultant of 18 N right.

Vector diagrams — forces are drawn as arrows, the length showing the size and the direction showing the direction. Arrows should start at the point where the force acts.

Example: a book on a table has weight pulling it down and the normal contact force pushing it up. These are equal in size and opposite in direction, so the resultant is zero and the book stays put. Take the table away and only the weight is left, so the book accelerates downwards.

A force is always exerted by something on something. "The force of motion" and "the force of the object" are not forces — if you cannot say what is pushing what, it is not a force and it does not belong on the diagram.