Forces
Effects of forces
What forces can do
A force is a push or a pull. Forces can:
- Change the shape of an object (compress, stretch, bend, twist)
- Change the speed of an object (speed it up or slow it down)
- Change the direction of motion of an object
Contact and non-contact forces
| Type | Examples | Objects must touch? |
|---|---|---|
| Contact forces | Friction, normal (reaction) force, tension, compression, air resistance (drag), upthrust | Yes |
| Non-contact forces | Gravitational force (weight), magnetic force, electrostatic force | No — act through a distance |
Key contact forces to know:
- Friction — opposes relative motion between surfaces in contact; always acts in the opposite direction to motion (or intended motion).
- Normal reaction (N) — the force a surface exerts on an object perpendicular to the surface; prevents the object passing through.
- Tension — a pulling force transmitted through a rope, string or spring.
- Upthrust — upward force exerted by a fluid on a submerged or floating object.
Resultant force
The resultant force (net force) is the single force that has the same effect as all the individual forces acting on an object combined.
Adding forces along the same line:
- Forces in the same direction: add magnitudes. \( F_\text{res} = F_1 + F_2 \)
- Forces in opposite directions: subtract magnitudes. \( F_\text{res} = F_1 - F_2 \) (resultant acts in the direction of the larger force)
- If \( F_\text{res} = 0 \): the forces are balanced — the object is in equilibrium.
Free body diagrams
A free body diagram shows all the forces acting on a single object as labelled arrows:
- The length of each arrow represents the magnitude of the force.
- The direction of the arrow shows the direction of the force.
- Each arrow starts at (or passes through) the object and is labelled with the force name and value.
- Only forces acting on the object are shown — not forces the object exerts on others.
Example — free body diagram for a book on a table
A book of weight 8 N rests on a table. Draw and describe its free body diagram.
Two forces act on the book:
- Weight = 8 N, downward (gravity acting on the book's mass).
- Normal reaction = 8 N, upward (table pushing up on the book).
Both arrows are equal in length, opposite in direction. Resultant = 0 N → book is in equilibrium (at rest).
In "describe the forces" questions, always name the force, give its direction, and state the cause. For a falling object: "Weight acts downward due to gravity; air resistance acts upward opposing the motion."