# Types of Forces and Net Force

> Pushes and pulls, contact and non-contact forces, free body diagrams, and how forces add up.

From HSC Physics on WhizzNotes, free study notes for NSW students: https://whizznotes.com/physics/forces-and-net-force/. Updated 2 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The stuck fridge
Josh and his dad need to move an old fridge across the kitchen. Josh pushes. Nothing happens. His dad pushes too. Still nothing. The fridge just sits there, as if glued to the floor.

"Friction," says Dad. He slides a flattened cardboard box under it. Now one gentle push and the fridge glides.

Nothing about the fridge changed. Its weight is the same. What changed was one of the forces acting on it. To understand why things move or stay still, you need to look at **all** the forces at once.

**In short:** A force is a push or a pull, measured in newtons (N). The net force is all the forces on an object added together with their directions: if it is zero the object stays still or keeps a steady velocity, and if it is not zero the object accelerates.

## What is a force?

A **force** is a **push or a pull**. Forces can:

- make a still object start moving
- make a moving object speed up, slow down or stop
- change the direction of motion
- change the shape of an object (squash, stretch, bend)

Force is measured in **newtons (N)**. An apple weighs about 1 N. A force is a **vector**, so it has a size and a direction. We measure force with a **spring balance** (newton meter).

## Contact and non-contact forces

| Contact forces (objects must touch) | Non-contact forces (act at a distance) |
|---|---|
| **Friction**: resists sliding between surfaces | **Gravity**: pulls masses together |
| **Air resistance (drag)**: friction from air or water | **Magnetic force**: between magnets and some metals |
| **Normal force**: a surface pushing back on an object | **Electrostatic force**: between charged objects |
| **Tension**: pull through a rope or string | |
| **Applied force**: a push or pull from a person or machine | |
| **Upthrust (buoyancy)**: push up from a liquid or gas | |

### Key idea: Friction is not always the bad guy
Friction stops you slipping when you walk, lets car tyres grip the road, and holds a nail in wood. Without it you could not even pick up a pen.

## Free body diagrams

A **free body diagram** shows one object as a dot or box, with an arrow for every force on it. Longer arrows mean bigger forces.

*Diagram: Free body diagram of a box on the floor being pushed: weight down, normal force up, applied push to the right, friction to the left*
*A box being pushed across the floor.*

## Net force (resultant force)

The **net force** is what you get when you add up all the forces on an object, taking direction into account.

- Forces in the **same** direction: **add**.
- Forces in **opposite** directions: **subtract**. The net force points the way of the bigger one.

### Worked example: The fridge again
Josh pushes right with 200 N. Friction pushes left with 200 N.

Net force $= 200 - 200 = 0$ N. The forces are **balanced**, so the fridge does not move.

With cardboard underneath, friction drops to 50 N:

Net force $= 200 - 50 = 150$ N to the right. The forces are **unbalanced**, so the fridge speeds up.

## Balanced and unbalanced forces

| Net force | What happens |
|---|---|
| **Zero** (balanced) | Object stays still, **or** keeps moving at constant speed in a straight line |
| **Not zero** (unbalanced) | Object accelerates: speeds up, slows down or changes direction |

### Watch out: Moving does not mean there is a net force
A car cruising at a steady 100 km/h on a straight road has **balanced** forces. The engine's driving force exactly equals air resistance plus friction. You only need a net force to **change** motion.

**Mind map**

```text
Forces
  Contact
    Friction
    Air resistance
    Normal force
    Tension
    Upthrust
  Non-contact
    Gravity
    Magnetic
    Electrostatic
  Net force
    Add same direction
    Subtract opposite
    Zero = balanced
  Effects
    Start or stop
    Speed change
    Direction change
    Shape change
```

### Year 11: Friction and inclined planes
The size of friction depends on how hard the surfaces are pressed together (the normal force) and how rough they are:

$$F_f = \mu F_N$$

where $\mu$ (mu) is the **coefficient of friction**. Static friction (not yet sliding) can be larger than kinetic friction (sliding). That is why it is harder to start pushing the fridge than to keep it moving.

On a slope at angle $\theta$, split the weight $mg$ into two parts:

- Down the slope: $mg\sin\theta$
- Into the slope: $mg\cos\theta$, which equals the normal force

So on a frictionless ramp, the acceleration is $a = g\sin\theta$.

### Year 12: Equilibrium and torque
An object in **equilibrium** has zero net force **and** zero net turning effect. The turning effect of a force is the **torque**:

$$\tau = rF\sin\theta$$

where $r$ is the distance from the pivot. For a seesaw to balance, clockwise torque equals anticlockwise torque. A long spanner makes a stuck nut easier to turn because a larger $r$ gives more torque for the same force.

## Quick summary

- A force is a push or pull, measured in newtons (N). Force is a vector.
- **Contact** forces: friction, air resistance, normal, tension, upthrust, applied.
- **Non-contact** forces: gravity, magnetic, electrostatic.
- **Net force** = all forces added with direction.
- Balanced (net force zero): still, or constant velocity. Unbalanced: acceleration.
- Free body diagrams show every force on one object as an arrow.

## Quiz

Try these questions from the chapter quiz.

1. Which is a non-contact force?
   - A. Friction
   - B. Tension
   - C. Magnetic force
   - D. Normal force
   - Answer: C. Magnetic force. Magnets can attract or repel without touching.

2. The unit of force is the:
   - A. Joule
   - B. Watt
   - C. Newton
   - D. Kilogram
   - Answer: C. Newton. Force is measured in newtons (N).

3. Forces of 40 N right and 15 N left act on a trolley. The net force is:
   - A. 55 N right
   - B. 25 N right
   - C. 25 N left
   - D. 55 N left
   - Answer: B. 25 N right. Opposite directions subtract: 40 − 15 = 25 N, in the direction of the bigger force (right).

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/physics/forces-and-net-force/#quiz