Forces 101
Every push and pull, decoded: name the forces acting on an object, tell contact from non-contact, weigh things with W = mg, and combine forces into a single resultant.
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Forces 101 💪
A **force** is simply a **push or a pull**. Forces make things speed up, slow down, change direction or change shape. They are measured in **newtons (N)** and always have a **direction**, so we draw them as **arrows**. Learn to name them and add them up.
Forces are vectors ➡️
Because a force has both a **size** and a **direction**, it is a **vector** — shown as an arrow whose length is the size and whose way it points is the direction. A force is measured with a **newtonmeter** (force meter). Several forces usually act on an object at once.
Label the forces
An interactive activity.
Contact and non-contact 🤝
Forces come in two families: • **Contact** forces need the objects to be **touching** — friction, air resistance, tension, and the normal (reaction) force. • **Non-contact** forces act **at a distance** — gravity (weight), magnetism and electrostatic force.
Which act at a distance?
Pick the TWO NON-contact forces (they act without the objects touching).
- Gravity (weight)
- Magnetic force
- Friction
- Tension
Match each force to what it is
- Friction
- Weight
- Tension
- Air resistance
- Opposes motion between two touching surfaces
- The pull of gravity on an object's mass
- The pulling force in a stretched rope or spring
- The drag from air on a moving object
Scalars and vectors ➗
Physical quantities are either: • **Scalars** — size only. Mass, speed, distance, energy and time. • **Vectors** — size **and** direction. Force, velocity, weight, acceleration and displacement. The key test: does a direction make sense for it? If yes, it is a vector.
Spot the vector
Which of these is a VECTOR quantity (it has a direction)?
- Force
- Mass
- Speed
- Time
Weight ⚖️
**Weight** is the force of gravity on an object. It is calculated with: **W = m g** (weight in N = mass in kg × gravitational field strength g = 9.8 N/kg). **Mass** (in kg) is the amount of matter and never changes. **Weight** (in N) is a force and changes with g — you weigh less on the Moon, but your mass is the same.
Work out the weight
An interactive activity.
Resultant force ⚔️
When several forces act along the same line, they can be replaced by a single **resultant force** with the same effect. • Forces the **same** way → **add** them. • Forces **opposing** each other → **subtract** them, and the resultant points the way of the bigger force. If the forces are balanced, the resultant is **zero**.
Find the resultant
An interactive activity.
The force rules
A force is a push or pull measured in _____. Weight is found from weight = mass × gravitational field _____. When two forces act in opposite directions along a line, you _____ them to find the resultant.