PHYSICS · Stage 7 · Chapter 2: Forces
Physics 2.1 Introduction to forces — Contact and Non-Contact Forces and Newtons
5 min read 6 parts Lesson 1 of 7
Lesson summary
2.1 Introduction to forces — Contact and Non-Contact Forces and Newtons — Alhussan International Academy — Physics (Stages 7–9) notes: Identify forces, represent them on diagrams, classify them as contact or non-contact and measure them in newtons.Force = a push or a pull that ca…
- Parachutist
- Accelerating car
- Air resistance and water resistance
- Reaction (normal force)
- Gravitational forces
- Electrostatic forces
Part 1 of 6What a force is
- Force = a push or a pull that can change the shape of an object or change the way it moves.
- Effects — a force can start an object moving, speed it up, slow it down, stop it, or change the direction of a moving object.
- You cannot see forces, but you can see what they do.
- Unit — force is measured in newtons (N), and a small apple weighs approximately 1 N.
- Vector — forces have a size and a direction.
Part 2 of 6Force arrows
- Drawing — you show the force on an object by drawing an arrow.
- The length of the arrow shows the size of the force.
- The direction of the arrow shows the direction of the force.
- The arrow starts from the object (from its centre in a diagram of the object).
- Parachutist — weight is shown downwards and drag upwards.
- Accelerating car — driving force is forwards, friction and drag are backwards, weight is downwards and the normal force is upwards.
Part 3 of 6Contact forces
- Contact forces = forces that only act when objects are in contact (touching).
- Friction — it acts when solid surfaces are in contact and acts to stop objects moving.
- Air resistance and water resistance — they act on objects moving through air and water and are types of drag.
- The moving object collides with the particles in the air or water and the collisions slow it down.
- Thrust — it pushes a car or plane forwards.
- Upthrust — the upward force on an object in contact with a fluid, which pushes up on any floating object.
- A hot air balloon rises because of the upthrust from the air around it.
- Tension — it acts when objects are pulled, like the force in a rope.
- Reaction (normal force) — a force that pushes you up in reaction to your weight pushing down, for example when you sit on a chair.
Part 4 of 6Non-contact forces
1Non-contact forces
= forces that act even when objects are not touching.
2Gravitational forces1 point
— they act between any objects that have mass.
- On Earth this force is called weight, it acts towards the centre of the Earth and always pulls you down.
3Electrostatic forces1 point
— they act between electrically charged objects.
- Rubbing plastic objects charges them with static electricity, so they can attract or repel other charged objects, and a charged plastic comb attracts pieces of paper.
4Magnetic forces1 point
— they act between magnets, and between magnets and magnetic materials such as iron, steel or nickel.
- A magnet can exert a force of attraction (a pull) or repulsion (a push) on another magnet.
| Force | Contact? |
|---|---|
| Friction | Yes |
| Tension | Yes |
| Gravity | No |
| Magnetism | No |
| Electrostatic | No |
Part 5 of 6Balanced and unbalanced forces
- Balanced forces — the resultant force is zero, so a stationary object stays at rest and a moving object continues at the same speed in a straight line.
- A book on a table has weight downwards and an equal normal force upwards.
- A car at constant speed has driving force equal to drag and friction.
- Unbalanced forces — the resultant force changes the motion, so the object speeds up, slows down or changes direction.
- Resultant of forces in a line — add forces acting in the same direction and subtract forces acting in opposite directions.
- Motion from a diagram — if the forward force is bigger, the object speeds up.
Part 6 of 6Measuring forces
- Force meter — a forcemeter (newtonmeter or spring balance) measures forces in newtons.
- Technique — zero it first, hold it in line with the force and read it at eye level.
- Friction — pull a block at a steady speed across a surface, so the pull equals the friction.
- Compare surfaces such as wood, carpet and sandpaper, and repeat each measurement for reliability.
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