Forces and motion, the playable version
Every box on your knowledge organiser is here as something you can poke. Play with each one first, then read the yellow line. That line is what goes on the sheet.
Forces come in pairs
You can't push something without it pushing back on you. Push a wall and the wall pushes your hand. A rocket shoves hot gas downwards, and the gas shoves the rocket upwards. The two pushes are the same size, in opposite directions, on two different objects. That is Newton's Third Law, and the two forces are called an interaction pair.
Try it: drag the engine power. Can you make the rocket lift off?
For your sheet: the two forces in a pair never cancel, because they act on different objects. The rocket only changes its motion when the forces on the rocket (push from the gas, pull from Earth) are different sizes. Those two come from different interaction pairs.
Mass and weight
People mix these up all the time. Mass is how much stuff you are made of, measured in kilograms. It doesn't care where you are. Weight is how hard gravity pulls on that stuff. It is a force, so it is measured in newtons, and it changes if gravity changes.
Try it: travel to another world. Watch which number changes and which one refuses to.
For your sheet: weight (N) = mass (kg) × gravitational field strength (N/kg). On Earth every kilogram is pulled with about 10 N, so a 4 kg bag weighs 40 N. The weight of an object changes if the gravitational field changes. Its mass does not.
Gravitational fields
Gravity can pull on you without touching you. The space around a planet where this happens is its gravitational field: a region where a mass feels a force. The arrows on a field diagram show which way that force points, which is always towards the centre of the planet.
Try it: tap anywhere in the space to let go of a rock.
Every rock falls along the arrows, wherever you drop it. It also speeds up as it gets closer, because the field is stronger near the planet.
The graph on your sheet: weight against mass
Put more mass on the scales and the weight goes up in step, so you get a straight line. How steep the line is tells you how strong gravity is.
For your sheet: the slope of a weight against mass graph is the gravitational field strength. It is the force on each 1 kg, in N/kg. Steeper line, stronger gravity.
Balanced and unbalanced forces
When the forces on something cancel out, we say they are balanced and the object is in equilibrium. Here is the part that surprises people: balanced does not mean stopped. It means nothing changes. A still object stays still. A moving object keeps going at the same speed in a straight line.
Challenge: get the trolley moving, then make the two pushes equal again. Does it stop?
For your sheet: objects in equilibrium either stay still or move at a constant speed, because the forces on them are balanced. Unbalanced forces change an object's speed or its direction.
Going round in circles
A satellite is always moving sideways, fast. Gravity pulls it towards Earth, at 90° to the way it is moving. A sideways pull like that doesn't make it faster or slower. It only bends the path, over and over, into a circle. So the speed stays the same but the direction keeps changing.
Try it: switch gravity off and see where the satellite goes.
For your sheet: for objects moving in a circle, such as an orbit, the force is at 90° to the velocity. Velocity means speed plus direction.
Distance–time graphs
This graph is a diary of a journey. Time runs along the bottom, and the line climbs as you get further from the start. Only one idea matters: the steeper the line, the faster you are going. A flat line means time is passing but you are not getting anywhere, so you have stopped.
Try it: choose how to move and watch the line draw itself.
For your sheet: average speed = total distance ÷ total time. You walk at about 1.5 m/s and run at about 5 m/s. Straight line = constant speed. Steeper = faster. Horizontal = stationary. Curved = accelerating. You find speed from the slope.
Check yourself
Seven quick questions. Getting one wrong is fine. Read the reason and it will stick.
Key words in plain English
- Newton's Third Law
- If A pushes B, then B pushes A just as hard the other way.
- Interaction pair
- Those two matching forces. Same size, opposite directions, on two different objects.
- Mass
- How much stuff something is made of. Measured in kg. Same everywhere.
- Weight
- The force of gravity pulling on a mass. Measured in N.
- Gravitational field
- The region around a mass where other masses feel a pull.
- Gravitational field strength
- The pull on each 1 kg. About 10 N/kg on Earth.
- Balanced
- The forces cancel out. Nothing about the motion changes.
- Unbalanced
- One force wins. Speed or direction changes.
- Equilibrium
- What we call an object when its forces are balanced.
- Resultant force
- The one force left over when you combine all the forces.
- Velocity
- Speed with a direction, like 5 m/s to the right.
- Acceleration
- Speed changing: getting faster or slower.
- Distance–time graph
- Shows how far something has gone as time passes. Slope = speed.
- Vector and scalar
- A vector has a size and a direction (force, velocity). A scalar has only a size (mass, time).