Forces: Unit Synthesis
Every example in this unit, from a spear-thrower flinging a spear to the Moon held in its orbit, comes down to one idea: a force is a push or a pull, and forces are what change the way things move.
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Q1 · Without looking at your notes, write as many kinds of force as you can in 2 minutes. Sort each one into contact or non-contact.
Q2 · A skateboarder rolls along flat ground and slowly comes to a stop. Which force slows them down, and what happens to their movement energy?
● Know
- The full set of forces in this unit: contact (friction, applied push, tension) and non-contact (electrostatic, gravitational, magnetic)
- That weight is calculated with W = mg and that forces are measured with a spring scale in newtons
- That magnets have N and S poles and that electromagnets use current in a coil
● Understand
- How balanced and unbalanced forces decide whether motion changes
- How forces transfer energy by doing work, and that energy is conserved
- How simple machines change the size of the force needed for a task
● Can do
- Draw and read a force diagram and decide if the forces are balanced
- Connect every force idea in the unit into one big picture
- Plan a fair-test forces investigation, naming the variables and a method
A force is a push or a measured in . Forces that cancel out are and cause no change in motion. The pulling force of gravity on mass is called . A lever is one example of a simple that changes the size of a force.
Every force in this unit is a push or a pull, and it does one of these things: it starts something moving, stops it, changes its direction, or changes its shape. The big sorting question is always the same, does the force need touching (contact) or can it act across a gap (non-contact)?
| Force | Contact or non-contact? | What it does in this unit |
|---|---|---|
| Applied push or pull | Contact | A direct push or pull from your hand, a foot or a rope. |
| Friction | Contact | Opposes motion between two surfaces; transfers movement energy to heat. |
| Tension | Contact | The pulling force along a stretched rope, string or cable. |
| Gravitational force | Non-contact | Masses attract; pulls objects down as weight and keeps the Moon and planets in orbit. |
| Electrostatic force | Non-contact | Like charges repel and unlike charges attract, acting across a gap. |
| Magnetic force | Non-contact | Like poles repel and unlike poles attract; the basis of magnets and electromagnets. |
A force diagram models all of these at once. Each force is drawn as an arrow on the object: a longer arrow means a bigger force, and the arrowhead shows the direction. If the arrows cancel, the forces are balanced and the motion does not change. If they do not cancel, the forces are unbalanced and the motion changes, the object speeds up, slows down or changes direction.
Gravity is a non-contact force: every object with mass attracts every other object with mass. On Earth, gravity pulls objects down, and that pulling force is what we call an object's weight. Weight is worked out with the rule:
Weight = mass times gravitational field strength, written as $W = mg$.
On Earth, the gravitational field strength $g$ is about 10 N for every kilogram. So a 2 kg bag of sugar has a weight of about $2 \times 10 = 20$ N. Notice that mass (in kilograms) stays the same everywhere, but weight (in newtons) depends on the gravity around the object, on the Moon the same bag would weigh less because the Moon's gravity is weaker.
The same pulling force reaches far out into space. The Earth's gravity pulls the Moon and keeps it circling, or orbiting, the Earth instead of flying off in a straight line. In the same way, the Sun's gravity keeps the planets in their orbits. Orbital motion is just gravity acting as a non-contact force across a huge distance.
Measuring forces: we measure a force, including weight, with a spring scale (also called a newton meter). The bigger the force, the more the spring stretches, and the scale reads the force directly in newtons.
Forces transfer energy. When a force moves an object along, scientists say the force does work, and doing work transfers energy. Stretch a rubber band and you store elastic potential energy; lift a ball up high and you store gravitational potential energy because you worked against gravity. Energy is never made or destroyed, it is only transferred from one store to another, this is the idea of energy being conserved.
Friction is the contact force that ties this together. It opposes motion, so a sliding object slows down, and the movement energy is transferred into heat in the two surfaces. Rub your hands together quickly and you feel that heat. The energy is not lost, it has just spread out as warmth.
Magnets and electromagnets. A magnet has two poles, a north (N) and a south (S). Like poles (N and N, or S and S) repel, and unlike poles (N and S) attract, all without touching. The magnetic force fills the space around a magnet as a magnetic field, which runs from the north pole around to the south pole and can be mapped with iron filings or a small compass. An electromagnet is made by sending an electric current through a coil of wire. It becomes stronger when you add more turns of wire, increase the current, or place an iron core inside the coil, and a big advantage is that it can be switched off by turning off the current, which is how scrapyard cranes pick up and then drop cars.
A simple machine changes the size of the force you need for a task. A lever (like a crowbar or a see-saw) lets a small effort move a large load by pivoting around a fixed point. A pulley uses a wheel and rope to change the direction of your effort, and a system with several wheels lets you lift a heavy load with a smaller pull. Simple machines do not give you energy for free, they trade a smaller force for a longer distance, but they make hard jobs possible.
Aboriginal and Torres Strait Islander Peoples' knowledge of forces. The woomera (spear-thrower) used by many Aboriginal Peoples across Australia is an elegant application of the lever idea. The woomera fits to the end of a spear and acts as an extension of the thrower's arm. By making the arm effectively longer, it acts like a lever that increases the speed the spear is released at, so the spear travels much faster and further than by hand alone. This careful understanding of forces, developed and refined over many thousands of years, made hunting far more effective and is a clear example of forces knowledge built into everyday tools. Aboriginal Peoples also built stone fish traps, such as those at Brewarrina (Baiame's Ngunnhu) on the Barwon River in New South Wales, where the walls use the pushing force of flowing water to guide and hold fish, another thoughtful use of forces.
A simple machine such as a lever or changes the size of the needed for a task. The woomera works like a by extending the thrower's . Energy is never made or destroyed, it is .
A loaded trolley is parked on a flat floor and does not move. You start to push it but it still does not move. Predict: what is the friction force doing while you push gently, and what must change for the trolley to finally move?
How close was your prediction?
Well done, you connected friction, balanced forces and the change to motion.
Key insight: friction can balance your push until your force grows larger than friction's limit.
Below are 8 everyday situations from the Forces unit. For each one, write whether the named force is a contact (C) or non-contact (NC) force, then name the specific force type.
| # | Situation | C or NC? | Force type |
|---|---|---|---|
| 1 | A footy boot gripping the grass as a player runs | ||
| 2 | Earth pulling the Moon to keep it in orbit | ||
| 3 | A rubbed balloon picking up small pieces of paper | ||
| 4 | Two like magnet poles pushing apart | ||
| 5 | A tug-of-war rope pulling a team forward | ||
| 6 | The weight of a school bag on a spring scale | ||
| 7 | A hand pushing a shopping trolley | ||
| 8 | A scrapyard electromagnet lifting a car |
Then pick situation 7 and draw a quick force diagram in your book showing the applied push, friction and weight as labelled arrows. Decide whether the forces are balanced or unbalanced if the trolley moves at a steady speed.
Question: How does the angle of a ramp affect the force needed to pull a load up it?
You will design a fair test using a toy car or block as the load, a board as the ramp, and a spring scale to measure the pulling force in newtons.
Variables:
- Independent variable (IV): the angle of the ramp, test at least 3 angles (for example 10 degrees, 20 degrees, 30 degrees, set with a protractor)
- Dependent variable (DV): the steady pulling force in newtons read off the spring scale as the load moves slowly up the ramp
- Controlled variables: the same load (same mass); the same ramp surface (same friction); pulling at a steady slow speed; the same spring scale; the same length pulled up the ramp
Method (4 steps):
- Set the ramp to the first angle and measure it with a protractor. Hook the spring scale to the load at the bottom of the ramp.
- Pull the load slowly and steadily up the ramp and read the steady force in newtons on the spring scale.
- Repeat steps 1 to 2 twice more at the same angle (3 trials) and calculate the average force.
- Change the ramp to the next angle and repeat for all angles.
Results table template:
| Ramp angle (degrees) | Trial 1 (N) | Trial 2 (N) | Trial 3 (N) | Average force (N) |
|---|---|---|---|---|
| 10 | ||||
| 20 | ||||
| 30 |
Expected finding: the steeper the ramp, the larger the pulling force needed, because you are working against more of the load's weight. A gentle ramp is itself a simple machine: a smaller force over a longer slope can raise the same load.
Write your plan and a hypothesis below using the format: "If the ramp angle is increased, then the pulling force will [change direction] because [scientific reason]." Then name two controlled variables and explain why each must be kept the same.
Q1. A skateboarder pushes off, rolls along flat ground, and slowly comes to a stop. Use the ideas of contact and non-contact forces, balanced versus unbalanced forces, and energy to explain what happens. (4 marks)
Q2. Explain the difference between mass and weight. A 4 kg melon is taken to the Moon, where gravity is weaker than on Earth. State what happens to its mass and to its weight, and use W = mg in your answer. (3 marks)
Q3. Plan a fair test to investigate how the angle of a ramp affects the force needed to pull a load up it. Identify the independent, dependent and two controlled variables, and write a hypothesis. (4 marks)
Answers
▾MCQ 1
B Gravitational, electrostatic and magnetic forces all act across a gap without touching, so they are non-contact forces. Friction, tension and an applied push all need objects to be touching, so they are contact forces.
MCQ 2
A Steady speed in a straight line means no change in motion, which means the forces are balanced (they cancel out). If the forces were unbalanced there would be a net force and the car would speed up, slow down or turn.
MCQ 3
C Weight is found with W = mg. With a 5 kg mass and gravity of about 10 N per kilogram, the weight is 5 times 10 = 50 N. The spring scale measures this pulling force directly in newtons.
MCQ 4
D Switching the current off turns the electromagnet off completely, so it cannot make it stronger. Adding more turns of wire, increasing the current, or adding an iron core all make an electromagnet stronger.
MCQ 5
B The woomera acts like a lever that extends the thrower's arm. By effectively lengthening the arm, the spear is released at a higher speed, so it travels faster and further. It does not remove friction, add gravity, or use magnetism.
Short Answer 1
Model answer: When the skateboarder rolls along, friction (a contact force between the wheels, bearings and ground, plus air resistance) acts backward against the motion. The forward push from pushing off has ended, so the only force along the ground is friction, this makes the forces unbalanced with a net force backward, which slows the skateboarder down. The skateboarder's movement (kinetic) energy is not lost, it is transferred into heat in the surfaces and a little sound, and energy overall is conserved. The skater stops once all the movement energy has been transferred away.
Short Answer 2
Model answer: Mass is the amount of matter in an object, measured in kilograms, and it does not change from place to place. Weight is the pulling force of gravity on that mass, measured in newtons, and it depends on the local gravity through W = mg. On the Moon the melon's mass stays 4 kg because it is still made of the same matter. Its weight becomes smaller than on Earth because g (the gravitational field strength) is weaker on the Moon, and W = mg gives a smaller force when g is smaller.
Short Answer 3
Model answer: Hypothesis: "If the ramp angle is increased, then the pulling force needed will increase, because a steeper ramp means you work against more of the load's weight." Independent variable: the ramp angle (for example 10, 20 and 30 degrees set with a protractor). Dependent variable: the steady pulling force in newtons read from a spring scale. Controlled variables: the same load (same mass), the same ramp surface (same friction), a steady slow pulling speed, the same spring scale and the same distance pulled. Method: set the angle, pull the load slowly up while reading the force, repeat 3 times and average, then change the angle and repeat.
The hook at the start showed three everyday events, a dropped pencil, a sliding pencil and a rubbed pencil, all caused by forces: gravity, friction and an electrostatic force. Every idea in this unit comes back to a single sentence: a force is a push or a pull, and forces change the way things move.
Look back at your Think First list of forces. Which forces did you miss? Which one connects most directly to the spring scale you use to measure forces in newtons, and what will you do to strengthen your weakest area before the unit assessment?
- A force is a push or a pull measured in newtons with a spring scale; forces are contact (applied, friction, tension) or non-contact (gravitational, electrostatic, magnetic).
- Force diagrams show whether forces are balanced (no change in motion) or unbalanced (motion changes); friction transfers movement energy to heat and energy is conserved.
- Weight = mass times gravity ($W = mg$); gravity keeps the Moon and planets in orbit; magnets and electromagnets repel and attract; simple machines like levers and pulleys change the size of the force needed.