Example 1: weight
What is the weight of a 3 kg melon on Earth?
- Weight = mass × 10.
- 3 × 10 = 30 N.
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IB DemystifiedMYP Sciences
Forces and their effects
Every time you kick a ball, open a door or stop your bicycle, you are using forces. Forces are pushes and pulls, and they explain why things start moving, stop, turn or change shape.
Recommended for MYP 1 · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Mass and weight
- Forces in the real world
- Worked examples
- Assessment tips
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- identify contact and non-contact forces
- describe what forces can do to objects
- explain the difference between mass and weight, and calculate weight
- use force diagrams to decide whether forces are balanced
- explain friction and air resistance, and when they are useful
- plan a fair test and analyse simple force data
Before you start
You will use these skills. If any feel shaky, review them first.
- measuring length and time
- reading scales and tables
- simple multiplication and subtraction
Key vocabulary
- Force
- A push or a pull, measured in newtons (N).
- Contact force
- A force that acts when objects touch, such as friction.
- Non-contact force
- A force that acts at a distance, such as gravity or magnetism.
- Weight
- The force of gravity on an object, in newtons.
- Mass
- The amount of matter in an object, in kilograms.
- Balanced forces
- Forces that are equal in size and opposite in direction.
Understanding the ideas
-
What is it?
A force is a push or a pull. Forces can make objects speed up, slow down, change direction or change shape. We measure forces in newtons (N) with a newton meter, which contains a spring.
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Why does it happen?
Some forces need contact, like friction and air resistance; others act at a distance, like gravity and magnetism. When the forces on an object are balanced, its motion does not change; when they are unbalanced, it speeds up, slows down or turns.
-
How do we know?
We can measure forces with newton meters and show them on force diagrams with arrows. Isaac Newton explained how forces change motion over 300 years ago, and his ideas are still used to design cars, bridges and rockets.
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Why does it matter?
Understanding forces helps keep us safe: tyres and shoes need grip, parachutes and playground surfaces reduce injuries, and engineers make vehicles streamlined to save fuel.
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What does it connect to?
Forces link to speed and motion, pressure, energy and simple machines in physics, to how muscles and bones move in biology, and to measuring and calculating in mathematics.
Mass and weight
- Mass is the amount of matter, measured in kilograms (kg). It is the same everywhere.
- Weight is a force, the pull of gravity, measured in newtons (N).
- On Earth, weight = mass × 10 (each kg weighs about 10 N).
- On the Moon, gravity is weaker: each kg weighs only about 1.6 N.
- So an astronaut's mass stays the same on the Moon, but their weight is much less.
Forces in the real world
Rickshaw and motorbike tyres need deep tread to grip wet roads. Cricket shoes have spikes for grip. Parachutes, airbags and soft playground surfaces all make stopping gentler to protect people.
Worked examples
Example 2: resultant force
A boy pushes a trolley with 40 N. Friction is 15 N. What is the resultant force?
- The forces act in opposite directions, so subtract.
- 40 − 15 = 25 N in the direction of the push.
Assessment tips
Questions on forces often use force diagrams, simple data and everyday situations. Expect to:
- Identify forces and say whether they are contact or non-contact.
- Calculate weight and resultant forces, with units.
- Explain balanced and unbalanced forces.
- Plan fair tests and describe patterns in results.
Common mistakes: giving weight in kilograms; saying an object at rest has no forces on it; adding forces that act in opposite directions; and forgetting the unit N.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: ramps and rolling cars
Partially guided investigation · about 45 minutes · groups
- Research question
- How does the height of a ramp affect how far a toy car rolls along the floor after leaving it?
- Scientific background
- A car released from higher up is moving faster at the bottom of the ramp. Friction from the floor then slows it down until it stops.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Toy car, ramp (a long board or book), pile of books to change the height, metre rule or tape measure, masking tape.
- Method
- Set the ramp at a height of 5 cm and mark a start line near the top.
- Let the car go from the start line without pushing it.
- Measure how far it travels from the bottom of the ramp.
- Repeat three times at each height (5, 10, 15, 20 cm) and work out the mean.
Safety. Keep the rolling area clear so nobody trips, and pick up the car and books after the experiment.
Then evaluate: why is it important not to push the car, and what else should stay the same?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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