Forces and motion
Pushes and pulls, gravity and weight, friction, air and water resistance, and balanced and unbalanced forces.
📌 Key points
- A force is a push or a pull. Forces can make things start moving, stop, speed up, slow down, change direction or change shape.
- Forces are measured in newtons (N) with a force meter (also called a newton meter).
- Gravity is a pulling force. The Earth’s gravity pulls everything towards the centre of the Earth.
- Mass is the amount of matter in an object, in kilograms (kg). Weight is the force of gravity on it, in newtons (N). On Earth, 1 kg weighs about 10 N.
- Gravity on the Moon is weaker, so you would weigh less there, but your mass would stay the same.
- Friction is a force between two surfaces that slows movement. Rough surfaces make more friction than smooth ones.
- Air resistance and water resistance slow down things moving through air or water. Streamlined shapes reduce them.
- Balanced forces are equal and opposite. They do not change the motion: the object stays still or keeps moving at the same speed.
- Unbalanced forces change the motion: the object speeds up, slows down or changes direction.
📖 Key words
- Force
- A push or a pull.
- Newton (N)
- The unit of force.
- Gravity
- The force that pulls objects towards each other, e.g. towards the Earth.
- Weight
- The force of gravity on an object, measured in newtons.
- Mass
- The amount of matter in an object, measured in kilograms.
- Friction
- A force between two surfaces that slows down movement.
- Air resistance
- A force that slows down objects moving through air.
- Streamlined
- Having a smooth shape that moves easily through air or water.
✏️ Worked example
A box is pushed to the right with a force of 30 N. Friction acts to the left with a force of 12 N. What is the overall force, and what happens to the box?
- The forces act in opposite directions, so subtract: 30 N − 12 N = 18 N.
- The overall force is 18 N to the right.
- The forces are unbalanced, so the box speeds up towards the right.
- Answer: 18 N to the right; the box speeds up.
Draw force arrows. Forces in the same direction add up; forces in opposite directions subtract. Equal and opposite forces are balanced.
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