GCSE Physics · Edexcel 1PH0 · Topic 2B
Newton's laws and forces
Key points
- Newton's first law: if the resultant force on an object is zero, it stays at rest or keeps moving at a constant velocity.
- Newton's second law: . A bigger resultant force gives a bigger acceleration; a bigger mass gives a smaller one.
- Inertia is the tendency of an object to keep its motion; inertial mass is the ratio .
- Weight is the gravitational force on a mass: , with N/kg on Earth. Mass does not change with location; weight does.
- Newton's third law: when two objects interact, they exert equal and opposite forces on each other. These pairs act on different objects and are the same type of force.
- A falling object reaches terminal velocity when air resistance grows to equal its weight, so the resultant force is zero.
- In circular motion the speed can be constant while the velocity changes, because the direction changes; a centripetal force acts towards the centre.
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Newton's three laws explain every change in motion you will meet at GCSE. Once you can identify the resultant force on an object, you can predict whether it speeds up, slows down, changes direction or carries on unchanged.
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