GCSE-ready- Home
- Lessons
- GCSE & IGCSE Physics
- Newton's laws and weight
Newton's laws and weight
🎬 The doodle video for this lesson is coming soon. Subscribe on YouTube to see it first.
Three laws explain every push and pull you will meet in GCSE Physics.
First law
If the resultant force on an object is zero, it stays still or keeps moving at a constant velocity.
To find the resultant force, add forces in the same direction and subtract opposite ones.
To find the resultant force, add forces in the same direction and subtract opposite ones.
Second law
force = mass × acceleration, or F = m × a
Force in newtons (N), mass in kg, acceleration in m/s2.
A bigger force or a smaller mass gives more acceleration.
Force in newtons (N), mass in kg, acceleration in m/s2.
A bigger force or a smaller mass gives more acceleration.
Third law and weight
When two objects interact, they exert equal and opposite forces on each other.
Weight = mass × gravitational field strength, W = m × g. Use the value of g the question gives you. On Earth it is about 10 N/kg.
Weight = mass × gravitational field strength, W = m × g. Use the value of g the question gives you. On Earth it is about 10 N/kg.
A 1200 kg car accelerates at 2.5 m/s2. Find the resultant force.
- F = m × a
- F = 1200 × 2.5
- = 3000 N
Answer: 3000 N
Zero resultant force means constant velocity. F = m × a. Interaction forces are equal and opposite. Weight W = m × g.
Check you have got it
Answer 6 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.