Newton's First Law — AQA GCSE Physics
Test yourself on Newton's First Law with AQA GCSE practice questions.
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Newton's First Law explained
A resultant force is what remains when every force on an object is added as a vector.
Read the full explanation
If that total is zero, velocity cannot change: a stationary object stays still, and a moving object keeps the same speed in a straight line. Zero resultant force is not the same as no forces at all. A car cruising on a level road has multiple forces acting on it: weight is balanced by the normal contact force, while the driving force is exactly matched by the sum of drag and friction. A hailstone at terminal velocity has its weight exactly matched by air resistance. In these cases, forces balance out, so the object neither speeds up nor slows down. Because velocity is a vector, this law also means direction cannot change: an object with zero resultant force never turns.
Your focus
- Decide from a free body force diagram whether the resultant force is zero, and predict the motion that follows.
- Explain why a hailstone stops accelerating once air resistance has grown to equal its weight.
- Distinguish an object with zero resultant force from one with no forces acting on it at all, identifying all horizontal and vertical forces on a cruising car.
Newton's First Law exam tips
Marking Points
- one mark for stating that the resultant force acting on the object is zero
- one mark for saying the object stays stationary, or continues at the same speed in the same direction
- one mark for naming the forces that balance, such as weight and normal contact force, or driving force and drag plus friction
- one mark for stating that the balanced forces are equal in size and opposite in direction
Examiner Tips
- 💡Write the words 'resultant force is zero' rather than 'the forces cancel out'; the mark scheme looks for that phrase.
- 💡Whenever a question says steady speed, constant velocity or terminal velocity, make zero resultant force your first sentence.
- 💡Identify all forces acting on an object, including vertical forces like weight and normal contact force, before describing the motion.
Common Mistakes
- writing that no forces act on an object moving at a constant velocity, instead of saying the forces are balanced
- claiming a resultant force is needed to keep an object moving at a steady speed
- treating a car going round a bend at a steady speed as having zero resultant force, when its direction is changing
- assuming only two forces act on a cruising car, ignoring vertical forces like weight and normal contact force