AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER
Newton's First Law
Forces · Lesson 14 of 20
Teacher copy - includes the notes for whoever is teaching from it.
Warm-up
Answer each one, then check.
1. What is a resultant force?
A single force with the same effect as all the forces
2. What are balanced forces?
Equal and opposite forces
3. What is velocity?
Speed in a given direction
4. What is friction?
A force opposing motion
5. What is constant velocity?
Same speed in the same direction
Learning Objectives
1. State Newton's First Law.
2. Apply it to objects moving at constant velocity and to objects whose speed or direction changes.
3. Explain that when a vehicle travels at a steady speed the resistive forces balance the driving force.
4. Explain inertia (Higher tier).
Newton's First Law
If the resultant force on an object is zero, a stationary object remains stationary and a moving object continues at the same speed in the same direction.
The velocity of an object will only change if a resultant force acts on it.
Balanced and Unbalanced Forces
|
No resultant force means no change in velocity. |
A car at constant speed with balanced forces, a braking car with a resultant force and a puck sliding on smooth ice.
Steady Speed
|
A car travels at a steady 20 m/s. The driving force is 1500 N. What is the resistive force? |
1. Steady speed
Velocity is constant, so the resultant force is zero
2. Forces balance
Resistive force = driving force
Answer: 1500 N, in the opposite direction.
Changing Velocity
|
A car brakes from 20 m/s to rest. Explain the motion using Newton's First Law. |
1. Velocity changes
So there must be a resultant force
2. Braking force
Greater than the driving force, acting against the motion
Answer: A resultant force opposes the motion, so the car decelerates.
Inertia (Higher)
|
Explain why a passenger moves forwards when a car brakes suddenly. |
1. Before braking
The passenger moves with the car at a constant velocity
2. Braking
The car slows, but no force acts on the passenger from the seat
3. Inertia
The passenger's body tends to continue at the same velocity, so moves forward relative to the car
Answer: Inertia: objects tend to keep their state of rest or uniform motion.
Key Ideas
Use the law in explanations.
▸ Zero resultant. Stationary or constant velocity.
▸ Non-zero resultant. The velocity (speed or direction) changes.
▸ Direction matters. A circular motion needs a resultant force towards the centre.
▸ Inertia (Higher). The tendency of objects to continue in their state of rest or uniform motion.
Key Terms
|
Newton's First Law An object keeps its velocity unless a resultant force acts on it. |
Inertia The tendency of an object to stay at rest or keep moving at constant velocity. |
|
Balanced forces Forces with a resultant of zero. |
Uniform velocity Constant speed in a straight line. |
|
Resistive force A force opposing motion. |
Driving force A force that makes a vehicle move. |
Your Task: Apply the Law
10 minutes
|
Explain each in terms of forces: (a) a cyclist coasts at constant speed on a flat road with no pedalling (hint: think of ideal conditions) (b) a book stays on a table (c) a ball rolls to a stop on grass. 1. Is there a resultant force? 2. Does velocity change? |
A good answer shows: (a) In ideal conditions with no resistive force the resultant is zero. (b) Weight and normal contact force are balanced. (c) Friction gives a resultant force opposing the motion.
Note: Ask what would happen on smooth ice.
Can I...?
☐ State the first law.
☐ Apply it to constant velocity.
☐ Apply it to changing velocity.
☐ Explain balanced forces on a car.
☐ Explain stopping in terms of forces.
☐ Define inertia.
☐ Explain passenger motion.
☐ Use resultant force.
Summary
✓ Resultant zero: stationary or constant velocity.
✓ Resultant non-zero: velocity changes.
✓ Steady speed: driving force equals resistive force.
✓ Inertia: tendency to continue the same motion.
|
EXAM FOCUS A car travels at a constant speed. Use Newton's First Law to explain what this tells you about the forces. (2 marks) The resultant force is zero. |
Exam Practice: Newton's First Law
Answer all questions. Use the mark allocation as a guide to how much to write. · 13 minutes
▸ Question 1 · 2 marks · State. State Newton's First Law of motion.
▸ Question 2 · 2 marks · Explain. A car travels along a level road at a steady speed. The driving force is 1500 N. State the resistive force and explain your answer.
▸ Question 3 · 3 marks · Explain. A lorry's driver takes his foot off the accelerator. The lorry slows down. Use Newton's First Law to explain why.
▸ Question 4 · 3 marks · Explain. A passenger in a car moves forwards when the car brakes suddenly. Explain why, using the idea of inertia.
▸ Question 5 · 2 marks · Explain. A puck slides at a constant velocity across very smooth ice. Explain why it does not slow down.
Question 1 · 2 marks · State
|
“State Newton's First Law of motion.” |
HOW TO ANSWER IT Command word: State. Worth 2 marks, so plan before writing.
Question 1 · mark scheme
2 marks available. Award a mark for each point made.
▸ Zero resultant: stationary stays stationary. 1 mark
▸ Moving object keeps the same velocity. 1 mark
▸ Model answer. If the resultant force on an object is zero, a stationary object stays stationary and a moving object continues at the same speed and in the same direction.
Question 2 · 2 marks · Explain
|
“A car travels along a level road at a steady speed. The driving force is 1500 N. State the resistive force and explain your answer.” |
HOW TO ANSWER IT Command word: Explain. Worth 2 marks, so plan before writing.
Question 2 · mark scheme
2 marks available. Award a mark for each point made.
▸ 1500 N. 1 mark
▸ Resultant force zero. 1 mark
▸ Model answer. 1500 N. The speed is constant so the resultant force is zero and the forces are balanced.
Question 3 · 3 marks · Explain
|
“A lorry's driver takes his foot off the accelerator. The lorry slows down. Use Newton's First Law to explain why.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 3 · mark scheme
3 marks available. Award a mark for each point made.
▸ Resistive forces remain. 1 mark
▸ Resultant force acts against the motion. 1 mark
▸ Velocity changes so the lorry slows. 1 mark
▸ Model answer. When the driving force stops, the resistive forces (friction and air resistance) become the resultant force, which acts against the motion and reduces the velocity.
Question 4 · 3 marks · Explain
|
“A passenger in a car moves forwards when the car brakes suddenly. Explain why, using the idea of inertia.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 4 · mark scheme
3 marks available. Award a mark for each point made.
▸ Passenger moving with the car. 1 mark
▸ Inertia: tendency to keep the same velocity. 1 mark
▸ Keeps moving forwards as the car slows. 1 mark
▸ Model answer. The passenger was moving at the same velocity as the car. Inertia is the tendency to continue at the same velocity, so the passenger keeps moving forwards while the car slows.
Question 5 · 2 marks · Explain
|
“A puck slides at a constant velocity across very smooth ice. Explain why it does not slow down.” |
HOW TO ANSWER IT Command word: Explain. Worth 2 marks, so plan before writing.
Question 5 · mark scheme
2 marks available. Award a mark for each point made.
▸ Negligible friction. 1 mark
▸ Resultant force zero so velocity constant. 1 mark
▸ Model answer. There is almost no friction, so the resultant force is about zero and the velocity stays the same.