Newton's First Law
Build confidence in Newton's First Law for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Newton's first law states that an object stays at rest or moves at constant velocity unless acted on by a resultant force.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Newton's first law states that if the resultant force acting on an object is zero, then a stationary object remains stationary and a moving object continues to move at the same speed in the same direction.
So an object continues at constant velocity unless a resultant force acts on it. Constant velocity means both constant speed and unchanging direction.
The tendency of objects to continue in their state of motion is called inertia. Inertial mass is a measure of how difficult it is to change an object's velocity, and it is defined as the ratio of force over acceleration. A larger inertial mass means a greater force is needed to produce the same change in velocity.
Revision notes
The law
If the resultant force on an object is zero, a stationary object remains stationary and a moving object continues at the same velocity.
Constant velocity means constant speed AND unchanging direction. A resultant force is required to change either.
Applying it
A car travelling at a steady speed on a straight road has balanced forces — the driving force equals the resistive forces.
If the driving force increases, the resultant force is no longer zero and the car accelerates. If it decreases, the car decelerates.
Inertia
Inertia is the tendency of objects to continue in their state of rest or of uniform motion.
Inertial mass measures how difficult it is to change an object's velocity, defined as the ratio of force over acceleration. A larger inertial mass needs a greater force for the same acceleration.
Key points
- Zero resultant force means constant velocity.
- A stationary object stays stationary.
- A moving object continues at the same velocity.
- Constant velocity means constant speed and direction.
- Inertia is the tendency to continue in the same state of motion.
- Inertial mass is the ratio of force over acceleration.
Worked examples
Example 1
State Newton's first law. [2 marks]
Model answer
If the resultant force acting on an object is zero, a stationary object remains stationary (1 mark), and a moving object continues to move at the same speed and in the same direction (1 mark).
Example 2
A car travels at a steady speed along a straight road. Describe the forces acting on it. [2 marks]
Model answer
The forces are balanced, so the resultant force is zero (1 mark). The driving force from the engine is equal in size to the resistive forces such as friction and air resistance (1 mark).
Example 3
Define inertial mass. [2 marks]
Model answer
Inertial mass is a measure of how difficult it is to change the velocity of an object (1 mark), and it is defined as the ratio of force over acceleration (1 mark).
Common mistakes
Saying an object needs a force to keep moving.
With zero resultant force it continues at constant velocity indefinitely.
Saying zero resultant means no forces act.
Forces may act but cancel each other out.
Forgetting direction in constant velocity.
Velocity is a vector, so direction must also be unchanging.
Confusing inertial mass with weight.
Inertial mass is force over acceleration; weight is mass times g.
Exam tips
- State both halves of the law — stationary and moving.
- Say the forces are balanced, not that there are none.
- Include direction when describing constant velocity.
- Define inertial mass as the ratio of force over acceleration.
Key terms
- Newton's first law
- Zero resultant force means constant velocity.
- Inertia
- The tendency to continue in the same state of motion.
- Inertial mass
- A measure of how hard it is to change velocity.
- Resistive force
- A force opposing motion, such as friction or air resistance.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.