1.3.5Forces and Their Effects
Newton’s First Law — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 13 of 16
Part 1 of 6The law
1Newton's first law
Newton's first law an object stays at rest, or continues to move at constant velocity, unless acted on by a non-zero resultant force.
2
Constant velocity means a steady speed in a straight line.
3
If the resultant force is zero, the velocity does not change.
4
Objects on Earth slow down only because of friction and air resistance.
- A puck on ice slows slowly because friction is small.
- A spacecraft in deep space coasts at steady speed with its engines off.
Part 2 of 6Inertia
Inertia
Inertia the tendency of an object to resist changes in its motion.
Inertia is a property of mass, not a force.
The greater the mass, the greater the inertia and the larger the force needed to change the velocity.
A loaded trolley is harder to start and harder to stop than an empty one.
Inertia depends on mass anywhere, not only on Earth.
Part 3 of 6Examples of inertia
Seat belts
- Seat belts in a sudden stop passengers keep moving forward at the car's original speed unless the belt exerts a force.
Tablecloth trick
- Tablecloth trick the dishes have too much inertia to move because friction acts for too short a time.
Coin on a card
- Coin on a card when the card is flicked away, the coin drops into the cup.
Ketchup bottle
- Ketchup bottle moving the bottle down and stopping it suddenly makes the ketchup carry on moving.
Part 4 of 6Constant velocity with forces acting
Constant velocity with forces acting
The law does not mean no forces act; it means the resultant is zero.
Car at steady 25 m/s
- Car at steady 25 m/s driving force equals the total friction and air resistance.
Parachutist at terminal speed
- Parachutist at terminal speed weight equals air resistance.
Book at rest on a table
- Book at rest on a table weight and contact force cancel.
Forces must balance in each direction, along and across the motion.
Part 5 of 6Using the law
1
If velocity is constant, the unknown force equals the opposing forces.
2
If an object changes speed or direction, there must be an unbalanced force on it.
3The first law is the special case of F
The first law is the special case of F ma with F = 0, giving a = 0.
4Skater example
- Skater example after pushing off, only tiny friction acts, so she glides at almost constant velocity.
- A heavier skater has more inertia, so she is harder to speed up and harder to stop.
Part 6 of 6More from the specification
1
Resultant force
- Resultant force the single force equal to the vector sum of all forces;
- zero resultant means constant velocity.
2
Newton's first law
- Newton's first law an object stays at rest or at constant velocity unless a resultant force acts.
3
Acceleration
- Acceleration a resultant force produces acceleration in its direction, F = ma.
4
Hooke's law
- Hooke's law a spring's extension is proportional to the force applied, up to the limit of proportionality.
5
Force-extension graph
- Force-extension graph a straight line shows proportionality and the gradient is the spring constant;
- extension equals length minus original length.
6
Extension
- Increase in length over the original length.
7
Length
- Resistance is directly proportional to the length of a wire.
8
Acceleration
- Rate of change of velocity.
9
Proportional
- Changing by the same factor as another quantity.
10
Elastic
- Returns to original shape when force is removed.
11
Spring constant
- Stiffness k in N/m.
12
Circular motion
- Motion along a circular path with a force towards the centre.
13
Original length
- Length of the spring with no load.
Key terms
| Term | Meaning |
|---|---|
| Newton's first law | No resultant force means no change in velocity. |
| Inertia | Resistance to changes in motion; larger for larger mass. |
| Constant velocity | Steady speed in a fixed direction. |
| Resistive force | Force opposing motion, such as friction or drag. |
| Equilibrium | State with zero resultant force. |
Avoid these mistakes
1
Saying a force is needed to keep an object moving at steady speed — a force is needed only to overcome friction.
2
Thinking an object at constant velocity has no forces on it — the resultant force is zero.
3
Thinking inertia is a force — it is a property of mass.
4
Saying a "force of motion" pushes a moving object — no such force exists.
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