1.4Motion, Forces and Energy
Effects of Forces — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
15 min read 31 parts Lesson 30 of 91 Unit quiz
Lesson summary
Resultant Forces and Newton's First Law — IGCSE PHYSICS notes: Combine forces along a line, decide whether they are balanced, and apply Newton's first law to rest and constant velocity.Definition — a force is a push or a pull on an object caused by an interaction with another obj…
- Effects of a force
- What it decides
- Vector nature
- Book on a table
- Equal and opposite
- Same direction
Part 1 of 7What forces do
Definition
- Definition a force is a push or a pull on an object caused by an interaction with another object.
Effects of a force
- Effects of a force a force can change an object's speed, direction, shape or size.
- Engine thrust makes a car speed up and brake friction makes it slow down.
- The Sun's gravity pulls a comet so it changes direction.
- Two opposing pushes on a spring make it compress and change shape.
Part 2 of 7Resultant force
Definition
- Definition the resultant force is the single force with the same effect as all the forces acting on an object.
- Forces along one straight line are combined into one net force.
What it decides
- What it decides the resultant gives the direction the object moves and the size of the net force.
Vector nature
- Vector nature force is a vector, so directions must be shown by signs.
- Choose one direction as positive and the opposite as negative.
- It does not matter which you choose if you are consistent.
Part 3 of 7Balanced and unbalanced forces
| Feature | Balanced forces | Unbalanced forces |
|---|---|---|
| Along a line | Equal and opposite | Do not cancel |
| Resultant force | Zero | Non-zero |
| Effect on motion | No change in velocity | Acceleration in the direction of the resultant |
| Example | Book resting on a table | Car braking |
- Book on a table
- the weight acts downwards and the table's contact force acts upwards.
- The two are equal in size and opposite, so the resultant force is zero.
Part 4 of 7Calculating a resultant along a line
1
Choose a positive direction, for example right is positive.
2
Give each force a sign according to its direction.
3
Add the forces together.
4
State the size and the direction of the answer.
Equal and opposite
- 5 N right and 5 N left give 5 + (−5) = 0 N.
Same direction
- 3 N and 7 N together give 10 N.
Opposite directions
- 7 N left and 3 N right give (−7) + 3 = −4 N, so 4 N to the left.
Part 5 of 7Newton's first law
1Statement
- Statement an object stays at rest, or continues in a straight line at constant speed, unless a resultant force acts on it.
- A stationary object stays stationary.
- A moving object keeps its constant velocity.
2Converse
- Converse if an object is at rest or has constant velocity, the resultant force on it is zero.
3Constant velocity
- Constant velocity this means no change in speed and no change in direction.
Part 6 of 7Examples of the first law
1Mug on a table
- Mug on a table it stays still until someone picks it up or knocks the table.
2Space debris
- Space debris it keeps going in a straight line at constant speed until it meets a planet's gravity or hits an asteroid.
3Car at steady speed
- Car at steady speed it continues until the driver brakes or accelerates.
- Engine force balances friction and air resistance, so the resultant is zero.
- More engine force speeds the car up, while more friction (brakes) slows it down.
Part 7 of 7More from the specification
More from the specification
Hooke's law and spring extension
- Hooke's law states that the extension of a spring is directly proportional to the applied load force, described by the equation F = kΔx, provided the limit of proportionality is not exceeded.
Force-extension graph analysis
- Plotting load against extension gives a straight-line graph passing through the origin;
- beyond the limit of proportionality, the line curves as the material undergoes permanent plastic deformation.
Measuring initial and extended length
- Spring extension is measured by subtracting the original unloaded length from the total stretched length recorded against a vertical millimeter ruler.
Centripetal force in circular motion
- An object moving in a circular path at constant speed experiences a resultant force acting perpendicularly towards the centre of the circle, changing its velocity direction without altering speed.
Hooke's law and spring extension
- Hooke's law states that the extension of a spring is directly proportional to the applied load force, described by the equation F = kΔx, provided the limit of proportionality is not exceeded.
Force-extension graph analysis
- Plotting load against extension gives a straight-line graph passing through the origin;
- beyond the limit of proportionality, the line curves as the material undergoes permanent plastic deformation.
Measuring initial and extended length
- Spring extension is measured by subtracting the original unloaded length from the total stretched length recorded against a vertical millimeter ruler.
Centripetal force in circular motion
- An object moving in a circular path at constant speed experiences a resultant force acting perpendicularly towards the centre of the circle, changing its velocity direction without altering speed.
Circular motion
- Motion along a circular path with a force towards the centre.
Extension
- Increase in length over the original length.
Length
- Resistance is directly proportional to the length of a wire.
Proportional
- Changing by the same factor as another quantity.
Elastic
- Returns to original shape when force is removed.
Key terms
| Term | Meaning |
|---|---|
| Force | A push or pull on an object caused by an interaction. |
| Resultant force | The single force with the same effect as all the forces acting on an object. |
| Balanced forces | Forces that cancel, giving a zero resultant. |
| Unbalanced forces | Forces that do not cancel, giving a resultant. |
| Newton's first law | Without a resultant force an object stays at rest or moves at constant velocity. |
| Constant velocity | Constant speed in a constant direction. |
Avoid these mistakes
1
Believing a moving object must have a resultant force — constant velocity needs zero resultant force.
2
Mixing up constant speed with constant velocity — velocity also needs a constant direction.
3
Adding opposing forces without signs — opposite directions subtract.
4
Giving only the size of a resultant — state the direction too.
Last updated:
\n