1.5.1Motion, Forces and Energy
Effects OF Forces — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 10 parts Lesson 39 of 91 1 quiz
Lesson summary
1.5.1 Effects of Forces — IGCSE PHYSICS notes: Describe how forces change speed, direction, shape and size, and summarise resultant forces, Newton's laws, springs, friction and circular motion.Force = a push or a pull on an object caused by an interaction with another object, mea…
- Balanced forces
- Second law (Extended)
- Load–extension graph
- Hooke's law (Extended)
- What the symbols mean
- Drag (air resistance)
Section 1 of 10What a force does
Force
Force a push or a pull on an object caused by an interaction with another object, measured in newtons (N).
A force can change the speed of an object.
- Thrust from an engine makes a car speed up and friction from the brakes slows it.
A force can change the direction of motion.
- The Sun's gravitational pull makes a comet change direction.
A force can change the shape or size of an object.
- Opposing forces on the ends of a spring make it compress or stretch.
Section 2 of 10Resultant force
Resultant force
Resultant force the single force that has the same effect as all the forces acting on an object.
It decides the direction of motion and the size of the net force.
Forces are vectors, so give each direction a sign.
- Forces in the same direction add and forces in opposite directions subtract.
Forces at right angles (Extended) combine with Pythagoras' theorem.
Section 3 of 10Balanced and unbalanced forces
Balanced forces
- Balanced forces the resultant is zero, so there is no change in velocity.
- The object stays at rest or moves at constant speed in a straight line.
Unbalanced forces
- Unbalanced forces the resultant is not zero, so the object accelerates in the direction of the resultant.
- A resultant in the direction of motion speeds the object up, opposite to the motion slows it down and at an angle changes its direction.
Example
- Example a book on a table has weight balanced by the contact force from the table.
Section 4 of 10Newton's laws of motion
1First law
- First law an object stays at rest or moves at constant velocity unless a resultant force acts on it.
2Second law (Extended)
- F = ma, so the acceleration is in the direction of the resultant force.
- F is the resultant force in N, m is mass in kg and a is acceleration in m/s2.
- Acceleration is directly proportional to force and inversely proportional to mass.
3Inertia
- Inertia the tendency of an object to resist changes in its motion, which increases with mass.
Section 5 of 10Springs and Hooke's law
Load–extension graph
- Load–extension graph a straight line through the origin up to the limit of proportionality, then it curves.
Extension
Extension stretched length − original length.
Hooke's law (Extended)
- F = kx, so extension is directly proportional to the load.What the symbols mean: F = force (N) · k = spring constant (N m−1) · x or e = extension (m)
- k is the spring constant in N/m and is the force per unit extension.
- The law applies only up to the limit of proportionality.
Investigating a spring:
- Clamp the spring vertically beside a metre rule and record its original length.
- Add a 1.0 N load and record the new length.
- Calculate the extension = new length − original length.
- Repeat, adding 1.0 N each time, up to about 8 N.
- Plot load against extension and find where the line stops being straight.
Section 6 of 10Friction and drag
Friction and drag
Friction
Friction a force between two surfaces that opposes motion and produces heating.
Drag (air resistance)
Drag (air resistance) friction on an object moving through a fluid.
Reducing friction
- Reducing friction use lubricants, smoother surfaces and rolling parts.
Increasing friction
- Increasing friction use rougher surfaces and a larger force pressing the surfaces together.
Resultant force on a moving object
Resultant force on a moving object driving force − friction and drag.
Section 7 of 10Circular motion (Extended)
1
A resultant force towards the centre (the centripetal force) is needed for motion in a circle.
2
This force acts at right angles to the motion, so it changes direction but not speed.
3
Speed can stay constant while velocity changes because the direction changes.
4
For a fixed radius the force increases with the mass and the speed.
5
For constant speed the force decreases as the radius increases.
Section 8 of 10More from the specification
1
Resultant force
- The single force that has the same effect as all the forces acting on an object;
- equal and opposite forces give a resultant of zero.
2
Newton's first law
- An object stays at rest or moves at constant velocity unless acted on by a resultant force.
3
Newton's second law equation
- F = ma links the resultant force, the masses involved and the acceleration produced.
4
Elastic behaviour
- An elastic object returns to its original shape when the load is removed;
- a ruler is used to measure extension against the applied load.
5
Load-extension graph
- A straight line through the origin shows extension is proportional to load, up to the limit of proportionality.
6
Resultant force
- The single force that has the same effect as all the forces acting on an object;
- equal and opposite forces give a resultant of zero.
7
Newton's first law
- An object stays at rest or moves at constant velocity unless acted on by a resultant force.
8
Newton's second law equation
- F = ma links the resultant force, the masses involved and the acceleration produced.
9
Elastic behaviour
- An elastic object returns to its original shape when the load is removed;
- a ruler is used to measure extension against the applied load.
10
Load-extension graph
- A straight line through the origin shows extension is proportional to load, up to the limit of proportionality.
11
Elastic
- Returns to original shape when force is removed.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Force | A push or pull measured in newtons. |
| Resultant force | Single force with the same effect as all the forces acting. |
| Balanced forces | Forces with a zero resultant; no change in velocity. |
| Unbalanced forces | Forces with a non-zero resultant; they cause acceleration. |
| Spring constant | Force per unit extension, in N/m. |
| Friction | Force opposing motion between surfaces. |
| Centripetal force | Resultant force towards the centre of a circular path. |
Section 10 of 10Exam technique
1
Thinking balanced forces mean the object is stationary — it may move at constant speed.
2
Using the driving force alone in F = ma — use the resultant force.
3
Using the total spring length as the extension — extension is the increase in length.
4
Thinking friction always acts forwards or only when sliding — it opposes motion or the tendency to move.
Marked practiceQuizzes for this lesson
Last updated:
\n