1. What is Force?
A force is a push or pull that can change the state of motion of an object, change its direction, change its speed, or produce deformation.
Moving a box away from you.
Drawing a bucket upward with a rope.
Kicking a stationary football can make it move.
Force has magnitude and direction
Force is a vector quantity: its effect depends on both how large the force is and the direction in which it acts. Two equal forces in opposite directions can cancel, while forces in the same direction can add.
SI unit
The SI unit of force is the newton (N). A force of 1 N gives a mass of 1 kg an acceleration of 1 m/s².
2. Balanced and Unbalanced Forces
Balanced forces
When the forces acting on an object have a zero resultant (net force), they are balanced. Balanced forces do not produce acceleration. The object may remain at rest or continue moving with constant velocity.
Unbalanced forces
When the resultant force is not zero, the forces are unbalanced. The object accelerates: its speed, direction, or both can change.
| Situation | Net force | Effect |
|---|---|---|
| Book resting on a table | Zero | No acceleration |
| Car moving at constant velocity on a straight road | Zero | No acceleration |
| Football being kicked | Non-zero during the kick | Acceleration |
| Car braking | Non-zero | Velocity changes |
3. Force of Friction
Friction is a force that opposes relative motion or the tendency of relative motion between surfaces in contact.
Why does friction occur?
Even apparently smooth surfaces have microscopic irregularities. When surfaces touch, these irregularities interact. Friction therefore depends on the nature of the surfaces and how strongly they are pressed together.
Walking, writing, gripping objects, braking vehicles, holding a nail in wood and preventing slipping.
Causes wear, heating and energy loss in machines and moving parts.
Reducing friction
- Lubrication using oil or grease.
- Using rollers, wheels or ball bearings.
- Making surfaces smoother where appropriate.
- Streamlining vehicles to reduce fluid resistance.
Increasing friction
- Grooves and treads on tyres.
- Rough shoe soles for better grip.
- Roughened surfaces where slipping must be prevented.
4. Inertia
Inertia is the tendency of an object to resist a change in its state of rest or motion. The greater the mass of an object, the greater its inertia.
Passengers may jerk backward when a stationary bus starts suddenly.
Passengers may jerk forward when a moving bus stops suddenly.
A passenger tends to move sideways when a moving vehicle turns sharply.
Mass and inertia
Mass is a measure of the amount of matter and is also a measure of an object's inertia. A heavier object generally requires a greater force to produce the same acceleration as a lighter object.
5. Newton’s First Law of Motion
Newton’s first law states that an object remains at rest or continues to move with uniform velocity in a straight line unless acted upon by an external unbalanced force.
This is also called the law of inertia.
Everyday examples
- A book remains on a table until an external force moves it.
- Passengers lean forward when a moving bus stops suddenly.
- Dust comes out when a carpet is beaten because the carpet moves suddenly while dust tends to remain in its state.
- A coin can fall into a glass when the card beneath it is flicked quickly.
6. Momentum
Momentum is the quantity of motion possessed by an object. It depends on both its mass and velocity.
where p = momentum, m = mass in kg, and v = velocity in m/s.
Momentum is a vector quantity because velocity has both magnitude and direction.
Example
A 2 kg ball moving at 5 m/s has:
If the direction of velocity changes, the momentum also changes even if the speed remains the same.
7. Newton’s Second Law of Motion
Newton’s second law connects force with the rate of change of momentum. For constant mass, it gives the familiar relation:
where F is net force in newtons, m is mass in kilograms and a is acceleration in m/s².
Derivation from momentum
Initial momentum = mu and final momentum = mv.
Rate of change of momentum:
because a = (v − u)/t.
Meaning of F = ma
| Change | Result |
|---|---|
| Mass increases, same force | Acceleration decreases |
| Force increases, same mass | Acceleration increases |
| Net force is zero | Acceleration is zero |
Worked example
A 5 kg object accelerates at 2 m/s². The net force is:
8. Newton’s Third Law of Motion
Newton’s third law states that when one object exerts a force on another object, the second object simultaneously exerts an equal-magnitude force in the opposite direction on the first.
Important features of an action-reaction pair
- The two forces are equal in magnitude.
- They act in opposite directions.
- They act on different objects.
- They occur simultaneously.
Examples
Your foot pushes the ground backward; the ground exerts a forward force on your foot.
A swimmer pushes water backward; water pushes the swimmer forward.
Exhaust gases are pushed backward; the gases exert a forward force on the rocket.
A projectile moves forward while the firearm experiences a backward reaction.
9. How the Three Laws Work Together
| Law | Main idea | Question it helps answer |
|---|---|---|
| First Law | Zero net force → no acceleration; inertia | What happens when the net force is zero? |
| Second Law | Net force produces acceleration: F = ma | How much acceleration results from a given net force? |
| Third Law | Forces occur in equal-and-opposite pairs on different objects | What force does the interacting object experience? |
10. Applications in Everyday Life
They help restrain passengers because their bodies tend to continue moving when a vehicle stops suddenly.
Friction between brake components and the wheel slows the bicycle.
Friction between soles and ground allows walking without continuously slipping.
Expelled gases provide the reaction associated with forward motion.
The force exerted by a bat changes the ball’s velocity; the ball exerts a reaction on the bat.
Sudden starting, stopping and turning demonstrate inertia.
Safety connection
Seat belts, helmets, vehicle restraints and controlled braking all reduce risks associated with changes in velocity and the forces involved in collisions.
11. Formula Sheet
| Quantity | SI unit |
|---|---|
| Force | newton (N) |
| Mass | kilogram (kg) |
| Acceleration | m/s² |
| Velocity | m/s |
| Momentum | kg m/s |
12. Quick Revision
13. Practice Questions
- Define force. Why is force called a vector quantity?
- State the SI unit of force and define 1 N.
- Differentiate balanced and unbalanced forces.
- Can an object be moving while the net force on it is zero? Explain.
- Define friction and state two situations where friction is useful.
- Give two methods of reducing friction.
- What is inertia? How is it related to mass?
- Explain why passengers jerk forward when a moving bus stops suddenly.
- State Newton’s first law of motion.
- Give two everyday examples of Newton’s first law.
- Define momentum and write its SI unit.
- Calculate the momentum of a 4 kg object moving at 3 m/s.
- State Newton’s second law of motion.
- Derive F = ma for a body of constant mass from the rate of change of momentum.
- A 10 kg object accelerates at 2 m/s². Find the net force.
- A force of 20 N acts on a 5 kg body. Find its acceleration.
- What happens to acceleration if the force is doubled while mass remains constant?
- What happens to acceleration if mass is doubled while force remains constant?
- State Newton’s third law of motion.
- Why do action and reaction forces not cancel each other?
- Explain how walking demonstrates Newton’s third law.
- Explain rocket propulsion using Newton’s third law.
- How does friction help a vehicle stop?
- Why are tyres provided with grooves/treads?
- Why are seat belts important when a vehicle stops suddenly?
- Differentiate inertia of rest, motion and direction with examples.
- A 2 kg ball moving at 6 m/s has what momentum?
- A net force of 12 N acts on a 3 kg object. Find acceleration.
- Explain why a heavier object has greater inertia.
- Explain how Newton’s three laws can be observed in a single everyday activity such as cycling or riding in a bus.
14. Interactive 30-Question Quiz
Choose one option for every question, then click Check Score.