Force and Laws of Motion
Concept-first revision with scientific reasoning, everyday links, diagrams, worked examples and exam-focused practice.
Chapter Snapshot
- Force can change motion, but balanced forces may produce no acceleration.
- Inertia is the tendency to resist change in motion and increases with mass.
- Newton first law connects inertia with unchanged motion in absence of unbalanced external force.
- Newton second law relates force to rate of change of momentum and gives F=ma for constant mass.
- Newton third law states interacting bodies exert equal and opposite forces on each other.
- In an isolated system, total momentum is conserved.
What You Will Learn
- Explain balanced and unbalanced forces
- Relate inertia to mass
- State and apply Newton three laws
- Understand momentum and change of momentum
- Derive the school-level relation F=ma
- Apply conservation of momentum
Why This Chapter Matters
Why does a passenger lurch forward when a bus stops? Why is a heavy trolley harder to accelerate than a light one? Why does a gun recoil? These are not separate facts. They arise from Newton's laws and momentum. Motion described how velocity changes; this chapter asks what causes that change. Newton's framework connects everyday events to quantitative mechanics.
Bridge from Earlier Learning
- Motion, velocity and acceleration
- Mass
- Direction and vectors
- Basic algebra
These ideas are reused and deepened in this chapter.
1. Force and Net Force
2. Inertia and Newton First Law
Inertia is the tendency to resist changes in rest or uniform straight-line motion. Newton's first law says an object remains at rest or in uniform straight-line motion unless an unbalanced external force acts. Mass measures inertia at this level: larger mass means greater resistance to acceleration.
3. Momentum
Momentum combines mass and velocity, p=mv. Because velocity has direction, momentum also has direction. A light fast object and a heavy slow object can have comparable momentum. Momentum is especially useful in short interactions such as collisions and recoil.
4. Newton Second Law
The second law states that rate of change of momentum is proportional to the applied net force and occurs in its direction. For constant mass, momentum change is m(v-u). Dividing by time gives m(v-u)/t = ma. In SI units the proportionality constant is chosen as 1, giving F=ma.
5. Newton Third Law
When one body exerts a force on another, the second exerts an equal and opposite force on the first. The forces act on different bodies, so they do not cancel on one object. Walking works because the foot pushes the ground backward while the ground pushes the body forward.
6. Conservation of Momentum
If external force on a system is negligible, total momentum before an interaction equals total momentum after. During a collision the bodies exert equal and opposite internal forces over the same time, producing equal and opposite momentum changes. Recoil is a direct example.
7. Safety and Momentum Change
Seat belts, airbags and padded surfaces increase the time over which momentum changes. For the same change in momentum, a longer stopping time means smaller average force. Safety design therefore often aims to lengthen deceleration time.
8. One Connected Framework
The first law describes zero net force, the second quantifies the effect of non-zero net force, and the third describes the paired nature of interactions. They should be understood as one system, not three unrelated statements.
Visualise the Concept
Newton laws connect forces with changes in momentum and interactions between bodies.
Formula / Relationship Sheet
Formula / Key Relations
Momentum
Direction follows velocity.
Newton second law
For constant mass.
Momentum form
Average over the time interval.
Conservation of momentum
For one-dimensional isolated interaction.
Worked Examples & Scientific Reasoning
It increases stopping time and reduces average force for the same momentum change.
Momentum conservation requires backward gun momentum to balance forward bullet momentum.
Observe Like a Scientist
Exam Thinking Strategy
Identify the body being analysed, then find the net external force. Use F=ma for acceleration and momentum conservation for isolated interactions.
Common Mistakes
Key Terms
Quick Revision
Quick Revision
- Force can change motion, but balanced forces may produce no acceleration.
- Inertia is the tendency to resist change in motion and increases with mass.
- Newton first law connects inertia with unchanged motion in absence of unbalanced external force.
- Newton second law relates force to rate of change of momentum and gives F=ma for constant mass.
- Newton third law states interacting bodies exert equal and opposite forces on each other.
- In an isolated system, total momentum is conserved.
- Revise the explanation behind each definition or formula, not only the wording.
Focused Practice
Which quantity measures inertia most directly at this level?
View Solution
Mass
State Newton first law.
View Solution
An object remains at rest or uniform straight-line motion unless acted on by an unbalanced external force.
A 3 kg body accelerates at 4 m/s². Find force.
View Solution
12 N
Find momentum of 0.2 kg ball moving at 15 m/s.
View Solution
3 kg m/s
Why do action-reaction forces not give zero acceleration to each body?
View Solution
They act on different bodies; each body acceleration depends on its own net force and mass.
Explain how Newton second law leads to F=ma for constant mass.
View Solution
Use momentum change m(v-u), divide by time, identify acceleration (v-u)/t and choose SI proportionality constant 1.
Final Chapter Summary
Chapter Summary
- Force can change motion, but balanced forces may produce no acceleration.
- Inertia is the tendency to resist change in motion and increases with mass.
- Newton first law connects inertia with unchanged motion in absence of unbalanced external force.
- Newton second law relates force to rate of change of momentum and gives F=ma for constant mass.
- Newton third law states interacting bodies exert equal and opposite forces on each other.
- In an isolated system, total momentum is conserved.
- A strong answer explains the mechanism or reason, then uses the correct scientific term, relation or example.