Dynamics 2: Newton's Laws of Motion, Complete O Level Physics Cheatsheet
O Level Physics, Chapter 4 · Read time: ~9 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
Types of forces and free body diagrams
Contact forces need the objects to touch: friction, air resistance, tension and the normal (reaction) force.
Non-contact forces act at a distance: gravitational, electrostatic and magnetic forces.
A free body diagram shows one object on its own with every force acting on it, drawn as labelled arrows from the object. Forces the object exerts on other things are left out.
Trap: Only draw forces acting on the object you're analysing. A common mistake is adding the force the object exerts on the floor, which belongs on the floor's diagram, not this one.
Newton's laws in action
Balanced forces (resultant = 0): the object stays at rest, or keeps moving at constant velocity in a straight line. A car cruising at steady speed has its driving force exactly balanced by friction and air resistance.
Unbalanced forces: the resultant force makes the object accelerate, decelerate or change direction. F = ma, so a bigger resultant gives a bigger acceleration and a bigger mass gives a smaller one.
Action-reaction pairs: when object A pushes on object B, B pushes back on A with an equal and opposite force. The two forces are the same type and act on different objects.
K323 doesn't ask you to state Newton's laws word for word, but it does expect you to apply them.
Trick: A book resting on a table has weight and a normal force acting on it. They balance, but they are **not** an action-reaction pair, because they act on the same object. The weight's partner is the book pulling the Earth upwards.
Trap: No resultant force does not mean no motion. It means no change in motion. An object moving at constant velocity has zero resultant force.
Resistive forces and terminal velocity
Friction and air resistance oppose motion. Air resistance increases with speed.
A skydiver jumps: at first weight is much larger than air resistance, so they accelerate. As speed rises, air resistance grows, the resultant force shrinks and acceleration decreases.
Eventually air resistance equals weight, the resultant force is zero and the skydiver falls at a constant terminal velocity.
Three forces on a point: the scale drawing method
If an object is at rest under three forces, the resultant is zero, so the three force arrows drawn tip to tail form a closed triangle.
To find an unknown force: choose a scale, draw the known forces tip to tail at the correct angles, then the line that closes the triangle gives the size and direction of the third force. Measure it and convert back using your scale.
Worked Example
A 1,200 kg car accelerates along a straight, level road. The engine provides a driving force of 3,000 N and the total resistive force on the car is 600 N.
(a) Calculate the resultant force on the car. [1]
Resultant force = 3,000 − 600 = 2,400 N, forwards
(b) Calculate the acceleration of the car. [2]
a = F / m
= 2,400 ÷ 1,200
= 2.0 m/s²
(c) As the car speeds up, the resistive force increases. Describe and explain what happens to the car's motion if the driving force stays at 3,000 N. [3]
As speed increases, the resistive force increases, so the resultant force decreases.
The acceleration therefore decreases, by F = ma.
When the resistive force reaches 3,000 N, the resultant force is zero and the car moves at a constant maximum speed.
Why this question is a good test of the topic: part (a) checks students use the resultant force rather than the driving force, and part (c) tests the same balanced-force reasoning as terminal velocity, applied to a new situation.
Frequently Asked Questions
What is the difference between contact and non-contact forces?
Contact forces need the objects to touch, like friction and tension. Non-contact forces act at a distance, like gravitational, electrostatic and magnetic forces.
What happens when the resultant force on an object is zero?
It stays at rest or keeps moving at constant velocity. It does not slow down.
How do I identify an action-reaction pair?
The two forces are equal, opposite, the same type, and act on two different objects. If both forces act on the same object, they are not a pair.
Why does a skydiver reach terminal velocity?
Air resistance increases with speed until it equals the skydiver's weight. The resultant force is then zero, so the speed stays constant.
Is momentum in the O Level Physics syllabus?
No. Momentum is not part of the K323 (SEC G3 Physics) syllabus. It is covered at A Level instead.
Free body diagrams still feeling abstract?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author.