Forces and Dynamics for Singapore O-Level Physics (6091): types of forces and free-body diagrams, friction and resultant force, Newton's three laws of motion, and the turning effect of forces (moments)

What this module covers

Forces and Dynamics is the heart of Newtonian mechanics in O-Level Physics (SEAB 6091). It explains why objects speed up, slow down, stay still or turn. The module identifies the common forces acting in everyday situations, shows how to add them into a resultant, links that resultant to acceleration through Newton's laws, and extends the idea of force to rotation through moments.

It builds directly on the kinematics of the previous module (the acceleration here is the same acceleration measured there) and underpins energy, pressure and electricity later. Each dot point below has full worked answers and practice questions.

Types of forces and free-body diagrams

Types of forces and free-body diagrams catalogues the forces you will meet, including weight, the normal contact force, friction, tension and air resistance. A free-body diagram isolates one object and draws every force on it as an arrow, with length showing magnitude and direction showing the line of action.

Drawing a clean free-body diagram is the first step in almost every mechanics problem, because it makes the forces, and therefore the resultant, visible.

Friction and the resultant force

Friction and resultant force shows how to combine forces along a line into a single resultant and explains friction as the force opposing relative motion between surfaces. When the driving force exceeds friction there is a resultant in the direction of motion and the object accelerates; when they are equal the resultant is zero and the velocity is constant.

For forces in a straight line the resultant is found by adding forces in one direction and subtracting those in the other:

Newton's laws of motion

Newton's laws of motion tie force to motion. The first law introduces inertia, the second gives the central equation $F = ma$, and the third describes equal-and-opposite force pairs acting on different bodies.

The turning effect of forces (moments)

Turning effect of forces and moments extends force to rotation. The moment of a force is

where $d$ is the perpendicular distance from the pivot to the line of action of the force. For an object in equilibrium, the principle of moments says total clockwise moments equal total anticlockwise moments, which is the key to balancing seesaws, beams and the human forearm.

How this module is examined

• Resolve to a resultant. Reduce the forces to one resultant before using $F = ma$.
• Quote the laws precisely. State each of Newton's laws in full, and never claim action-reaction pairs cancel on one object.
• Balance the moments. For equilibrium problems set clockwise moments equal to anticlockwise moments about a sensible pivot.

Check your knowledge

Recall and calculation questions across the module. Attempt them, then check the worked solutions.

1. State Newton's first law of motion. (2 marks)
2. A resultant force of $24\ \text{N}$ acts on a mass of $6.0\ \text{kg}$. Calculate the acceleration. (2 marks)
3. A child of weight $300\ \text{N}$ sits $1.2\ \text{m}$ from the pivot of a seesaw. Calculate the moment of her weight about the pivot. (2 marks)
4. Explain why action and reaction forces do not cancel each other out. (2 marks)
5. A car travels at constant velocity. State what this tells you about the resultant force on it. (1 mark)