Forces and turning effects
Forces, free-body diagrams, Newton's laws, friction, terminal velocity, moments, centre of gravity and stability for G3 Pure and O-Level Physics.
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Before you begin
Forces explain why things start, stop, speed up, slow down, turn and topple. The lessons start with what a force is and how to draw every force on one object, then use Newton’s laws to link the resultant force to the motion, including friction and falling through air. The second half turns to turning effects: moments, the principle of moments, centre of gravity and stability. The last lessons find unknown forces with a scale drawing.
Be comfortable with: scalars and vectors and kinematics, especially acceleration and velocity–time graphs.
Learning goals
- Distinguish contact forces from non-contact forces
- State that mass measures the amount of matter in a body
- Describe a gravitational field as a region where a mass experiences gravitational force
- Define gravitational field strength as gravitational force per unit mass
- Apply weight = mass × gravitational field strength
- Distinguish mass from weight
- Describe the effect of balanced and unbalanced forces on a body
- Describe ways a force may change motion
- Identify action–reaction pairs on interacting bodies
- Draw free-body diagrams for force systems in at most two dimensions
- Solve three-force static equilibrium graphically
- Apply resultant force = mass × acceleration
- Relate mass to resistance to change in motion
- Explain the effects of friction on motion
- Describe falling with and without air resistance, including terminal velocity
- Describe a moment as a force's turning effect in everyday examples
- Apply moment = force × perpendicular distance from the pivot
- Apply the principle of moments to new situations and problems
- Treat the weight of a body as acting at a single point, its centre of gravity
- Explain qualitatively how centre-of-gravity position affects stability
Syllabus statements covered
- identify and distinguish between contact forces (e.g. friction, air resistance, tension and normal force) and non-contact forces (e.g. gravitational, electrostatic and magnetic forces)
- state that mass is a measure of the amount of matter in a body
- state that a gravitational field is a region in which a mass experiences a force due to gravitational attraction
- define gravitational field strength, g, as gravitational force per unit mass placed at that point
- recall and apply the relationship weight = mass × gravitational field strength to new situations or to solve related problems
- distinguish between mass and weight
- apply Newton's Laws to: (i) describe the effect of balanced and unbalanced forces on a body (ii) describe the ways in which a force may change the motion of a body (iii) identify action-reaction pairs acting on two interacting bodies (stating of Newton's Laws is not required)
- identify forces acting on a body and draw free body diagram(s) representing the forces acting on the body (for cases involving forces acting in at most two dimensions)
- solve problems for a static point mass under the action of three forces for two-dimensional cases by a graphical method
- recall and apply the relationship resultant force = mass × acceleration to new situations or to solve related problems
- show an understanding that mass is the property that resists change in motion of the body (inertia)
- explain the effects of friction on the motion of a body
- describe the motion of bodies with constant mass falling in uniform gravitational field with or without air resistance, including reference to terminal velocity
- describe the moment of a force about a pivot in terms of its turning effect and relate this to everyday examples
- recall and apply the relationship moment of a force (or torque) = force × perpendicular distance from the pivot to new situations or to solve related problems
- state the principle of moments for a body in equilibrium
- apply the principle of moments to new situations or to solve related problems
- show an understanding that the weight of a body may be taken as acting at a single point known as its centre of gravity
- describe qualitatively the effect of the position of the centre of gravity on the stability of objects
Lessons
Work through them in order.
Dynamics (Newton's laws)
- What is a force?Describe what a force can do and classify common forces as contact or non-contact.
- Inertia and massExplain inertia and why a larger mass is harder to start, stop or turn.
- Gravitational field strength and weightDefine gravitational field strength, calculate weight with W = mg and tell mass from weight.
- Free-body diagramsDraw a free-body diagram that shows only the external forces acting on one object.
- Balanced forces and Newton's first lawFind the resultant force on one body and use Newton's first law to explain rest and constant velocity.
- Newton's third law: action–reaction pairsIdentify Newton's third-law pairs as equal and opposite forces acting on two different bodies.
- Unbalanced forces and Newton's second lawUse the resultant force in F = ma to calculate acceleration, including slowing down.
- Friction and how it affects motionExplain how friction changes the resultant force, and when friction is useful or wasteful.
- Air resistance and terminal velocityExplain how air resistance grows with speed until a falling object reaches terminal velocity.
Turning effects (moments)
- Moment of a forceCalculate the moment of a force from the force and its perpendicular distance from the pivot.
- Equilibrium and the principle of momentsApply the principle of moments and zero resultant force to objects in equilibrium.
- Centre of gravityLocate the centre of gravity of an object, including a plane lamina found with a plumb line, and use it to take moments.
- Stability and states of equilibriumTell stable, unstable and neutral equilibrium apart, and explain how a low, wide base prevents toppling.
Graphical force skills
- How to add forcesAdd forces along a line using signs, and add forces at an angle with a scale drawing.
- Three forces in equilibrium: the triangle of forcesSolve three-force equilibrium problems by drawing a closed triangle of forces to scale.
- Constructing force triangles with unknown tensionsUse one known weight and two string directions to locate the missing corner, then measure both tensions.
Practise and check
Or choose
Topic reference
Quick reference
| Quantity or rule | Symbol | Equation or statement | Unit |
|---|---|---|---|
| weight | W | W = mg | newton (N) |
| gravitational field strength | g | g = F/m; about 10 N kg⁻¹ near the Earth | N kg⁻¹ |
| resultant force | F | vector sum of all the forces on one object | newton (N) |
| Newton’s second law | Fᵣₑₛᵤₗₜₐₙₜ = ma | newton (N) | |
| moment of a force | M | M = Fd, with d the perpendicular distance from pivot to line of action | newton metre (N m) |
| equilibrium | resultant force = 0 and resultant moment = 0 | ||
| principle of moments | sum of clockwise moments = sum of anticlockwise moments, about the same point | newton metre (N m) |
Core knowledge to remember
- A force is a push or pull of one object on another. It is a vector, measured in newtons. Contact forces need touching; gravitational, electrostatic and magnetic forces act at a distance.
- Mass is the amount of matter in a body and a measure of its inertia; it is the same everywhere. Weight is the gravitational force on the body and changes with g.
- If the resultant force on an object is zero, it stays at rest or keeps moving at constant velocity. If not, it accelerates in the direction of the resultant force, with Fᵣₑₛᵤₗₜₐₙₜ = ma.
- When A pushes or pulls on B, B pushes or pulls on A with an equal and opposite force of the same type. The two forces act on different objects, so they never cancel.
- Friction opposes sliding between surfaces. It changes the resultant force, and it heats the surfaces.
- Air resistance on a falling object grows with speed. When it equals the weight, the object falls at its terminal velocity.
- An object is in equilibrium only if the resultant force and the resultant moment are both zero.
- The weight of an object acts at its centre of gravity. The object topples when the line of action of its weight falls outside its base, so a low centre of gravity and a wide base make it more stable.
- Three forces in equilibrium, drawn head-to-tail to scale, form a closed triangle.
Common mistakes
- Using the applied force instead of the resultant force in F = ma.
- Saying a moving object needs a forward resultant force to keep moving. A force is needed only to change its velocity.
- Calling an object’s weight and the normal contact force on it an action–reaction pair. Both act on the same object.
- Mixing up mass in kilograms and weight in newtons.
- Saying terminal velocity means zero velocity. It means zero acceleration.
- Using the distance along a bar instead of the perpendicular distance from the pivot to the line of action.
- Saying an object topples when its centre of gravity is outside the object. It topples when the line of action of its weight falls outside its base.