Forces and balanced motion

Learning objectives

• Combine forces along one line.
• Recognise balanced forces.
• Distinguish mass from weight.

The big idea

A force is an interaction between objects. Add forces with directions to find the resultant. Balanced forces give no acceleration, so an object may remain at rest or keep moving steadily.

Mass measures inertia and is recorded in kilograms. Weight is the gravitational force on a mass and is measured in newtons. Friction can oppose sliding without being the only force present.

1. Interactions, not possessions

A push, gravity and friction are interactions between objects. Start a force diagram by choosing one object. A book on a table has downward weight and upward contact force; the table is not supplying an upward force merely because objects must always have two forces.

2. Combine directions carefully

A trolley pushed right by 8 N and left by 3 N has a 5 N resultant right. That resultant changes velocity. If the two pushes become equal, the trolley does not necessarily stop: it can continue at a steady velocity when the other forces also balance.

3. Mass and weight behave differently

Mass describes inertia and is measured in kilograms. Weight is a gravitational force, W = mg, in newtons. A 2 kg object has about 20 N weight where g is 10 N/kg. Moving it to a place with weaker gravity changes its weight but not its mass.

A method you can reuse

1. Choose one object and draw its interactions.
2. Assign directions and label force magnitudes.
3. Combine opposite forces by subtraction.
4. Use the resultant to predict a velocity change.

Equations and relationships

Investigate and explain

Use a simulation to keep mass fixed while changing the resultant force; compare the change in speed each second.

Compare equal and unequal pushes, then investigate how friction changes the resultant. Check whether the simulation is modelling static or sliding friction.

Open the interactive investigation

Check your understanding