- Define and explain Friction and Drag in your own words
- Use key terms such as air resistance accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Friction and Drag: exploring how surfaces and air resistance oppose motion — and when that helps.
Exploring how surfaces and air resistance oppose motion — and when that helps.
Key ideas
Forces are pushes, pulls and twists
Forces act between objects and are measured in newtons. Contact forces need touching — friction, tension and the normal push of a surface. Non-contact forces act at a distance — gravity, magnetism and static electricity. Drawing them as arrows shows both size and direction at a glance.
Balanced forces mean no change in motion
When forces are equal and opposite they cancel, giving a resultant force of zero. A book resting on a table and a car cruising at steady speed both have balanced forces. Unbalanced forces leave a non-zero resultant, which accelerates the object in that direction — this is the heart of Newton's second law.
Key term — air resistance: The drag force of air pushing against a moving object, which grows as the speed increases.
A sledge is pulled forwards with 80 N while friction pulls backwards with 30 N. What is the resultant force?
Draw the forces as arrows: 80 N forwards, 30 N backwards. Forces in opposite directions subtract: 80 − 30. 80 − 30 = 50. The resultant acts in the direction of the bigger force.
Answer: 50 N forwards — so the sledge accelerates forwards.
- Saying balanced forces mean the object is stationary Balanced forces also allow steady-speed motion — a car at a constant speed has balanced forces.
- Thinking friction is always a nuisance Friction lets tyres grip roads and brakes stop cars — engineers only reduce it where it is unhelpful.
Practice
The forces are balanced — air resistance upwards equals weight downwards, so the resultant force is zero.
W = m × g = 5 × 10 = 50 N.
0 N — the forces are balanced, so the rope does not accelerate.
Non-contact — a magnet pulls iron without touching it.
Quick check
Which of these best defines "air resistance"?
A car needs a bigger engine force to keep a constant speed uphill than on flat ground. Why?
Would your mass change on the Moon? Would your weight?
- Friction and Drag: exploring how surfaces and air resistance oppose motion — and when that helps.
- Forces are pushes, pulls and twists: Forces act between objects and are measured in newtons.
- newton: The unit of force, symbol N.
- Watch out for: saying balanced forces mean the object is stationary