1.3 Drag forces and friction
Summary: Move your finger across the desk. Does the surface feel smooth or rough? Even really smooth surfaces apply a force, allowing things to move across them. Tags: science oxford-science student-book-7 physics Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
Think back
- What is the force that affects objects moving through air?
- What is the force that affects objects moving through water?
- What does friction do to moving objects?
Key idea Friction grips objects and makes it more difficult to move them. It also slows down moving objects. We can reduce friction by lubrication.
Key idea Water resistance and air resistance are drag forces. Drag forces slow down moving objects. We can reduce drag forces by making moving objects more streamlined.
Key words friction, lubrication, water resistance, air resistance, Drag Force, streamlined
Move your finger across the desk. Does the surface feel smooth or rough? Even really smooth surfaces apply a force, allowing things to move across them.
What is friction?
Surfaces like a metal slide in a playground look and feel really smooth. However, if you take a close look, you will see that they are rough. You can see this in Figure 1.
Figure 1Even an object that seems smooth can be uneven close up. This can cause friction.
When a book is resting on the table, you can push on it, but it may not move. Friction holds on to objects and makes it more difficult to move them. As you increase the force by pushing harder, the book will start to move. If you remove the force, the book slows down and stops. The force of friction also slows down moving objects.
A Compare the friction on a slide with the friction on a road.
Is friction useful?
Friction can be a good thing. You need friction to walk. This is because the friction between your foot and the road produces the force needed to move you forward. The brakes on your bike and in a car work because of friction.
How can you reduce friction?
One way to reduce friction is by using oil or grease. This is called lubrication. When you oil the chain of your bike, the surfaces move past each other more easily. Snowboarders wax their boards to reduce the friction between their board and the snow.
B Suggest why the hinges of a door need to be lubricated.
What are drag forces?
Figure 2 shows dolphins swimming through the water and a surfer paddling through the water. The dolphins and the surfer will both experience water resistance.
Figure 2When you move through water, you experience water resistance.
A snowboarder jumping through the air will experience air resistance. Water resistance and air resistance are drag forces. If you think about the particles in the air and in the water, you can explain drag forces. Figure 3 shows how particles in air and water cause drag.
Figure 3A moving object is in contact with air or water particles. Full text alternative to the content
As an object moves through air or water, the particles are pushed out of the way. This produces a drag force, which slows it down.
C Name the drag force acting on an aeroplane in flight.
Are drag forces useful?
Parachutes are used to slow down drag-racing cars and skydivers. The contact between the parachute and the air produces a drag force.
How can you reduce drag forces?
Speed cyclists will tuck their arms in close to their body as they cycle. They will even make sure that their thumbs are as close to the handlebars as possible. This makes them more streamlined, which reduces the force of air resistance.
Working scientifically A company wants to compare different materials for making parachutes. Name three ways that they could make it a fair test.
Stretch zone
- Copy and complete the sentences. The force of air resistance/friction acts between two touching solid surfaces that are moving across each other. When the surfaces are rough/smooth, they will hold on to each other. There are two drag forces: air resistance/Gravity and magnetism/water resistance. Drag forces are produced when objects push bonds/particles out of the way.
- Explain why you do not lubricate the brakes on a bicycle.
Stretch zone
- Explain why diving birds pull their wings in when they are about to enter the water.