1. Air and Water Resistance
11 November 2018 Objectives: To describe the effects of upthrust on a floating or sinking object. To describe the effect of drag on motion through a fluid.

2. Starter (5 mins)
Using a large tank of water, ask the students to predict which objects will float and which will sink. Drop the objects into the water to test the predictions. Ask the students to draw force diagrams for an object that floats and one that sinks.

3. Forces in water upthrust thrust friction weight What is thrust?
The force that pushes an object forward.
What is upthrust?
The force that pushes an object upwards in a liquid or a gas, also called buoyancy.
upthrust
thrust
friction
weight

4. Ferry forces upthrust or buoyancy air resistance thrust friction
Label the forces acting on the ferry as it sails to France.
upthrust or buoyancy
air resistance
thrust
friction
weight

5. Changing forces
The ferry arrives at Calais and is loaded with travellers.
What happens to the weight of the ship?
What happens to the upthrust on the ship?
Which is the largest force - weight or upthrust?
increases
increases
both equal

6. More about friction
Friction always tries to slow moving objects down. We say it opposes motion.
Friction is created whenever two touching objects or surfaces move past each other.
Friction also occurs when things move through air. This is called air resistance or drag.
There is a maximum value for the frictional force which depends on:
the force pushing the two surfaces together;
the state of the surfaces in contact.

7. Explaining air resistance
Air resistance is a type of friction that occurs whenever an object moves through the air, and is caused by the frictional forces acting between the air and the object.
400 N
If the area of contact between the air and object is reduced, the object is said to be streamlined.
300 N
Which of these cars is more streamlined?

8. Practical
Students can then measure the upthrust acting on an object by measuring the apparent reduction in weight when it is submerged.
The effect can be explained by the idea that the particles in the liquid are pushing back on the object.

9. Practical
The students can then carry out an experiment to measure the effect of water or air resistance:
use a catapult to fire small objects with parachutes of different surface areas and measure the distance they travel.
To test the effect of water resistance, they can drop different shaped objects made from modelling clay through a cylinder of water and time how long it takes the objects to reach the bottom.

10. Plenary
answer the Structured Question ‘Forces’ question 2d.