Air and Water Resistance 26 October 2021 Objectives

Air and Water Resistance 26 October 2021 Objectives: To describe the effects of upthrust on a floating or sinking object. To describe the effect of drag on motion through a fluid.

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.

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

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

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 both equal

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: l the force pushing the two surfaces together; l the state of the surfaces in contact.

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. If the area of contact between the air and object is reduced, the object is said to be streamlined. Which of these cars is more streamlined? 400 N 300 N

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.

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.

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