Session 2: How can we change the friction force – Step 1 Challenge and conception of the experiments

Objective 1: To experiments how to change a friction force.

Objective 2: To develop with the student an experiment in order to compare several test of friction forces, and the effect of changing parameters.

Explanation

From all the concepts we have discuss up to now, friction is the most approachable in terms of intuition, at least in terms of slowing down a movement. This session will be dedicated to experimenting how to slow down a movement generating a frictional force.

Material

Cars: Criteria to choose the cars: the car should not be too light, otherwise they are instable, but they should not be too heavy, because they should roll quite far when you push them. After trying different cars in common toy shops, we concluded that the best cars are the ones designed for babies (18 mth). Since they are not very strong and not very accurate in general those cars roll well.
Car ramp, simple slope: Either you build it yourself or you take a kid’s car ramp. The main point of a good ramp is that the passage between the floor and the ramp should be as smooth as possible. This is why most of the time the ones you can buy are thin and curved.
Sheets of paper
Straws
Adhesive paper+

Maximum duration

45 minutes

Main question to be asked

How to reduce the displacement of this car?
How to build an experiment to compare our tests?

Introduction/Starters

Look with your students at these two pictures and ask them why cyclists and speed skiers have those special equipments. Also ask them to look at the position they take and discuss it.

Main activities

A. Challenge

Give to your students in groups of 3, one ramp and one car. Ask them first to check the distance the car reach if they release it on the slope.

The challenge consists to transform the car to reduce as much as possible the displacement of the car without touching the wheel system, and without changing too much its weight

B. Structured experiments

Students have performed experiments, where they could only compare their own tests because they have all vary the parameters in several ways. Propose them for the next session to organise a contest but fixing parameters.

Discuss with them what are the parameters that have to be fixed, and which ones they will vary to study the impact of the friction force.

If they want to compare their tests they should release their cars the same way: from the same height on the ramp, and very parallel to the border
They can vary the shape of the surface and the value of the surface
They can also make volume and discuss how to determine the surface

Conclusion/Plenary

By adding area that resists to the air, the car is slowed down in its movement, and its displacement is reduced.