New draft Experiences and Outcomes
Our Physical World - Forces & Motion

First Level - Through my experience of different materials which I use, I can talk about the need to conserve the Earth’s resources at home and in school and what I can do to help.  SCN 101A
Second Level - By carrying out investigations into friction I can explain how it affects movement, and can use my understanding of friction to design or improve a product.  SCN 222L
                     

Smart Driving

2. What a Drag!
How does the shape of an object affect its speed?

This investigation will explore the relationship between the shape of an object and its air resistance. Drag is the resistance any object feels while moving through a liquid or a gas. If you hold your hand out the window of a moving car, drag is the wind resistance pushing against it. Nearly 60% of the fuel a car uses to drive at a constant 55 m.p.h. (90 k.p.h.) goes into overcoming wind resistance. So reducing a car’s drag can greatly improve its fuel consumption. Pupils can then relate their findings to the shapes of cars and so their fuel efficiency.

Unaerodynamic or aerodynamic? Frugal or thirsty?

In this activity, we deliberately make a small cart un-aerodynamic by making a large baffle for it from Corriflute or stiff cardboard. The air resistance on the cart (known as aerodynamic drag) then becomes sufficiently great to lengthen, by a noticeable amount, the time taken by the cart to run down a slope. Pupils should be able to relate the cart with the baffle to vehicles with poor aerodynamics.

Materials

Wooden plank, 170 x 30 cm
Brick (or box), to support the plank
Meccano® cart with 2 axles and 4 wheels
Corriflute® or stiff cardboard sheet, 50 x 50 cm
Polystyrene wedge, about 1 cm thick, to support Corriflute
Brass weight, 200 g
Adhesive tape (or Sellotape®)
Stopwatch

A cart made from Meccano, carrying a 200 g load, was allowed to run freely from a standing start down an inclined plane. (Figure 1).

               Figure 1 - Meccano cart with weight taped on             Figure 2 - Meccano cart with large baffle.

Method

For our inclined plane we used a plank of wood 1.7 m long. The exact length doesn’t matter, but should be over 1.0 m. The slope should be at an inclination of between 3° and 10° and preferably at the gentler end of this range. For a 1.0 m length of slope, a vertical rise of 7 cm is suitable.

The pupils should make a few practice runs to learn what to do. If working in pairs, the one with the stopwatch should count down “3, 2, 1, Start”, signalling when the other lets go the cart at the top of the slope. It really is crucial that the cart makes a standing start and is not given a helping shove because that just messes up the results.

Typical results are:
Time of descent without baffle: 3.2 s
Time of descent with baffle 4.4 s
The results should be repeated because the different times will be found to have a spread of about 0.6 s. If the average is taken, the difference should be convincing.

Notes
The size of the baffle may seem huge compared with the cart. We found that sizes smaller than around 50 cm square had a negligible effect. Using a very light cart made, for example, of balsa wood might allow a smaller baffle to be used. Sharp pupils will realise that the 200 g weight is necessary for a proper scientific test. If the cart without the baffle weighs less than the cart with the Corriflute fitted, how do they know it was the baffle rather than the different weight that affected the time?

See the YouTube videos - Reducing Aerodynamic Drag & Terminal Velocity

The sounds in the sand dunes