the drawing pad just a little. What happens when the
tractor goes downhill?
Results
Your tractor runs easily downhill. If you watch care-
fully, you will see that it even goes a little faster than
when you hold the cardboard level. On a downhill
drive, the engine has to expend less energy. Can you
use this effect to get your tractor to drive on a bumpy
surface?
Experiment 17
>
You will need: bright sunlight, solar model,
bumpy ground that drops away (e.g. gravel path, pav-
ing stone with rough surface, or lawn)
Instructions
Run your solar model in bright sunlight down a
slightly sloping, bumpy surface. How does its perfor-
mance change?
Results
When it’s going downhill, your solar vehicle moves
along quite easily. The bumps in the ground no longer
present any serious obstacles. The engine power is
the same as when it drives over flat, bumpy terrain,
but with the downward slope the engine doesn’t
have to expend as much energy to drive the tractor.
Instead, it can use its power to move the tires over
little obstacles.
An Uphill Drive
From riding a bicycle, you not only know that it’s a
lot easier to go downhill, but also that you need a lot
more energy to go uphill. If it’s too steep, you even
have to get off and walk. The strength of your body
is no longer enough to conquer the incline. Is it like
that with your solar model too? We’ll use the inclined
plane to investigate that, too.
Experiment 18
>
You will need: light, solar model, large drawing
pad or other strong cardboard
Instructions
Have your tractor drive up an inclined plane. Use the
back side of a drawing pad. During the drive, slowly
lift the front end of the drawing pad. What happens
when the solar tractor drives uphill?
Results
When your tractor drives uphill, it goes slower. And
when the slope is too great, it won’t move. The power
of the engine is not enough to turn the wheels.
So the speed depends on the slope. Or, put different-
ly: with a slope, the engine’s force has to be applied
not only to moving the tractor forward, but to moving
it upward as well.
Driving with the Battery
So far, we have been running the tractor on solar
energy. Now let’s see whether its driving performance
changes when we use the battery instead of the solar
module.
Experiment 19
>
You will need: solar model, battery, large drawing
pad or other strong cardboard, flat and slightly slop-
ing bumpy ground
Instructions
In your model, switch out the solar module for the
battery. Make sure that the battery is fully charged.
Carry out experiments 14 through 18 one more time
and observe how the vehicle performs.
Results
The driving performance of your model hardly
changes. The engine runs just as well with the energy
of the solar module as it does with the energy of a
1.5-volt battery.
Your Engine’s Performance
The engine and solar module in this experiment kit
are not strong enough to make fast, powerful vehicles
— like those remote-controlled cars that run on big
rechargeable batteries. For that, you would need
much larger and more expensive solar modules.
But the photo cells are good enough to power a ve-
hicle in bright sunshine on flat, even ground or down
a bumpy slope.
Your engine is not without power, though. In the next
experiments, we will find out what it is capable of.
19
