GCSEC: Downhill Race

DOWNHILL RACE

Choose any two of the objects and race them down the slope. To make the race fair, one person might have to hold and release both objects, rather than trying to have two people release their object at exactly the same time.

Conduct races between other pairs of objects. See if you can discover the most important characteristic for predicting the winner.

Gallileo proved that objects free fall at the same speed or velocity, regardless of their weight (if air resistance is not a factor). Obviously, not all objects roll down a slope at the same velocity.

Objects that are not moving prefer to remain stationary due to their inertia. When objects begin free falling, gravity overcomes their straight line or linear inertia. Gravity has a uniform effect on linear inertia, because all parts of an object are moving equally in the same direction.

When objects begin rolling, gravity must overcome their rotational inertia. As demonstrated in the BALANCING STICK exhibit, rotational inertia is determined by both an object's weight or mass and the distance that mass is from the point of rotation. Rings have much more rotational inertia than solid disks because all of the ring's mass is much further from the center, its point of rotation.

Race cars use special lightweight wheel rims that have less rotational inertia.

You can recreate Galilleo's experiment quite easily. Collect several objects that weight different amounts and a short board. Working outside or over a padded surface, place the objects side by side on the board. Hold the board as high as your chest and quickly tilt it forward so that all of the objects fall from the same height and at the same time.

Gallileo claimed that all objects fall at the same rate, regardless of their different weights. A piece of paper falls much more slowly than a tennis ball. Was Gallileo wrong?

Drop a tennis ball and piece of paper at the same time and the piece of paper is much slower. Crumple the paper into a ball about the same size as the tennis ball and drop them both again. The tennis ball probably still wins, but not by as much. Crumple the paper into a tighter ball and race them again. The tennis ball still wins, but the race is even closer. Now crumple the paper into a very small ball and race them. Now the paper and the tennis ball are even. You have not changed the weight of the paper, only its shape. Air resistance is all that keeps a piece of paper from falling as fast as a tennis ball.

Would you expect two identical objects would always reach the end of this exhibit at exactly the same time? Why or why not?

Experiments such as this one often have a small amount of error built in. Scientists look for ways to minimize the built in error. Some sources of error in this exhibit include:

the ramp may not be perfectly uniform
the diameters and weights of the objects are intended to be the same but are probably not identical
the objects might not be started at exactly the same time
the time needed to roll down the slope is short.

Mechanical watches, such as pocket watches, have a round balance wheel that is essential for keeping time. In hot weather that balance wheel will expand. Fine watches have bimetallic strips attached to the balance wheel which curl towards the center in hot weather. Why?