Students can build in pairs or in a larger group, up to four students is best.
It is helpful to have more than one building a run, so that one can hold the track in position while the other tapes.
The Play-Debref-Replay method of science teaching works great for this activity (see the resource).
The challenge: build a roller coaster for marbles, so that they run along the entire track and drop in the cup. Include at least one of each of these elements: a loop, a hill and a curve.
Additional challenge for older students: keep within a defined area.
Students freely experiment to build tracks, encouraged to test it out continually if they are not. They will quickly learn what works and what does not.
Start as high as you can.
Loops that are smaller will slow the marble down less.
Banking a corner (tipping up one side) might help keep a marble on track.
After some time gather for each group to show their run, show cool features of their track, and explain how they overcame challenges.
Inject some basic concepts and terms as appropriate:
Potential energy is the energy the marble has from being high (most students start their track high, giving the marble a lot of potential energy to start it of).
Kinetic energy is energy the marble has as it moves. It needs a lot of kinetic energy (i.e. must be going fast) to make it through a loop, or around a corner.
Energy is conserved as the marble moves, but is transformed between different kinds of energy. Starting high and stationary it has all gravitational energy, which is converted to kinetic energy as it rolls downwards. As a marble moves along a track its energy changes from potential to kinetic and back. At the top of a hill it has a lot of potential energy, which changes to kinetic energy as it speeds up down the hill (and the PE drops). As it moves up a hill, it slows so loses KE, but gains PE as it gains height. Energy is conserved throughout. The marble gradually loses all of its energy to friction with the track. Friction generates thermal (heat) energy and sound, so when the marble comes to a stop all its initial gravitational energy has been converted to thermal and sound energy.
Students could name the sections of their track and draw a graph of the PE and KE going up and down along the length.
In a loop, if the marble is going fast enough, it will stay on the track. The marble wants to keep going in a straight line, so pushes against the track. The track keeps curving over, pushing back against it. A marble in a tighter loop will acceleration more than a marble in a wider loop. If the marble is not going fast enough, it will not push against the track enough and fall off.
Allow students to work on their track some more, using ideas from other groups, and possibly inspiration from real roller coaster elements e.g. corkscrew, inclined loop (https://en.wikipedia.org/wiki/Roller_coaster_elements)
As they work, talk to them in terms of energy.
Background information on real roller coasters:
A real roller coaster has no engine and once elevated to its start point is entirely driven by the force of gravity. At the start of the ride, which must be the highest point, the roller coaster has a certain amount of potential energy, and will not gain any more energy, but transfer PE to KE and back again, while losing both to friction.
Weightlessness and heaviness that you feel in a roller coaster car is from the combination of gravity and acceleration when the roller coaster changes direction. More than 1g at the bottom of a hill (gravity+acceleration down). Less than zero gs at the top of a hill (from deceleration being greater than the force of gravity). Weightlessness can also be explained in this way: when the roller coaster car starts descent from the top of a hill, the riders and seat are both falling due to gravity, and without the force of seat pressing on them, the riders feel like they are weightless.