Looping the loop

Demonstration

A fun demonstration of the relationship between energy stored gravitationally, and energy stored kinetically; it can also be used to consider the forces involved during a complete loop through a vertical circle.

Apparatus and Materials

• Flexible curtain rail, 3m to 4m long
• Ball bearing
• Toy hot wheels kit (OPTIONAL)

Health & Safety and Technical Notes

Be very careful over working at heights when setting up this apparatus and when using it. No one should climb on stools or benches.

Read our standard health & safety guidance

Toys such as 'Hot Wheels' allow these demonstrations to be set up easily at floor level.

It is rewarding to construct a version of this apparatus, very well mounted with the top section detachable. A convenient method of mounting is to glue blocks of wood 3 cm x 3 cm x 3 cm at 30 cm intervals around the curtain rail. The blocks are screwed to the track with counter-sunk screws (so that the steel ball does not hit the screw-heads as it goes round the rail). The blocks could be drilled so that the holes fit the ends of clamps attached to retort stands with bosses (see illustration). Alternatively, 10 cm nails can be put through the holes in the blocks.

Bend the curtain rail so that the ball can 'loop the loop'. This is more effective with a 3.5 m length or even a 4 m one. The initial fall should be as steep as possible and the loop needs to be tight.

If you plan to discuss circular motion: the size of the gap should be such that while the ball is in the gap the parabolic motion it gains is approximately equal to the circular motion it would have had if the track was there.

A "Roller Coaster Physics" kit is now (October 2011) available from the supplier: Data Harvest. This allows the user to build a variety of models of roller coaster and investigate several related concepts. It includes a 62-page teachers' guide, with lesson plans and design briefs. Order number KX78880.

Procedure

1. Use a tray of sand, or a good wicket keeper, to catch the ball.
2. Use the apparatus to explore what happens when the ball is released from different positions on the track. (See teaching notes.)
3. Extension for circular motion discussion: remove the top piece and release the ball from different heights until it completes the circle.

Teaching Notes

• It is great fun to challenge students to set the steel ball off at the right height, h (assuming that no energy is dissipated, the ball bearing just goes round the loop when h = 5R/2). You might ask them why you need to store more energy gravitationally at the start than is needed to lift the ball to the height of the top of the loop.
• Ask: "What makes the ball go round a circle? What pushes or pulls the ball with a real force to make it do that?"
• Point out that there must be some inward force towards the centre of the loop

• Ask: "What provides the force at the top of the loop B?" (the track and gravity). After removing a bit of the track in step 3, the ball will still go round the loop, provided the speed is right. Too fast and it flies upwards along a projectile (parabolic) path and too slow and it falls downwards along a projectile (parabolic) path.
• Ask: "What provides the force at the sides halfway up the loop (A and C)?" (The track.) The effect of gravity pulls vertically and only slows the ball down a bit.
• Ask: "What provides the force at the bottom of the track (D)?" (The resultant between the inward push of the track and the downward force of gravity.)