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Rolling Down Hills

Public summary: 

Find out about rotational mechanics by racing jam jars of differing weights downhill. Plus.

Racing jam jars down a gentle slope, to see how filling them affects the speed.
Useful information
Kit List: 

In the box there should be:
Empty jam jar
Jar half full of water
Jar full of water
Full jar of jam

You will also need:
Gentle slope - Large wooden board. There is one dedicated to this purpose, but if that cannot be found, the archbridge base should work instead if not in use.
A couple of things of obvious different weights to drop that won't break. Two rolls of gaffa of different sizes works nicely.
Foam block/thick pad/sleeping mat - something soft to place at the bottom of the ramp to stop the jars escaping. The foam covering lungs works. Alternatively use an opened t-shirt box to catch the jars. Hitting the t-shirts gives them a soft landing. Some padding around the sides of the ramp is needed in case the jars roll off the ramp.
(Optional) Some sort of release mechanism to release all the jars at the same time.

Packing Away: 

Store jars in stores.

Frequency of use: 

Race the jam jars down a slope to see which one is faster.

In a Nutshell

Show people that rotational motion is a bit weird - you have to account for the distribution of mass not just the overall properties.


Not much to do. Place the ramp on the floor (if using a board then use a couple of jars/ spare box or find something else to hold the back end up). If using an opened t-shirt box to catch the jars, place at the bottom of the ramp, or if using foam, place about 10cm past the end of the ramp (taping it down helps if its small).

The Experiment

Gain attention by telling people they'll be racing jars down a hill. Explain what's in the jars.

First, bring out the drop test objects and ask kids which one is heavier (they can feel this for themselves) and ask which one will hit the ground first if they're dropped from the same height at the same time. Both should hit at the same time (demonstrate a couple of times, from different heights if necessary). Get them to tell you what force pulls things down. The heavier something is, the stronger the gravity. BUT, the heavier something is, the harder it is to get it to move (it's not as easy to pick up something heavy). These effects cancel each other out so all objects fall under gravity at the same rate.

Now ask them to think about what will happen on the ramp - if gravity doesn't care how 'heavy' things are, should the weight of the jars make any difference? Will they all reach the bottom at the same time? Feel free to prompt some more (why do you think X, what might cause Y) if the group is old enough to engage.

Let the kids pick up the jar of jam and the empty jar to get a feel for the weights, and then race! (if the kids are eager and you can get them to release when you say, then they can hold the jars and let go when you tell them to. Alternatively, a pole or stick can be used as a release mechanism). The jar of jam should beat the empty jar.

It looks like the heavier jar wins but this makes no sense since we've already seen gravity doesn't care about weight. What is the difference between falling and rolling? When falling (or sliding on the slope), the jar stays one way round (in one orientation) whilst moving. When rolling, as well as moving down the slope, the jar is also spinning. This is what changes things. Gravity needs to put in extra effort to get the jars spinning as well as going down.

Get the kids talking about it a bit and then ask them if they've been on a roundabout/ merry-go-round/ turntable thing at a park. Does it just start spinning on its own without any pushing? What about pushing the thing if someone else is on it - is it easier to make it spin fast if the other person is in the middle or at the edge? (You can also try to connect this to Spinny Chair experiment if it is also out.) Exactly the same thing is happening here with the jars. The empty jar has almost all of its mass concentrated in a very thin layer of glass a long way from the axis of rotation whereas the one full of jam has a lot of mass quite close to the middle. It's therefore much 'easier' to make the full jar spin than the empty jar (could be worth mentioning, to reinforce the idea that weight is irrelevant, that if there were no friction the jars would all slide (not roll!) down the hill in the same amount of time).

Next try the jar of jam vs the jar of water. The jar of water wins, why? What’s the difference between the water and the jam? Answer: Jam is sticky, but water isn’t. When the jar of the water rolls down, the water doesn’t have to spin, only the jar, whereas in the jar of jam both the jar and jam are spinning. This means the jar of water can go faster.

The interesting case of the half-full jar and the jam jar can now be the ultimate race. The jam jar has the extra weight, but the half-full water jar doesn’t have to spin as much, so you can get the kids to bet on which will win. This one is actually quite close, so you’ll have to see which wins on the day!

In conclusion for linear motion the distribution of mass is irrelevant, only the total, whereas for rotational motion it's not just the mass that matters, its position is important as well.

The wikipedia page on 'moment of inertia' has more information and a nice gif animation of (front to back) a solid cylinder, cylindrical shell, ball and spherical shell racing down a slope.

Risk Assessment
Date risk assesment last checked: 
Wed, 02/01/2019
Risk assesment checked by: 
Grace Exley
Date risk assesment double checked: 
Wed, 02/01/2019
Risk assesment double-checked by: 
Risk Assessment: 

Rolling cake tins, and jam jars down slopes with different weight distributions. Describing what is happening with a bar with weights at each end:

Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Rolling objects The objects rolling into people, and tripping them up. 4 3 12 Make sure the base of the slope is not heading towards a gangway.
Call first aider in case of injury.
2 3 6
Jam jars The objects bouncing off things and breaking, then possibly making sharp objects as a result which can cause cuts. 3 4 12 Put something soft at the base and around the sides of the slope to catch the objects. Consider wrapping the jars in sellotape.
In case of breakage, close experiment until broken glass and spilled water is cleared up.
Call a first aider in the event of an injury.
2 3 6
Liquid in jars Slip hazard from spilled water if jars break. 3 3 9 Keep children away from any breakages and dispose of safely and quickly. Put something soft at the base and around the sides of the slope to catch the objects.
Call first aider in case of injury.
2 3 6
Objects on surfaces Jars falling off the bench and hitting people. 3 3 9 Do the experiment on the ground.
Call first aider in case of injury.
2 3 6
Experiment photos: