Balancing 2
Meg and Mo still need to hang their marbles so that they balance, but this time the constraints are different. Use the interactivity to experiment and find out what they need to do.
Problem
Meg and Mo have been distracted from their game of Marbles by the balance on which they hang their pouches of marbles between games.
They discover that they can make their pouches balance with Meg's pouch $60$cm from the centre, and with $10$ marbles in Mo's pouch.
Use the interactivity to explore this further.
Full Screen VersionFill in the table below showing how many marbles Mo needs in her pouch to balance Meg's pouch when it is in various different places away from the centre.
| Meg | Mo |
| 60cm | 10 marbles |
| 30cm | |
| 90cm |
Without using the interactivitiy, can you now work out how many marbles Mo would need in her pouch to balance with Meg's?
| Meg | Mo |
| 120cm | |
| 12cm | |
| 48cm | |
| 24cm | |
| 75cm |
Try to explain how you worked it out.
They take a break from playing with the balance to play another quick game of Marbles, after which they both have different numbers of marbles and someone has sneakily altered the balance.
They find that when they hang their pouches on the new balance, the two pouches balance when Meg has $12$ marbles, and Mo has her pouch $21$cm from the centre.
Use the interactivity to explore this further.
This time, Mo is going to move her pouch as Meg adds and removes marbles from her own pouch. Fill in the table below to show where Mo needs to move her pouch.
| Meg | Mo |
| 12 marbles | 21cm |
| 24 marbles | |
| 16 marbles | |
| 8 marbles |
Without using the interactivitiy, can you now work out where Mo would need to place her pouch to balance with Meg's?
| Meg | Mo |
| 30 marbles | |
| 15 marbles | |
| 2 marbles | |
| x marbles |
Try to explain how you worked it out.
Getting Started
Use the interactivity to help you complete the tables.
Look at your results in the table.
What happens if you double something on the left hand side?
What if you treble it, or halve it?
Student Solutions
Congratulations to Kyla and Ashley from the North London Collegiate School for Girls who explained how to work out the answers:
For every 60cm that Meg's bag is moved away from the middle, Mo's bag has to have 10 marbles. So if you put Meg's bag 30cm away from the middle the answer for Mo's bag has to be half the amount of marbles that she had to start with, because 30cm is half of 60cm (the distance you had to start with).
You do the same thing if you are multiplying. It is all to do with ratio.
Alice and Alex from The Mount School in York sent in a really lovely solution to the question, in which they explain their calculations.
| Meg | Mo |
| 60cm | 10 marbles |
| 30cm | 5 marbles |
| 90cm | 15 marbles |
| Meg | Mo |
| 120cm | 20 marbles |
| 12cm | 2 marbles |
| 48cm | 8 marbles |
| 24cm | 4 marbles |
| 75cm | 12.5 marbles |
For every 30cm there are 5 marbles.
30cm * 4 = 120cm
5 marbles * 4 = 20 marbles
120cm / 10 = 12cm
20 marbles / 10 = 2 marbles
12cm * 4 = 48cm
2 marbles * 4 = 8 marbles
12cm * 2 = 24cm
2 marbles * 2 = 4 marbles
60cm...10 marbles; 15cm ...2.5 marbles
75cm...10 + 2.5 = 12.5 marbles
| Meg | Mo |
12 marbles | 21cm |
| 24 marbles | 42cm |
| 16 marbles | 28cm |
| 8 marbles | 14cm |
|
24 marbles... 42cm
so 30 marbles... 42cm +10.5cm = 52.5cm
15 marbles... 52.5 / 2 = 26.25cm
8 marbles ...14cm
Teachers' Resources
This problem offers an interactive environment that introduces students to moments of force by asking them to consider how a balance is affected by altering the weights and the distances from the centre.
This problem is a follow up to Balancing 1.
Balancing 3 develops the ideas further.