# Primary Science

## PSLE Science: Elastic Spring Force Graph

As parents of primary school kids or students preparing for PSLE, it’s important to understand the concept of elastic spring force. Whether it’s the spring in a toy or the physics behind a jack-in-the-box, the force exerted by compressed or stretched springs can have a significant impact on the behavior of objects.

By understanding the principles of elastic spring force, you can help your child better understand the science behind the world around them and prepare them for success in their academic pursuits.

In this article, we will dive deeper into the concept of elastic spring force and explore the knowledge learnt on actual PSLE questions.

## What is elastic spring force?

Elastic spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it.  The elastic spring force restores/ reverts the object to its original position. The type of force exerted by the spring, whether push or pull, depends on how it is attached.

A jack in the box toy has a compressed spring attached to the puppet which exerts an elastic spring force on the puppet, and the puppet exerts a push force against the lid of the box.

As the handle is turned more, the spring is compressed more and thus there is more elastic spring force exerted upon the puppet, until the push force overcomes/ is greater than the frictional force between the lid and the puppet, and the puppet jumps out.

Total length = Original length + Extension of spring

Be careful when reading questions and graphs! Take note of the key words in the question and the x / y axis, is it “extension”, or “ “total”?

There are two factors that affect elastic spring force:

1. How much the elastic object is stretched or compressed.

The more stretched/ compressed elastic object is, the more elastic spring force acting on it.

2. The stiffness of an elastic object.

The stiffer the elastic object, the more elastic spring force it exerts as compared to a less elastic object when it is stretched/ compressed to the same length.

Let’s practice!

### PSLE 2022 Section A Q27

Three springs, X, Y and Z, have the same length. When the springs are hung using three identical blocks, the results are as shown.

Which graph shows the relationship between the elastic spring force and the extension of the springs X, Y and Z?

The three springs X, Y and Z are of the same total length (4cm) when hung. However, the total mass attached to each spring is different.

Spring X is holding three identical blocks, Spring Y is holding two identical blocks, and Spring Z is holding one block. This shows that the stiffness of the springs are different. Spring X is the stiffest spring, followed by Spring Y, then lastly spring Z is the least stiff spring.

If they were of the same stiffness, the springs should look like this:

Spring X should have been the longest spring, due to having the most mass stretching it.

Thus from our original question’s diagram, we can conclude that since Spring X is the stiffest spring, X exerts the most elastic spring force.

The answer is graph 3, as at the same extension, the elastic spring force of the stiffest spring X should be the greatest, followed by Y, then Z.

### PSLE 2022 Section B Q35

Mingxin conducted an experiment on a string made of material R using the set-up below.

He added a mass of 100 g and measured the length L of the string. He repeated this until the mass reached 1000 g. Next, he repeated the experiment with another string of the same thickness but made of material S to make a comparison. His results are as shown.

#### (a) State the mass added when the length L of string S is 63 cm. [1]

Ans: 400 grams.

Tip: Annotate on the graph using a ruler and pencil!

#### (b) State the increase in length L of string R when the mass is increased from 0 to 800 g. [1]

Ans: 2 cm

Note: The question is asking for “increase in length L”, meaning the extension only!

62 cm – 60 cm = 2cm

#### (c) Explain why the experiment is not a fair comparison. [1]

Ans: The original length L of string R and string S are different. Thus we cannot compare and conclude that any changes in the length of string R and S are due to the different materials only, so the results are not reliable.

Note: Directly state what is different about the two strings other than the changed variable.

Identify the changed variable- the different materials S and R.

Identify the measured variable- length L of the string

Format: We cannot compare and conclude that any changes in the (Measured variable) is only due to the (Changed variable), so the results are not reliable.

Do not state “to ensure a fair test”!

#### (d) Suggest why there were no results for the string made of materiaI S beyond 800 g. [1]

Ans: String S broke as it passed its maximum elasticity.

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