IOP Institute of Physics

# Experiments

Forces and springs: Stretching and bouncing

1. Show them the PEANUTS label on the snake tin.

Aims/Facts

• Establish what the children know about springs.
 Activity Discussion Explain that you have had a problem with the lid and not been able to open it for a while. Explain that one problem with peanuts is that they get worms in them. Then open the tin, pointing it slightly forwards so that the snake or worm springs forwards.See Apparatus List Ask the children what has caused the worm to spring out. They will quickly explain that there is a spring inside.Continue the discussion to find out what they know. Show Slide 2 which is animated. Repeating the demonstration often helps. Include the words pushes and pulls in the discussion.

2. Hand two lengths of ‘fashion string’ to each class group and ask each group to pull one of the strings and then compare it with the unstretched one.

Aims/Facts

• Plastic and elastic materials introduced.
 Activity Discussion Then hand out a similar length of fishing pole elastic and ask them to gently stretch this.See Apparatus List Discuss the changes in length of the fashion string which stays stretched and the elastic which springs back when released.See safety notesExplain that they show plastic and elastic behaviour.Show the animated Slides 3 and 4 to help the discussion.

3. Slide 5

Aims/Facts

• The greater the force the greater the extension.
 Activity Discussion Show a long length of fishing pole elastic and pass the loop at one end to the nearest child. Keep hold of the loop at the other end. Explain that they will have to be very sensible and not let go of the elastic. Also, they must not pull very hard. Then ask the child with the elastic to pass the end to the next child whilst you keep hold of the elastic. This should be repeated so the elastic is threaded round the class. Throughout the game, keep hold of your loop and before the tension becomes large stop the game by releasing the tension slowly.See Apparatus ListSee safety notes The elastic stretches to many times its original length. All the children can feel that the force on the elastic is increasing as the length increases.Emphasise that the children must carefully pass the elastic back the way it came. Discuss the fact that the elastic shrinks back to its original length.

4. Show the class some ‘stretchimals’ or similar elastic toys.

Aims/Facts

• Reinforce: the greater the force the greater the extension.
 Activity Discussion Show Slide 9.Show Slides 6 and 7.Hand two stretchimals with different shapes to each group.Ask them to stretch them by slowly increasing the force applied. Show Slide 8.See Apparatus ListSee safety notes Discuss what they find out. Take the opportunity to discuss their understanding of a force.

5. Explain that it is very useful to be able to estimate the size of a force.

Aims/Facts

• Developing estimating skills.
 Activity Discussion Slide 10 gives some useful values that they can remember. This could be extended if newtonmeters are available to check estimates.

6. Slide 11

Aims/Facts

• Different thicknesses require different forces to stretch them by the same amount.
 Activity Discussion Ask the groups try to apply the same pull to the two differently shaped stretchimals. Show Slide 12 and ask if the blue lizard will be longer or shorter than the red frog when the 10 newton force pulls on it. Point out that the ‘greater the force, the greater the increase in length’ applies for all the shapes. But the thinner the toy the longer the extension for a particular size of force.

7. Show Slides 13 and 14

Aims/Facts

• Revision
 Activity Discussion Take the opportunity to re-cap. Then use Slide 14 to reinforce the ideas.

8. Slide 15 – Springs

Aims/Facts

• Springs have elastic properties.
 Activity Discussion Explain that springs have elastic properties.Ask a volunteer to grip one end of a ‘slinky’ spring firmly and not to let go until you say.Holding the other end of the slinky, walk back across the front of the class so that the ‘slinky’ stretches.See Apparatus List Springs change length when a force is applied and return to their original length when the force is removed. Larger forces cause larger changes than small forces.Shaking the end of the end of the spring so that a transverse wave is seen helps to get across a message that ‘springs can make things happen’.

9. Ask volunteers to stand in a line and hold four or five similar springs.

Aims/Facts

• The larger the force on the spring the larger the extension.
 Activity Discussion Hang objects of different weights on the springs. Compare the amount the springs are stretched.See Apparatus List Use this as an opportunity to estimate the force on each spring, e.g. an apple will exert a force of about one newton etc.

10. Show a toy that has a spring in it such as the woodpecker that pecks as it slides down the rod.

Aims/Facts

• Springs can make things happen.
 Activity Discussion Show the animated Slide 16.See Apparatus List This is an example of a spring making a to-and-fro movement possible. It is called an ‘oscillation’.

11. Ask for examples of where springs are used.

 Activity Discussion Demonstrate examples of uses of springs in toys, wind-up toys, sports equipment e.g. a tennis racquet etc and other items that you have brought.See Apparatus ListSee safety notes Use the discussion to point out that there are numerous examples and many are hidden.

12. Point out that some springs are springy when they are squashed rather than stretched.

Aims/Facts

• Some springs are compressed when forces are applied.
• Unlike elastic which only works in extension.
 Activity Discussion We say they are ‘compressed’.Show examples of objects that use springs using Slides 17 and 18 and hand round some examples for groups to inspect.See safety notesAsk what happens to the shape of a cushion or mattress as they sit on a chair or get into bed. Discuss examples.Ask why the bed shown in the cartoon (in Slide 18) must be very hard.They soon notice that the mattress is absolutely flat so the weight of the character is not enough to squash the springs.

13. Ask if they have been on a bouncy castle and what would happen if it was too hard or too soft.

Aims/Facts

• Some springs are more firm than others.
 Activity Discussion Discuss examples of springy surfaces that are firm such as the surface of a play area.

14. Show Slide 19

Aims/Facts

• Springs are useful.
 Activity Discussion Discuss the examples.

15. Medical application: stents

Aims/Facts

• Springs save lives.
 Activity Discussion Explain that springs save lives.Show Slide 20.Explain that stents are tiny springs made of very special metal that remembers its shape. When a stent is squashed it later springs back to its original shape. Explain that some people may have a blockage which prevents the blood flowing round the heart. A stent can be inserted which keeps the artery open so the blood can flow.Discuss the problems. That the metal must not corrode, that the spring must be very, very small.

16. Show Slide 21 (animated so the steps can be shown individually.)

Aims/Facts

• Physics (science) saves lives.
 Activity Discussion Show Slide 22.Explain that a special tube, called a catheter, is threaded all the way through the arteries in the body from the leg to the heart. The catheter can be seen throughout this process by special scans that have been developed using physics. When the catheter reaches the correct position, a tiny balloon pushes on the stent so it opens the artery. The stent then stays in place as the catheter and balloon are withdrawn. During discussion emphasise that this is now a very common procedure that helps to reduce the likelihood of heart attacks.Make the point that many of the techniques now used by the medical profession have been developed using physics.