Standing waves on a spring

You might have seen/done this one before, but the teachers I was showing it to found it quite useful, so I thought I would post it on talkphysics. The nodes and anti-nodes are a lot clearer when the amplitude of the oscialltions is higher, but I was told the signal generator broke the previous vibrator when set too high, so I was a bit cautious with this one.

There are several teaching and learning points with this demo. For example, you could get the students to calculate the speed of the wave along the spring, as we know the frequency from the signal generator and can measure the wavelength (for example by measuring the length of the stretched spring with a ruler).

Once we know the speed of the wave, we could ask the students to predict the next frequency in which a standing wave will form.

What else would you use this demonstration for? What other questions would/could you ask?

How high does Doodle Jump?

I was playing a new update of Doodle Jump on my iPhone this morning and I suddenly thought it would make a pretty cool lesson on motion and it could actually apply to both GCSE and AS Physics, depending how you phrase your questions.

The idea is to find out how high the little alien in Doodle Jump actually jumps. There is no reference in the game to suggest what scale the screen has, so I used Vernier Video Physics and arbitratily set the distance between the block on which the alien was jumping (see video below) and the last but one block above the creature to be 10m.

I then tracked the position of the alien for one jump and the analysis of the velocity graph shows the gradient is not right, i.e. g is not close enough to 10m/s2.

So, I went back and changed the scale to be 4.5m between the two blocks mentioned above. That seemed to have done the

trick, as the gradient of the velocity – time graph in the video below is about 10m/s2.

If you look back at the Displacement – Time graph for the vertical axis, you can now see that the little alien jumps to a height of about 2.3m. That is quite something for a little fella like him.

Obviously, we are assuming the alien is jumping near the surface of the earth, or at least a planet with the same g.

Teaching Forces on a trampoline!

It can be tricky to find good examples to show how forces add up to give a resultant force. In particular, sum of vector forces in AS Physics is something that takes practice in order for students to grasp. So, when one of my boys enjoyed a ride on one of those trampolines where they strap you to two elastic ropes to make you jump very high I thought it would be useful to share this photo with you. The tensions from the two ropes pull him at the same angle on either side, but he jumps up vertically. Why does this happen? You can ask students. Then force arrows could be drawn and look at their vertical and horizontal components to see that the horizontal components are balanced and the vertical components add up, etc…

What other useful concrete examples do you use with your students?

 

Sorry the photo got uploaded on its side instead of the right way up, but you should be able to easily rotate it on a PPT presentation, or you could mess with you students and tell them it was taken at the Equator 😀 and see what they say!