Newton III in action

On Friday, we examined the importance of Newton’s 3rd law of motion.  In our discussions, different explanations for the motion of jets and rockets were proposed and considered.  The front runners were;

  1. at launch, the ground pushes back against a rocket
  2. during flight, air pushes back against a plane

Unfortunately, the lack of ground and air (or any other gas) meant that neither of these models were able to explain the propulsion of an object in space.  It was at this point we remembered Newton’s 3rd law of motion (or here with non-rocket examples) from Standard Grade.

You’ve got to be careful with Newton’s 3rd law of motion, it’s easy to get confused. Bonus question: What’s wrong with this explanation?

I found a photograph that provides a stunning visualisation of Newton’s 3rd law in action during the launch of a DeltaIV rocket.  You can read the details of setting up for this photo here.


image courtesy of Ben Cooper,

The photo was taken at very short range (about 30m) from the launch site and clearly shows hot gases being forced out of the exhaust at high speed. When a rocket forces out gas, the expelled gas pushes back on the exhaust with an equal force.  Since the exhaust is part of the rocket’s structure, the entire rocket is propelled in the opposite direction to the gas.

It is this pushing back on the exhaust that provides thrust for a rocket.  It doesn’t matter if the rocket is on the launch pad, in mid air or outer space. As long as it can push gas out of the exhaust, it will be able to propel itself forwards using Newton’s 3rd law of motion.

We don’t normally get a clear view of the hot gases being forced out of a rocket in launch photographs.  A lot of the smoke seen in images like the one shown below is actually steam.


NASA/courtesy of

There are two main sources of steam during launch.  The most obvious is the burning of fuels but NASA also soaks launch platforms with water just before and after launch so that the massive sound waves don’t damage the vehicle being launched.  There is a wikipedia article on the use of water during space shuttle launches.

download your copy of the SQA data booklet

The blue SQA data booklet we use in class is also available online as a pdf file.  It’s a good idea to get familiar with the page(s) for your course as you get to use a copy of the booklet for unit assessments, NABs, prelims and the external exam in May.

Download your own copy of the booklet for use at home by clicking on the link below.

drawing scale diagrams

Here is an example of a vector problem where we use a scale diagram to find the displacement of an object.

You can watch the video using the player, click on the download button to save it to your computer or watch it in iTunes if you have subscribed to the podcast.

Vector Scale Diagrams from mr mackenzie on Vimeo.

If you want to improve the accuracy of your scale diagrams;

  • use a sharp pencil
  • measure the vector lengths as accurately as you can
  • make sure the point of your vector arrow is on the end of the measured line
  • start each new vector right at the tip of the previous vector

Free physics software

I’ve had an email from Yenka to say that they are now offering many of their products for home use – free of charge!  I think Yenka is the new name for Crocodile Clips, the company who made Crocodile Physics and Crocodile Technology.  Their software allows you to create your own experiments to learn about

  • motion – SG transport, Int2 unit1, Higher unit1, AH unit1
  • electricity – SG using electricity, Int2 unit1, Higher unit2, AH unit2
  • light and sound – SG health physics, Int2 unit3, Higher unit3, AH unit3
  • electronics – Int1 applied practical electronics, SG electronics, Int2 unit2, Higher unit2

They also have some chemistry products you might find helpful.

The software is free but you can only use it at home.  Why not download it and see if you find it useful.