projectiles – moving in 2 directions at once

One of the things we spoke about today was the difficulty of dealing with horizontal and vertical velocities one at a time when we analyse the motion of a projectile. I have uploaded this diagram to see if it will help you see how the horizontal and vertical velocities change as we look at different parts of the trajectory. You can click on the diagram to see a larger version if you like.

Hopefully you can see the arrow I have drawn in for the velocity of the projectile at any point – it’s the brown arrow. I have added blue and green arrows to this.

The green arrows show the horizontal motion of the projectile. The green arrows stay the same length throughout the flight because there is no unbalanced force acting in this direction (Newton’s 1st Law told us that balanced forces result in a constant speed).

Looking again at the diagram, you should notice that the blue arrows show the vertical velocity of the projectile. The blue arrows change as we move along the trajectory. For the first half of the diagram, the blue arrows point up because the projectile continues upwards towards it maximum height (called the apex). As the projectile travels towards the apex, gravity acts to slow it down – we have deceleration due to gravity. This is why the blue arrows get shorter – the length of the arrow shows the size or magnitude of the velocity. Once it has passed the apex, the projectile falls downwards. The diagram shows that the blue arrows point downwards during the second half of the journey. As the projectile falls, it’s vertical velocity increases (it is speeding up) and this is shown by the blue arrows getting longer. This is acceleration due to gravity.

Notice that all the time that the blue arrows change their length and direction, the green arrows always have the same length and point in the same direction. Horizontal velocity is constant in projectile motion.

We can analyse horizontal motion quite easily using the relationship between speed, distance and time.

To analyse the vertical motion, we need to use the equations of motion.

Large Hadron Collider – lock up your protons!

Some of you have been asking why people are talking about the world ending on Wednesday.  Funny how this question has arisen whenever I hand out homework!  I’ve spoken about the Large Hadron Collider and some of you may have seen the BBC coverage on Big Bang Night last week.  They also have some special programmes scheduled to commemorate the day the LHC is switched on, including a particle physics radio programme with Alan Alda, Ben Miller, Eddie Izzard, Dara O’Briain and John Barrowman but then I could have told you that Physics is the new rock’n’roll 😉

I’ve also found a rap video about the LHC.  The clip seems to stream slowly so you might want to start it and hit pause to let it load fully before playing.  Enjoy!

physics in a lift

We had some fun in the school lift today, trying to confirm what we had read about apparent weight.  Although the effect was small, we managed to spot the same effect as was demonstrated by David jumping off a desk with a set of 100g masses hanging from a Newton balance.  

I found a link that puts some words around our findings.  Follow this link to find out more.

image: “elevator” by Beard PaPa

Projectile motion

We spent a lot of time looking at projectiles and projectile problems in class so this should be a concept that you are comfortable with.  If you are still a little unsure, there is a good summary page here.  The page contains interactive questions to check your progress – just make your selections and press the green “=” symbol each time to find out if you were right.

Here is another site with a decent summary.  It has a flash animation showing how motion in the x and y directions are treated independently and explains the method to solve projectile questions.

remember vectors?

We’ve been dealing with vectors since the very start of the Higher Physics course.  Before the holidays, we looked at displacement, velocity and acceleration as vectors.  Over the last 2 weeks, we have discussed velocity in the context of projectile motion and looked at the vector nature of forces.  

Some of the problem solving tasks we attempted towards the end of last week suggested people were a little rusty on vectors so I have looked out some youtube revision materials to jog the memory.

Sometimes clips go into too much detail for our course or start introducing alternative methods that we have not used in class.  To prevent confusion, I’ve suggested how much of each video is relevant for the Higher course.


The first 6 minutes of this video clip introducing vectors may be helpful.


Watch this one up to 2:50 for a good review of our June work on displacement.