The following is the title of the Junior Science Physics investigation for 2016:
Investigate and compare the quantitative effects of changing
(a) the pendulum length and
(b) the mass of the pendulum bob on the period (time of oscillation) of a simple pendulum oscillating through a small angle.
This investigation is a slight variation on a leaving cert physics investigation so if your teacher doesn’t teach physics then you may be at a slight disadvantage when it comes to understanding some of the nuances of the experiment.
This is my attempt to level the playing pitch.
Just understanding what you’re being asked to do is tricky, so let’s simplify it a little.
- The words investigate and compare mean pretty much the same thing in this context. So for now let’s just use investigate. But we will come back to significance of the word ‘compare’ again at the end.
- The word quantitative is just telling us that we will need numbers for our results. So if we just remember that then we don’t need to refer to it again.
- The term simple pendulum refers to a pendulum that consists of a single string with a mass at the end.
- The word mass is similar to the word weight. In your notes on Forces in Junior Cert Physics we explain what’s different about them, but if you’re just trying to get your head around this investigation you could consider them to mean much the same thing for now.
- The pendulum consists of a piece of string and a metal ball (the technical term is a plumb bob) so if you want to change the mass of the pendulum then you need to need to use a heavier or a lighter ball.
- The period (time of oscillation) refers to the time for one complete swing; all the way over and all the way back to the starting point.
- The phrase oscillating through a small angle means that when we pull the pendulum back, we shouldn’t pull it back too far before we release it. What does ‘too far’ mean? Well you could pull it back until the string is horizontal, and that would definitely be too far. So try for not more than about 45 degrees.
- The term period (time of oscillation) means the time it takes the ball to go through one full cycle. So you would start your clock when you release the ball, and then stop the clock when the ball gets back to the starting point.
- There’s a part (a) and a part (b), and by putting both parts in one big long sentence it only serves to confuse what we’re being asked to do. So whoever put this together didn’t do a particularly good job.
Let’s re-write it as follows:
(a) How does changing the length of a pendulum affect the time it takes the pendulum to do one full swing?
(b) How does changing the weight of the pendulum affect the time it takes the pendulum to do one full swing?
Now let’s come back to this word ‘compare’ again. What it means is, “Does increasing the length of the pendulum have the same effect as increasing the mass (on the time it takes the pendulum to do one complete swing)”?
When looking at how the time for one oscillation varies with mass, the variables are obviously mass and time. So the length needs to be kept constant. This can be trickier than it might appear.
Is the string oscillating about a fixed point? Look closely at top of the string to check. If it’s not then the length isn’t constant. What could you do to try and keep the top of the string fixed?
Where do you measure the length of the pendulum from?
Answer: From the top of the string to the center of mass of the ball.
At junior cert level you could probably just estimate where the center of the ball is, but at leaving cert level you should be more precise – can you think of how you would do this? If so then include it in your comments at the end of the booklet.
But what if the heavier ball is also larger than the old ball – will this change the overall length of the pendulum?
How could we use a larger ball and still keep the overall length constant?
Guess what will happen before you carry out the investigation. You can then refer back to this at the end in the comment section. Did you guess correctly or were you surprised by what you found out? It turns out that a lot of science is what we call counter-intuitive; this just means that a lot of the time what we find isn’t what we thought we’d find (‘against common-sense’). Which is why we always need to test our predictions. After all, it’s not exactly obvious that the earth is round, or that humans evolved from just a bunch of chemicals now is it?
So what do you think will happen to the time for one full swing if you use a longer string?
What do you think will happen to the time for one full swing if you use a heavier ball?
In the Electricity chapter in Junior cert physics there is an experiment where you investigate the relationship between current and voltage (potential difference) for a metallic conductor. When you plotted a graph with current on one axis and volts on the other you (should have) got a straight line through the origin.
A common exam question asks you what is the relationship between current and voltage, and how do you know?
The answer is that ‘they are proportional’ – we know this because we get a straight line through the origin.
In our experiment to investigate how the time taken for one swing varies with the length, do we get a straight line through the origin?
If the answer is yes, what does this mean?
If the answer is no, what does it mean?
It turns out that investigating the motion of a pendulum was one of the first and most important experiments in all of Science. To see why (and to get some hints on how to carry out your investigation) look at the video here.
And good luck with it
It’s a nice investigation; pity all the good is taken out of it by how it’s assessed: