Teaching Gravitational Fields

I love teaching the gravitational fields topic. It always feels like the first ‘grown up’ physics topic we visit at the start of year 13.  Like, finally we’ve laid all the ground work to be able to look at something complex, but beautifully described in the equations.

That’s probably not entirely fair on the entire gcse and AS curriculum! But it is how it feels to me. 🙂 It also can provide a good discussion on where the gravitational fields topic goes next, what was the next big development? I usually use a picture like this on an intro slide to provoke the questions:

It’s at this stage that student use of the word ‘gravity’ really shows up as an inadequate term. Do you mean gravitational force, or gravitational field strength? At least they can see more easily why I’m asking for a more specific term rather than earlier in school where they struggle to see why I want more info than just ‘gravity’. *frustrated face*

It’s a fabulous topic, but not a very practical one for obvious reasons. We already looked at measuring g in year 12 – more on that experiment in a future blog post! So, I need to make sure I don’t just talk at the students and then get them to answer some questions, and repeat.

What is the gravitational field strength on the surface of the Clanger’s planet?

One of my favourite activities to do after teaching gravitational field strength is to get students to watch this video:

and then work out the gravitational field strength of the Clanger’s planet. It’s great because they have to make assumptions about the density of the planet, and they need to estimate it’s radius from the curvature they see in the clip, using the astronauts height as a ruler.

After they have worked out a value for g, you can discuss if it seems reasonable based on the fact that the astronaut could run himself into orbit. What does this tell you about the actually density of the planet  compared to your assumption?

So much physics in the Clangers! Who knew? I should point out that I found that video on youtube years ago and the suggestion to use it in a physics lesson is on the video. I can’t take any credit for coming up with the idea.

Apollo Data

I have enjoyed using the Apollo data with my class this year. The task is detailed on the wonderful Teaching Advanced Physics (TAP) pages by the Institute of Physics.  These pages are a fantastic resource for all A level physics teachers. They are especially useful for new teachers and teachers who might not be physics specialists who have ended up with A level lessons.

The Apollo data, as presented in the TAP task, is easy to understand. It lets students inspect the Earth’s gravitational field and see if it does drop off as distance squared. The analysis can be done by spreadsheet (make sure you figure out how to get excel to plot the type of graph you want first – it can be done but isn’t immediately obvious how to do it!) or by pen and paper.

 Gravitational Potential

Due to a spec change, I didn’t need to cover gravitational potential this year. A shame because I like the comparison between gravitational and electric fields later on. I feel like I’ve taught it this year with a bit missing. I decided not to cover it anyway for time reasons (sad, but true) but I mentioned its existence.  🙂

I’ve still got Kepler’s third law and satellites to teach, so may add this topic in a few weeks.

Happy gravitational fields teaching!




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2 Responses to Teaching Gravitational Fields

  1. audsley says:

    Re: not liking the word gravity being used a lot.

    I know I’m probably fighting a losing battle with that one. It seems to be the preferred word in lots of places. Gravity force *ick*


  2. Excellent blog! Can’t wait to try out the Clangers video!


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