Lesson video

Hello, my name is Mr. Forbes, and I'll be leading you through the lesson today.

It's a lesson from the movement by force unit, and it's all about measuring speed practically.

That means you're gonna carry out an experiment, measure some distances and some times, and use that data to calculate speed.

By the end of this lesson, you're going to be able to measure the speed of a trolley rolling down a ramp.

You're gonna be able to do that as accurately as possible by making sure you measure time accurately and distance accurately.

So when you're ready, let's get on with the lesson.

The key word you need for the lesson here: the dynamic trolley is just a little trolley you roll down a ramp.

Gradient is another word for slope, so a high gradient is a steep slope.

A timer is anything you can use to measure time, such as your phone or an electronic timer we use in physics.

A meter rule is just a meter rule.

And average speed is the speed of an object over a certain distance, it's the distance divided by time.

And here's a set of definitions of those keywords.

You can pause the video here to read through them and you can return to this slide at any point.

So the first thing you need to know is what a dynamics trolley is.

And it's one of these.

It's a simple wheel vehicle we use in physics for quite a lot of our experiments.

It's a standard size, and it rolls smoothly over surfaces.

It's got very low friction and that allows it to keep on going quite a distance on this very small forces.

And the second piece of equipment is a simple ramp.

A ramp is just a slope surface, something like that.

A wooden board.

Sometimes you can buy specialist ones, but any simple slope surface will do.

When you put a dynamic trolley on it, there's a forward force that acts on it that'll make the trolley roll.

There is a small force of friction as well, but that's so small that we're going to basically ignore it during this experiment.

As we put the trolley on, it will start to roll down and speed up as it goes.

It speeds up because that forward force is always there, always causing an acceleration.

The average speed of those trolley is gonna be able to be worked out from the distance and time measurements that we take.

Now during this experiment, we are going to use different gradients.

The gradients is the steepness of the slope, and we can adjust that by just tilting the ramp.

We can get a small ramp like that with a low gradient and a small forward force, a medium sized gradient giving us a bigger forward force, and a larger gradient that's gonna give us quite a large forward force.

Now, even though I've described that as a large gradient, we really don't want to make those gradients too steep, otherwise it'll be very difficult to time the trolley.

It'll move too fast.

Okay, first check here.

It's gonna be in two parts.

First of all, the true and false part.

Using a ramp with a greater gradient will make the trolley roll faster: is that true or false?

Pause the video and make the selection and then restart.

Okay, welcome back.

That should have been true.

So well done if you got that.

Now I'd like you to justify that.

I'd like you to give me your reason.

Is it because the higher the gradient of the ramp, the lighter the trolley is; or the higher the gradient of the ramp, the greater the forwards force?

Pause the video, make a selection, and then restart.

Okay, well, your answer should have been the higher the gradient of the ramp, the greater the forwards force.

So well done if you've got that.

In the experiment, you're gonna need to measure a distance.

And the simplest way to do that is to use a meter rule, because it's a standard fixed length and it's fairly accurate.

So we're gonna use just one of those wooden or plastic meter rules.

What we do is we get the ramp and we put two marks on it.

A mark for the start line and mark for the finish line, just using the ruler to measure those distances out.

There's the start line, and there's the finish line.

You're going to need a timer to time how long it takes for the trolley to roll through that distance of one meter.

So you can position the trolley at the star line here and allow it to roll to the finish line.

To measure that time accurately, you'll want to start the timer exactly the point where you release the trolley.

And do that, you should use something like a countdown, three, two, one go, or something like that.

You've also got to be able to stop the timer precisely when the trolley reaches the finish line.

So well, to do that, the best thing to do is to look directly across the finish line so you can see when the front of the trolley reaches it, and then stop the timer straight away.

Okay, which two of these will help you measure the roll time accurately?

Pause the video, make your selection, and then restart when you're happy.

Okay, the two correct answers: we're looking directly across the finish line so you can stop the time at just the right time, and counting down to release of the trolley so that you can start the timer at just the right time.

Well done if you've got both of those.

So here's a set of instructions for the experiment.

I like to read through those carefully before you carry it out.

And to help you, I'll show you a video of someone carrying out an experiment and doing one of the tests.

<v Instructor>In this investigation,</v> we're going to be measuring the average speed of the trolley that's rolling down a ramp.

If you look at the ramp, you'll notice that there's two straight lines being drawn onto it, one where the trolley is and one at the bottom of the ramp.

These have been measured with a meter ruler and they're exactly one meter apart.

So what we're going to do is we're going to time how long it takes the trolley to move from the top line one meter to the line.

So here goes.

Three, two, one.

(trolley rattling) So it took our trolley 2.

74 seconds to travel down that one meter down the slope.

Now you'll have noticed that the trolley sped up as it went down the slope, at least at the start.

And so what we've measured is the average speed.

It's not gone at a steady speed all the way down, we've just got an average speed.

Now, it's always useful to repeat a measurement to double check if you've got it correct.

So let's try again and see if we get a similar time.

Three, two, one.

(trolley rattling) So there we go, exactly 2.

74 seconds again.

I'm actually very surprised at that.

I would've expected to be a little bit different because of timing errors of the person who was letting go of the trolley not quite starting at the right moment and not quite getting it right at the other end either.

Let's just have a look at a few more measurements and see what times we get for the next few runs of the trolley down the ramp.

Three, two, one.

(trolley rattling) Three, two, one.

(trolley rattling) Three, two, one.

(trolley rattling) Three, two, one go.

(trolley rattling) And so apart from the first two results, they're all slightly different.

And that can be put down to the timing errors when we start the timer and when we stop the timer at the end.

And we can now use those measurements to calculate the average speed of the trolley going down that ramp.

And to do that, we use the equation, speed equals distance divided by time.

Remembering of course that the distance we're measuring is exactly one meter.

<v ->once you're happy that you've understood everything</v> you can then get on with the experiment and collect your results.

Okay, you should have carried out the experiment by now.

And here's a results table.

Your table should look something like this, but it won't be exactly the same.

You should have three steepness, low, medium, and high, and at least three times for each of the trolleys.

Now your data won't look exactly the same as mine, and my table also has two extra columns for some calculations we're gonna be doing in the second part of the lesson.

So well done if your table on your results look anything like this.

Okay, we're gonna move on to the second part of the lesson and we're gonna use the data we collected in the experiment to find some average speeds.

Okay, we're gonna find the average time it took the trolley to roll down the ramp and the type of average we're finding is actually called the mean.

And the mean is when you add the values together and divide by how many values there are.

So we're gonna add the three different read time measurements here and then divide by three because there's three of them.

Do that song, and that gives me a mean time of 3.

26 seconds, which are right into my table just there.

Right, to test if you understand how to calculate mean, I'd like you to calculate the mean value of these four time values, please.

Pause the video, work out your answer, and then restart when you're happy.

Welcome back.

Let's have a look at the answer.

It was 2.

44 seconds.

When you get 2.

44 like this, there were four readings, so we add all four values together, then we divide by four, and that gives us an answer of 2.

44 seconds.

Well done if you've got that.

So now we're at the point where we can calculate the average speed of the trolley.

We've got the mean times and we know the distance 'cause we measured out the distance during the experiment.

It traveled one meter each time.

So for the lowest slope, we can calculate the average speed by using distance divided by mean time.

The distance was one meter, the mean time was 3.

29 seconds, so doing that calculation gives us an average speed of 0.

3 meters per second.

Now it's time for you to calculate the average speed of your trolley.

If you don't have any data, you can use the data here.

I'll like you to pause the video, calculate those average speeds, and then restart.

Welcome back.

I'm gonna use my data to calculate the average speeds so I can calculate the mean times, fill them in here.

Got 3.

33 seconds for the low gradient, and then use that on the speed equation to calculate the average speed.

Speed equal distance divided by time.

Average speeds, 1 divided by 3.

33, and that gives me an average speed of 0.

30 meters per second.

And I can repeat those same calculations with the other two gradients of ramp, even high.

Okay, we've reached the end of the lesson now, and here's the summary of the information.

You can find the average speed of a dynamics trolley as it rolls down a ramp by measuring the distance and the time.

Higher gradient of the ramp will increase the size of the force on the trolley and that will affect its speed.

You can measure the distance with a meter rule, and you can measure the time with the timer, and you can improve timely measurements by carefully observing the start and the finish line and doing things like counting down before you release the trolley.

So well done at reaching the end of lesson, and I hope to see you in the next one.

Bye.