Question Video: Changing the Subject of a Formula

Video Transcript

The displacement, 𝑠, of an object travelling with uniform acceleration, 𝑎, can be calculated using the formula 𝑠 equals 𝑢𝑡 plus a half 𝑎𝑡 squared, where 𝑢 is the initial velocity and 𝑡 is the time. There’re two parts to this question. The first part, Make 𝑎 the subject. And the second part, calculate the acceleration of an object that has started from rest and travelled 200 meters in eight seconds.

So if we have our formula, which is 𝑠 equals 𝑢𝑡 plus half 𝑎𝑡 squared. And if we want to make 𝑎 the subject, well, the subject means we want the 𝑎 on its own. So the first thing we’re gonna do is subtract 𝑢𝑡 from both sides of the equation. That’s cause that will leave us with a half 𝑎𝑡 squared on the right-hand side. So now, when we subtracted 𝑢𝑡 from both sides of the equation, we get 𝑠 minus 𝑢𝑡 is equal to a half 𝑎𝑡 squared. And now, because we’ve got a half 𝑎𝑡 squared on the right-hand side, what we’re gonna do is multiply both sides of the equation by two. And that’s because that will remove the half that we’ve got removed the fraction.

So when we do that, we get two multiplied by 𝑠 minus 𝑢𝑡 is equal to 𝑎𝑡 squared. That’s because if we have a half multiplied by two, you get one. So then, all we need to do is divide by 𝑡 squared because that will leave 𝑎 on its own. And once we do that, we’ll have two multiplied by 𝑠 minus 𝑢𝑡 over 𝑡 squared is equal to 𝑎. So therefore, having made 𝑎 the subject, we’ve got 𝑎 is equal to two multiplied by 𝑠 minus 𝑢𝑡 over 𝑡 squared.

Great. Now, let’s move on to the second part. And in the second part, we need to calculate the acceleration of an object that has started from rest and travelled 200 meters in eight seconds. So now to solve any question like this, what I’d do first is write down the information I’ve got. So we know that 𝑢 is equal to zero because this is the initial velocity. And we know that because we’re told that the object starts from rest. And for an object to start from rest, that means that it’s not gonna have any velocity at the beginning. So 𝑢 is equal to zero.

Then, the next thing we know is that the distance, which is 𝑠, is equal to 200 meters. And that’s because we’re told the distance travelled is 200 meters. Then, we also know the time because this is eight seconds and the acceleration 𝑎 is what we’re looking for. So we haven’t got that yet. Okay, now we’ve got our values, we can just substitute them into our equation. And the equation we’re gonna use is 𝑎 equals two multiplied by 𝑠 minus 𝑢𝑡 over 𝑡 squared because what we’re trying to find is 𝑎, which is the acceleration.

So now, to work out the acceleration, we’re gonna have 𝑎 is equal to two multiplied by. Then, we’ve got 200 cause that’s our 𝑠, cause that’s our distance minus 𝑢, which is zero multiplied by our time, which is eight. And then this is all divided by our time squared, which is eight squared. So this gives us that 𝑎 is equal to 400 divided by 64. And that’s because we had two multiplied by. And then, you’ve got 200 minus zero multiplied by eight. Well, 200 minus zero cause zero multiplied by anything is just zero gives us 200. Two lots of 200 is 400 and then divided by eight squared and eight squared is 64.

Then, if we simplified it by dividing both the numerator and denominator by four, we’re gonna get 100 over 16. And then once more divided by four, we’re gonna get 25 over four cause 100 divided by four is 25. 16 divided by four is four. So then, we can see that four goes into 25 six times with one remainder. So we get six and a quarter. And when we turn it into a decimal, it’s 6.25. Then, we know that the units are gonna be meters per second squared because that’s the measurement we’re gonna use for acceleration.

So we can say that if we make 𝑎 the subject, we get 𝑎 equals two multiplied by 𝑠 minus 𝑢𝑡 over 𝑡 squared. And if we calculated the acceleration of an object that has started from rest and travelled 200 meters in eight seconds, then the result is 6.25 meters per second.