Video: Finding the Mass of an Object by Using Its Weight

Timmy uses a Newton meter to measure the weight of a brick. He gets a value of 14.7 N. How can he find the mass of the brick from this value?

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Video Transcript

Timmy uses a Newton metre to measure the weight of a brick. He gets a value of 14.7 newtons. How can he find the mass of the brick from this value?

Okay, so what we’ve been told in this question is that Timmy has used what’s known as a Newton metre. And we’re measuring the weight of a brick here. We’ve been told that the Newton metre tells us that the weight of the brick is 14.7 newtons. And so let’s say that 𝑊, which we’ll say is the weight of the brick, is 14.7 newtons.

Now, the question asks us, how can we, or rather how can Timmy, find the mass of the brick from this value? And therefore, to answer this question, we need to remember the relationship between mass and weight. Well, the relationship that we’re looking for tells us that the weight of an object, in this case our brick, is equal to the mass of the object, 𝑚, multiplied by the local gravitational field strength, 𝑔.

Now notice this question doesn’t actually ask us to find the value of the mass. We just need to state how we can find the mass if we know the weight of the brick. And the way that we can do this is to rearrange this equation to solve for the mass. We can do this by dividing both sides of the equation by the local gravitational field strength 𝑔 because this way, on the right-hand side, we’ve got 𝑔 in the numerator and the denominator.

And so what we’re left with is the weight of the brick divided by the local gravitational field strength as being equal to the mass of the brick. And so the answer to the question, how can Timmy find the mass of the brick if he knows the weight, is that he must divide the value that he has got for the weight by the local gravitational field strength 𝑔 because, as we saw in the equation, the mass is equal to the weight which he already knows, divided by 𝑔. And hence, we found the answer to our question.

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