Fish in a Bucket

This week's experiment goes back to fishing trips from my childhood. As we caught fish, we put them into a large tub of water. I loved watching them swim around and around. I was told that if you weighed the tub, that it would not register the weight of the fish, unless they touched the sides or bottom of the tub. Was that right? Let's find out.

You will need:

  • a bathroom scale
  • a bucket of water
  • a brick or large rock
  • a piece of wood

Start by putting the bucket of water onto the scales, and recording the weight. Then pick up the brick and hold it under the surface of the water. Don't let it touch the side of the bucket. Look at the scale again. Has the weight changed?

Yes, the weight goes up. So the first part of what I was told was wrong. The tub of water did weigh more with the fish in it, even if they were not touching the tub. Well that certainly makes sense.

Next, touch the brick to the side of the bucket. Did it change? No. Touching the side of the bucket does not cause the weight to change. Again that makes perfect sense. Contact between the brick and the bucket (or the fish and the tub) does not make any difference. OK that makes perfect sense too.

Now, what if you placed the brick on the bottom of the bucket? Would the weight change then?

Don't try it yet. Instead, think about it for a minute. (Hint: Think about why some things float and some things sink.)

Alright, now try it. No, I did not say read what will happen. I said try it.

Did you try it? Good, then I don't have to tell you what happened, right? But, I will anyway. When you placed the brick on the bottom of the bucket and released it, the weight increased. Why?

When you put the brick into the water, you should have noticed that it seemed to weigh less in your hand. That is because the water was supporting part of its weight. The weight that you felt decreased by exactly the weight of the water that the brick displaced. The weight of the bucket increased by the same amount. That was the first weight gain for the bucket.

When you put the brick on the bottom of the bucket, your hand was no longer supporting part of its weight, so the weight of the bucket increased enough to register the full weight of the brick. That was the second weight gain.

Just one more question. What if you put the brick on a scale, and then submerged both of them in the bucket of water? How much weight would the scale show for the brick? Think about it. Then try it, and then go on to the next page.