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Hi, I'm Katie in Mrs. Nowell's fourth grade class at A.B. Combs Elementary School in Raleigh, North Carolina.

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Hi, how may I help you?

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Hi, we heard that you were working on an experiment to learn more about pulleys.

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That's right. We wanted to find out how pulleys work.

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Can you tell us about your experiment?

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Sure. First, each group was given 500 gram mass, two double pulleys, a cup, some string, and some pennies.

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We hooked the 500 gram mass to the bottom of the pulley system that we constructed.

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Their combined mass is called the load.

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What did you do with the pennies?

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We kept adding pennies to the cup until the load was lifted.

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We then placed our cup on a balance to find the total mass that it took to lift the load.

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Once we collected our data from two strings, we repeated with three and four strings.

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Did each group in the class get the same data?

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Not exactly, so we shared our data and took the class average of the total mass that it took to lift the load for each number of strings.

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When you analyzed your data, were you able to come to any conclusions?

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Yes. After putting the class averages up on the board, it was pretty obvious that the more strings we used, the less mass we needed to lift the load.

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How did you calculate how much mass you needed for each string?

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We counted the number of strings that supported the load and divided the combined mass by the string count.

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We saw right away that there was a problem when we did our first calculation.

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What was the problem?

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Our mass being lifted was 545 grams when we had only one string.

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It should have taken 545 grams to lift it.

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Instead, it took an average of 571 grams.

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We also found the same thing in each of our other calculations.

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Could you have collected your data incorrectly?

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That's what we thought at first, or that we had made an error in our arithmetic.

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Our teacher gave us a hint.

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We realized that there was friction in our pulley system.

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Friction slows things down and makes it more difficult to lift.

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That's why you needed more mass to lift the load.

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We concluded that you can find approximately how much mass is needed to lift the load by dividing by the number of strings,

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but you definitely have to consider friction.

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Thanks so much for your help.

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We won't forget about friction.

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You're welcome, and good luck on your project!