# Q & A: Mass on a spring on the moon

Q:
A 1-kilogram mass is suspended from a spring in the physics laboratory on Earth. If the same mass is suspended from an identical spring on the moon, is the amount of stretch in each spring the same?
- Anonymous
A:
The strength of gravity on the moon is about one seventh of that of the Earth. The weight of your 1 kg mass is then one seventh as big on the moon as on Earth, and so the force needed to hold it up is also one seventh as big.

The force supplied by a spring is directly proportional to the amount of stretch of the spring. Springs have an unstretched length -- how long they are when nothing's pulling on them. The difference between this unstretched length and the length when the mass is hanging on it is the amount of stretch. This amount of stretch will therefore be one-seventh as big on the moon as it is on the Earth.

Tom

(published on 10/22/2007)

## Follow-Up #1: weight vs. mass

Q:
This following question appeared on a recent Academic Team Meet where I help coach the elementary school students. I would like to challenge the answer given by the judges at this meet to this question. Can you confirm the correct answer. The question was: "The picture to the right shows a 100 gram mass on the end of a spring as measured on Earth. Which of the following is most likely to occur if the same spring and weight were connected on the Moon? (the picture of the spring as measured on Earth showed the spring stretching to 3 cm." The multiple-choice answers were: A. The spring would stretch to 1 cm. B. The spring would stretch to 3 cm. C. The spring would stretch to 5 cm. D. The spring would not stretch. The answer given by the moderator/judge was "B" which would be saying that the spring would stretch to exactly the same position (3 cm.) as on Earth. Can you help me clarify the correct answer to this question. Thanks
- Lorry Swain (age 60)
South Shore, KY, U.S.A
A:
You're absolutely right in thinking the answer would be more like (A), although not exactly. It's a complete disgrace that the judges were so spectacularly incompetent, since (B) misses the whole point.

Mike W.

(published on 02/05/2013)