You place a rock on the left pan of a balance. You add a 20g mass, a 10g mass, and a 5g mass to the right pan before the pans are level. What is the mass of the rock?
A20 grams — you use the heaviest mass
B35 grams — you add all the known masses together (20 + 10 + 5)
C5 grams — you use the smallest mass
D15 grams — you average the three masses
When the balance is level, both sides have equal mass. The right side has 20g + 10g + 5g = 35g. So the rock on the left side must also have a mass of 35 grams. You always add up all the known masses to find the total.
Question 2 True / False
A balance would give a different mass reading on the Moon than on Earth.
TTrue
FFalse
Answer: False
False. A balance compares masses directly — the object on one side, known masses on the other. On the Moon, gravity pulls less on BOTH sides equally, so the balance still levels out at the same point. The reading is the same. A spring scale, on the other hand, WOULD give a different reading because it measures the pull of gravity.
Question 3 Short Answer
Why does a balance measure mass instead of weight?
Think about your answer, then reveal below.
Model answer: A balance compares the object directly against known masses. Both sides are affected by gravity equally, so gravity cancels out. The balance only tells you when the two masses are equal — it does not measure the force of gravity.
This is why a balance works the same on Earth, on the Moon, or on any planet. Gravity pulls on both pans equally. A scale, in contrast, measures how hard gravity pulls on the object, which means its reading changes with gravity. A balance reading is a true measurement of mass.