Week 9 · Lesson 3 Gravitational Forces: contact and field forces, Earth’s gravitational field, mutual attraction, tides, orbits, mass and weight · 3 lessons
3
Mass is not weight
Science Prep 1 Term 1, Unit 2: pp. 47–54
GoalTell mass from weight and use weight = mass × g with small numbers.
New words
mass · the amount of matter in an object, measured in kgweight · the pull of gravity on an object, measured in newtonsg · the pull of gravity on each kilogram: about 10 N/kg on Earth
Explanation
Mass is the amount of matter in an object. It is measured in kilograms and is the same everywhere. Weight is the pull of gravity on that mass. It is measured in newtons and changes from place to place. The rule is weight = mass × g. Near the Earth’s surface g is about 10 N/kg (the exact value is 9.8), so a 3 kg object weighs about 3 × 10 = 30 N. On the Moon, g is one sixth of the Earth’s value, so the same object weighs much less, but its mass is still 3 kg. Higher above the ground, g gets a little smaller. Astronauts in orbit are not outside gravity: at the height of the space station, gravity is still about nine tenths as strong as at the ground. They float because they and the station are falling around the Earth together, so they feel a very small apparent weight, called microgravity.
Worked examples
A school bag has a mass of 2 kg. Its weight on Earth is 2 × 10 = 20 N, which is what the spring balance shows. On the Moon its mass would still be 2 kg, but its weight would be about 20 ÷ 6, which is about 3 N.
Place
Mass
Weight
Earth
60 kg
600 N
Moon
60 kg
100 N
A person has a mass of 60 kg. On Earth the weight is 60 × 10 = 600 N. On the Moon, with g about 10 ÷ 6 N/kg, the weight is about 100 N. The mass is 60 kg in both places.
A kitchen scale shows 500 g for a bag of oranges. It reports the mass in grams, even though it really feels the pull of gravity. The mass, 0.5 kg, would be the same on the Moon.
Class activity
Rubber-band balance, with an adult. Hang a rubber band from a hook and tape a ruler beside it. Hang washers or coins in a small bag from the band. Add one washer at a time and measure the stretch each time. Draw a table: the more weight, the more stretch. Then weigh some fruit on a kitchen scale and say whether the scale shows mass or weight.
Practice
1
A 4 kg object is on Earth (g = 10 N/kg). What is its weight?
4 × 10 = 40 N.
2
An object weighs 90 N on Earth. What is its mass?
90 ÷ 10 = 9 kg.
3
What happens to the mass of a rock if it is taken to the Moon?
It stays the same. Only its weight changes.
4
Why do astronauts in orbit seem to float?
They are falling around the Earth, so they feel a very small apparent weight, even though gravity still acts.
