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Clarifying the common confusion between the amount of matter in an object and the gravitational pull on it.
If you traveled to the Moon today, you would suddenly be able to dunk a basketball like a pro—but you wouldn't actually be any 'thinner.' Why does the universe change your weight but not your size?
Mass is the measure of the amount of matter in an object. Think of it as a count of every single atom that makes you, 'you.' Because those atoms don't disappear when you travel, your mass is constant—it stays the same no matter where you are in the universe. In the metric system, we measure mass in kilograms (kg). If you have a mass of on Earth, you still have a mass of floating in the vacuum of space.
Quick Check
If an astronaut has a mass of 80kg on Earth, what is their mass on the Moon where gravity is much weaker?
Answer
80kg. Mass does not change based on location because the amount of matter remains the same.
Weight is not the same as mass; it is a force. Specifically, it is the measure of the gravitational pull acting on an object. Because gravity is a force, we measure weight in Newtons (N), not kilograms. Your weight depends entirely on the gravity of the planet you are standing on. On a large planet like Jupiter, the 'tug' is very strong, so you would weigh more. On a small body like the Moon, the 'tug' is weak, so you weigh less.
1. Identify mass: 2. Identify gravity: 3. Multiply:
Quick Check
Which unit is used to measure weight in physics?
Answer
Newtons (N).
The value of (gravitational field strength) changes depending on the mass of the celestial body. While Earth's is , the Moon's is only about . This is why you feel 'lighter' on the Moon. Even though your mass (the atoms in your body) is identical, the force pulling you down is nearly 6 times weaker!
Let's compare a student on Earth and the Moon.
1. On Earth: 2. On the Moon:
The student's mass is in both places, but they weigh significantly less on the Moon.
An alien explorer lands on a mystery planet. Their scale says they weigh . If their known mass is , what is the gravity () of this planet?
1. Start with the formula: 2. Rearrange to solve for : 3. Plug in values: 4. Calculate:
If you move an object from Earth to deep space where there is zero gravity, what happens to its mass?
Calculate the weight of a cat on Earth ().
Weight is a force, while mass is a property of matter.
Review Tomorrow
In 24 hours, try to explain to a friend why a scale would show a different number on Mars even if you didn't lose any 'stuff' from your body.
Practice Activity
Find three items in your house with a mass listed in kg (like a bag of flour or a dumbbell). Calculate what their weight would be in Newtons on Earth using .