Mass vs Weight

Activities to try

1. 1

   Two instruments, one rock, different planets

   Why it works: The kilogram reading describes the object itself (its matter); the newton reading describes the gravitational pull on it. Mass is constant with location; weight changes because g changes. W = m×g is a proportional relationship, not a magic triangle.

   1. Place the SAME object (e.g. a 2 kg rock) on two instruments side by side: a two-pan (equal-arm) balance that compares the rock against standard kilogram masses, and a spring scale/newton meter that reads in newtons. On Earth note the rock balances 2 kg of standard masses and the spring scale reads about 20 N. (A digital scale that shows kg actually senses force and is set up for Earth's g, so on the Moon it would read too low. Only a balance that compares against standard masses gives the same reading everywhere.)
   2. Change the location from Earth to the Moon to Jupiter (via a planet selector, or by changing the value of g). Watch what each instrument does: the balance stays at 2 kg everywhere, while the spring scale changes — about 20 N on Earth, about 3 N on the Moon, about 50 N on Jupiter.
   3. Write down, for each location, which reading changed and which stayed the same, and state in words why: the amount of matter (mass, kg) never changed, but the gravitational pull on it (weight, N) did.
   4. Answer a prompt such as 'Would an astronaut float away here?' and connect your answer back to W = m×g — smaller g means smaller weight, but the same mass.
   5. Build a short ratio/double-number-line for weight across the planets to show weight is proportional to g while mass is held fixed.
2. 2

   Labelled instruments: which quantity, which unit

   Why it works: A balance and a spring scale do NOT measure the same thing: one measures the amount of matter (kg), the other the gravitational force (N). Units are not interchangeable.

   1. Examine a two-pan balance (used with standard masses) and a spring scale/newton meter side by side; for each, reveal (e.g. tap-to-reveal) the quantity it measures and its unit — two-pan balance → mass in kg; spring scale/newton meter → weight (force) in N.
   2. State in your own words why both show numbers but measure different things.
   3. Sort everyday examples by unit: which quantities belong in kg (amount of matter) and which in N (force).
3. 3

   Astronaut interview: weight, mass and 'no gravity in space'

   Why it works: Location changes weight (the gravitational pull) only by changing g, not mass (the amount of matter). Near-zero weight in deep space is different from the apparent weightlessness of free fall in orbit, where gravity is still strong; in every case mass is unchanged and 'floating' does not mean having no mass.

   1. Take part in a spoken or written exchange in the role of (or interviewing) an astronaut: 'Do you weigh less up here? What about your mass? Is there any gravity at all?'
   2. Explain whether the astronaut's mass changes on the Moon or in deep space (far from any planet or star), and whether their weight changes, giving reasons in terms of g.
   3. Address the 'there is no gravity in space' idea: in deep space, far from any large mass, the gravitational pull is tiny so weight is near zero while mass is unchanged; the Moon has gravity about a sixth of Earth's, so weight there is reduced, not zero.
   4. As an optional clarification of the 'no gravity in space' tail (kept qualitative), discuss why astronauts seen 'floating' on the International Space Station are NOT in a region without gravity: gravity there is still strong, and they float because they are in continual free fall — their mass has not changed.