Name(s) ______________________________________________________ Hr. _____
Physics: Practicing Calculating Weight
Calculating Weight is a very important concept in physics. In our country, forces (including weight) is
measured in pounds (lbs.). However, in the metric system, forces are measured in a unit called the
NEWTON (N). We need to be able to go back and force between our system and the Metric system.
Weight (in Newtons) is a product of the mass of the object (in kilograms) times the acceleration due to
gravity (9.8 m/s/s on the Earth). The weight vector is always directed downward.
W = m∙g
Helpful conversions and equation:
 1000 grams = 1 kilogram
 4.45 N = 1 lb (pound)
 2.2 lbs = 1 kg (YOU CAN ONLY use this conversion on the Earth!)
1. I weigh around 165 lbs on the Earth. Determine (a) my weight in
Newtons and (b) my mass in kilograms.
(answers: I weigh around 734.25 Newtons and have a mass of around 74.9 kilograms)
2. The new Martian rover, Curiosity, has a mass of around
900 kg.
(a) What is the weight of Curiosity on the Earth (in Newtons)?
(b) What is the weight of Curiosity on the Mars (in Newtons) if
the gravity (‘g’) of Mars is 3.6 m/s/s?
(c) How many pounds would it weigh on Mars?
(d) Its mass would not change from the Earth to Mars? Why not?
(answers: Curiosity weighs 8820 Newtons on the Earth and a weight of 3240 Newtons on Mars. On Mars it would weigh 728 lbs. Mass stays the
same regardless of the planet because mass is based on the atoms and molecules and this doesn't change. Weight does change because weight is
based on the product of mass and the gravity of the planet. Every planet has a different gravity.)
Practice calculating weight, p. 1
3. My Honda fit has a weight of 2540
lbs.
(a) What is its weight (on Earth) in Newtons?
(b) What is its mass in kilograms?
(answers: My car has a weight of 11,303 Newtons on the Earth and a mass of 1153.4 kg)
4. This medicine ball has pounds and kilograms:
(a)
Are the numbers correct? Is 15.4 lbs the same as 7
kgs?
(b)
What would be the mass of the ball on the moon?
(c)
Change the mass to grams.
(answers: these numbers are correct at least if you round; Its mass is 7 kgs anywhere. In grams, 7000 grams. To go from kilograms to
grams, multiply by 1000.)
5. The spring scale on the right shows the weight of 1 kilogram on
the Earth. What would the scale say on the Moon and on
Mars? The acceleration of gravity on the moon is 1.6 m/s/s and
on Mars is 3.6 m/s/s.
(answers: On the Earth, this one kg mass weighs 9.8; on the moon it weighs 1.6 Newtons and on Mars, it weighs
3.6 Newtons.)
Practice calculating weight, p. 2