Gravity, Weight and
Mass
Mass vs Weight
Mass, Weight and Gravity

Mass is a measure of matter

Mass is constant

Weight is a force

Weight is not a constant

Weight of an object depends on gravity
Calculating weight with mass and gravity

The weight of an object depends on its mass and the strength of gravity
Two meanings for ‘g’

Represents the acceleration of gravity in free fall which is 9.8m/s2
Or

Represents strength of gravity which is 9.8N/kg.

An object is weightless when it has zero gravity
Physics Problems

A 10-kilogram ball is supported at the end of a rope. How much force
(tension) is in the rope?

You are asked to find force.

You are given a mass of 10kg

W=mg

W=(10kg)(9.8N/kg)=98N

**Tension force in the rope is 98 Newtons
Physics Problems

How much would a person who weighs 490 N (110 lbs) on Earth weigh on
Jupiter? Since Jupiter may not have a surface, on means at the top of the
atmosphere. The value of g at the top of Jupiter’s atmosphere is 23 N/kg.

You are asked for the weight

You are given the weight on Earth and the strength of gravity on Jupiter.

Use W = mg

First, find the person’s mass from weight on Earth:
m = (490 N) ÷ (9.8 N/kg) = 50 kg.
Next, find the weight on Jupiter:
W = (50 kg) × (23 N/kg) = 1,150 N (259 lbs)