Mass, Weight, and Gravity
 Mass is a measure of
matter.
 Mass is constant.
 Weight is a force.
 Weight is not
constant.
Mass, Weight, and Gravity
 The weight of an
object depends on
the strength of
gravity wherever the
object is.
 The mass always
stays the same.
Calculating weight with mass
and gravity
 The weight of an object depends on its mass
and the strength of gravity.
 The formula gives the weight (Fw) in terms of the
mass of an object, m, and the strength of
gravity, g.
Two meanings for “g”
 The symbol g stands for the acceleration of gravity in
free fall, which is 9.8 m/s2.
 Another meaning for g is the strength of gravity, which is
9.8 N/kg.
 Sometimes it is more natural to discuss gravity in N/kg
instead of m/s2 because objects may not be in motion but
they still have weight.
 The two meanings for g are equivalent since a force of
9.8 N acting on a mass of 1 kg produces an acceleration
of 9.8 m/s2.
Gravity, acceleration and weightlessness
 An object is weightless when it experiences no net force
from gravity.
 If an elevator is accelerating downward at 9.8 m/sec2,
the scale in the elevator shows no force because it is
falling away from your feet at the same rate you are
falling.
Gravity, acceleration and weightlessness
 Airplane pilots and race car
drivers often describe forces
they feel from acceleration
as g forces.
 These g forces are not really
forces at all, but are created
by inertia.
 Remember, inertia is
resistance to being
accelerated.
Using weight in physics problems
 Like other forces, weight is measured in
newtons or pounds.
 Very often, weight problems involve equilibrium
where forces are balanced.
 The other common type of weight problem
involves other planets, or high altitudes, where
the strength of gravity (g) is not the same as on
Earth’s surface.
Calculating force required
to hold up an object

1.
2.
3.
4.
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 10 kilograms.
The force of the weight is Fw = mg and g = 9.8 N/kg.
The word “supported” means the ball is hanging motionless
at the end of the rope. That means the tension force in the
rope is equal and opposite to the weight of the ball.


Fw = (10 kg) × (9.8 N/kg) = 98 N.
The tension force in the rope is 98 newtons.
Calculating weight on Jupiter
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.
1.
2.
You are asked for the weight.
You are given the weight on Earth and the strength of
gravity on Jupiter.
Use Fw = mg.
First, find the person’s mass from weight on Earth:
3.
4.



m = (490 N) ÷ (9.8 N/kg) = 50 kg.
Next, find the weight on Jupiter:
Fw = (50 kg) × (23 N/kg) = 1,150 N (259 lbs)