Download Gravity - Mr. Cramer

Gravity and Weight
Mass and Weight are different.
Gravity affects weight,but not mass.
Weight is a Force.
First of all…what is gravity?
• What holds the book on
his head? Gravity.
• All objects in the
universe attract each
other through the force
of gravity.
What is mass?
Mass is the amount of matter present in an
object.
The amount of matter present does not change
with location.
Mass for our purposes, will be measured in kg.
Remember:
The formula for force: F = ma
uses mass measured in kilograms.
Mass is measured on a balance
Balances
Are used to measure mass. How much matter is
there. When a balance is used, equal
amounts of matter (same mass) balance each
other out.
Gravity does not affect mass because it acts
equally on each side of the balance.
Gravity affects Weight
Weight is actually a force.
Weight is a measure of the gravitational force
exerted by an object.
Weight is a product of mass and gravitational
force.
If gravity changes, weight changes.
If gravity changes, mass does not change.
Weight on Earth
Your weight is a reflection of the gravitational
force exerted on you by the Earth.
Because Earth is much more massive than you,
(and everything else on Earth) the
acceleration due to gravity is a constant.
Called free-fall acceleration, it equals 9.8 m/s²
on Earth.
Calculating Weight
Since F = ma
And weight (w) is a force
Weight (in Newtons) = mass (m) x free fall
acceleration due to gravity (g, = 9.8 m/s²)
SO: w = mg
Weighing in Newtons
Suppose a person weighs 50 kg
To calculate her weight in Newtons, use the
formula w = mg
w = 50 kg x 9.8 m/s²
w = 490 N (110 pounds)
How many Newtons???
A) a 5 kg bag of potatoes?
B) a 20 kg child?
C) a 35 kg dog?
D) a 140 kg football player?
How many Newtons?
A) a 5 kg bag of potatoes? 5 x 9.8 = 49 N
B) a 20 kg child? 20 x 9.8 = 196 N
C) a 35 kg dog?
35 x 9.8 = 343 N
D) a 140 kg football player? 140 x 9.8 = 1372 N
But what if g is NOT = 9.8 m/s²
Gravity is based on mass. If mass changes, the
acceleration due to gravity changes too.
Where would this happen???
On the Moon, for example.
Gravity on the Moon
is 1/6 the gravity on Earth, because the Moon’s
mass is 1/6 the mass of the Earth.
g on the moon is 1.6 m/s².
So look at those weights again…
How many Newtons on the Moon?
A) a 5 kg bag of potatoes?
B) a 20 kg child?
C) a 35 kg dog?
D) a 140 kg football player?
How many Newtons on the Moon?
A) a 5 kg bag of potatoes? 5 x 1.6 = 8 N
B) a 20 kg child? 20 x 1.6 = 32 N
C) a 35 kg dog? 35 x 1.6 = 56 N
D) a 140 kg football player? 140 x 1.6 = 224 N
What about other planets?
Planet
Mass (kg)
Value of g
Earth
5.97 x 10 ^24
9.8 m/s/s
Venus
4.87 x 10 ^ 24
8.9 m/s/s
Mars
6.42 x 10 ^23
3.7 m/s/s
Neptune
1 .02 x 10 ^26
11.0 m/s/s
Compare weights
• Earth
• 9.8 m/s²
Where would your weight
be greater?
• Venus
• 8.9 m/s²
a) Earth or Venus?
• Mars
• 3.7 m/s²
• Neptune
• 11.0 m/s²
b) Venus or Mars?
c) Mars or Neptune?
d) Neptune or Earth?
Compare Weights
• Earth
• 9.8 m/s²
Where would your weight
be greater?
• Venus
• 8.9 m/s²
a) Earth or Venus?
• Mars
• 3.7 m/s²
• Neptune
• 11.0 m/s²
b) Venus or Mars?
c) Mars or Neptune?
d) Neptune or Earth?
Summary
Mass is different from weight. Weight is a force.
g = acceleration due to gravity
Mass does not change no matter the value of g.
Weight is dependent on the value of g.
Weight = mass x g