Examine the forces
exerted on objects by
gravity
Standard IV – Objective 2
Force = Mass x Acceleration
(Newton)
Work = Force x Distance
(Joule)
Power = Work/Time (Watt)
Gravity – what is it?
Simply put, it is the
force that pulls
objects toward Earth
Universally put, it is
the mutual force
between all objects in
the universe
What affects gravity?
Gravity is affected by distance and mass
The greater the distance between objects,
the lower the force of gravity
The greater the mass of the object, the
greater its gravity
Gravity
All objects have a force that attracts them towards
each other - this is gravity
Even you attract other objects to you because of
gravity, but you have too little mass for the force to be
very strong (gravity only becomes noticeable when
there is a really massive object like a moon, planet or
star)
When you jump, the gravitational force of the Earth
pulls you down, but your gravitational force also pulls
the Earth towards you!
You just don’t notice this effect because your mass is
so much smaller than Earth’s
Mass
A measure of the amount of matter in an
object
Measured in grams or kilograms
It stays the same for an object no matter
where that object is in the universe
Weight
The force of gravity on an object at the
surface of a planet is its weight
When you step on a scale, you are
measuring the force with which Earth is
pulling you so your weight varies
throughout the universe
Measured in Newtons (1 N = 1 kg x 1
m/s2)
Gravity
Earth’s gravity pulls objects towards its center - it has a force of 9.8 N/kg
The moon has a gravity of just 1.6N/kg (as it is much smaller)
Acceleration due to gravity
9.8 m/s/s (also written as 9.8 m/s2) at Earth’s
surface
Weight = mass x acceleration due
to gravity
An apple weighs about 1 newton
A 50-kg person weighs 50 kg x 9.8m/s2 or 490
newtons (on Earth’s surface)
Free fall
When the only force acting on a falling object is
gravity, the object is in free fall
An object in free fall accelerates as it falls
A falling object falls 9.8 m/s faster every second
that it falls
Free fall continues
Therefore, after 3 seconds an object is
falling at a speed of 29.4 m/s, after 4
seconds it is falling at 39.2 m/s
All objects fall at the same rate regardless
of mass
Air resistance
• Even though all objects are supposed to fall
at the same rate, we know they do not
• All objects falling through air experience a
type of fluid friction called air resistance
• Air resistance (being friction) is opposite to
the direction the object is falling and eventually
prevents the object from accelerating faster
•The maximum velocity any object can reach in
free fall is called TERMINAL VELOCITY
More air resistance
The greater the object’s surface area,
the greater the air resistance
When an object can not accelerate
any more because of air resistance, it
is said to be at terminal velocity
In Earth’s atmosphere, leaves fall
slower than rocks
In a vacuum, they both fall at the
same rate
Projectile motion
Thrown objects are under the same gravitational
acceleration as objects falling straight down so
eventually they will fall straight down also
For example, a cannon ball shot horizontally and
one dropped vertically from the same height as
the cannon will both hit the ground at the same
time
Earth
Gravity on Earth = 9.8 N/kg
Say you have a mass of 100kg on Earth
Can you work out your weight?
Remember: weight (N) = mass (kg) x gravity (N/kg)
Your weight on Earth is 100kg x 9.8 N
Your weight = 980 N
Moon
Gravity on moon = 1.6N/kg
Say you have a mass of 100kg on Earth
What is your mass on the moon?
Remember, mass is the amount of stuff there is (you
still have two arms, two legs etc… on the moon)
Mass = 100kg on moon
What is your weight on the moon (weight = mass x
gravity)?
Your weight on the moon is 100kg x 1.6 N
Your weight = 160N
Your mass & weight
See if you can work out your mass and
weight on the Earth and on the moon
Remember:
 Gravity on Earth = 9.8 N/kg
 Gravity on the moon = 1.6 N/kg
Gravity
Complete the gravity on other
planets worksheet
Gravity
Although your mass is always
the same wherever you are,
you would weigh the most on
Jupiter, and the least on Pluto
Weight Bingo
Questions
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
On Mercury (3) the mass of 10kg =
On Venus (9) the mass of 5kg =
On Earth (10) the mass of 5kg =
On Mars (4) the mass of 4kg =
On Jupiter (23) the mass of 2kg =
On Saturn (9) the mass of 9kg =
On Uranus (8) the mass of 10kg =
On Neptune (11) the mass of 50kg =
On Pluto (0.4) the mass of 90kg =
On Mercury (3) the mass of 30kg =
On Venus (9) the mass of 15kg =
On Earth (10) the mass of 25kg =
On Mars (4) the mass of 100kg =
On Jupiter (23) the mass of 10kg =
On Saturn (9) the mass of 20kg =
30
45
50
16
46
81
80
550
36
90
135
250
400
230
180
Questions
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
On Uranus (8) the mass of 16kg =
On Neptune (11) the mass of 11kg =
On Pluto (0.4) the mass of 50kg =
On Mercury (3) the mass of 70kg =
On Venus (9) the mass of 45kg =
On Earth (10) the mass of 12kg =
On Mars (4) the mass of 60kg =
On Jupiter (23) the mass of 5kg =
On Saturn (9) the mass of 52kg =
On Uranus (8) the mass of 95kg =
On Neptune (11) the mass of 54kg =
On Pluto (0.4) the mass of 80kg =
On Mars (4) the mass of 21kg =
On Jupiter (23) the mass of 29kg =
On Venus (9) the mass of 65kg =
128
121
20
210
405
120
240
115
468
760
594
32
84
667
585