Lesson 3

1 What did you do last time? 2 newton spring inertia force push pull PRINT the word next
to the image on the next slide.
PH-lesson L3 pus for ine pul spr new
4 Why is FORCE measured in Newtons? An Englishman named Sir Isaac Newton
was a “founding father” of physics.

As the story goes, Newton, a young scientist, was staying isolated in the country to avoid being exposed to the plague of 1665.

He was relaxing under a tree in an apple orchard enjoying the fresh country air
when he saw a ripe apple fall off a branch
on to the ground.
5 What would you think about if you saw an apple fall? Perhaps it would depend on what was going through your mind at the time. If you were hungry and liked apples, you might think, “Mmmm, I’ll take a bite.” 6 Newton had been thinking about
the mysterious FORCE that holds
the Moon in orbit around the Earth.

With this in mind, Newton wondered
if the FORCE that holds the Moon
in orbit is the same FORCE that makes the apple fall to the ground?

Newton’s pondering of this
question eventually led to his answer:
Every apple is attracted to every other apple. The FORCE of Gravity Newton described gravity as the force of attraction between any two objects in the universe. Every apple is attracted to the Earth. 8 The greater the mass of the object,
the greater the FORCE of ATTRACTION.
The Earth has a very LARGE mass.
It exerts a large FORCE of ATTRACTION
The massive Earth pulls everything DOWN towards its center.
This pull of the Earth on all objects with a mass is called gravity!
The pull of gravity is why we do not float away.
on apples, on you and on everything else nearby.
9 Gravity 10 When you measure your weight on a bath scale,
you are measuring the pull of the Earth's gravity on YOU!
You measure
the pull of the Earth's gravity
on you in POUNDS.
Would you weigh
the same on the Moon,
as you do on the Earth
No, you will WEIGH LESS on the Moon
than on Earth.

The Earth is bigger and
more massive than the Moon.
So the Earth's gravity will be greater.

You weigh about SIX times more
on Earth than on the Moon!
Click for
11 On which planet would
you weigh the most
What is the MOST massive
object in the solar system
Our star - the Sun Jupiter
12 Your Weight on Other Planets Your weight on another planet =
your weight on Earth x mass-of-the-planet
divided by the planet's radius2
13 Your Weight on Moons and on Types of Stars On which of these moons or type of star would you weigh the most and why a neutron star: because this type of collapsed star has a very small radius. a red giant: because this type of aging star has a very large radius! On which moon or type of star would you weigh the least and why 14 If the gravity of the Earth is so much greater than the gravity of the Moon, why don’t all
the rocks fly off the Moon and hit the Earth?

The distance between two objects
affects the PULL of Gravity.

The greater the distance between
two objects, the less the PULL of Gravity.

The distance between the Earth and Moon
is very large. The Moon rocks are just
too far away to be pulled down to Earth!
15 SIMULATION: Gravity, Mass and Distance 1. Does the force of gravitational attraction between the balls
get bigger, as you increase
the mass of one of the balls?

2. Does the smaller ball feel
a greater force of gravitational attraction than the larger ball?

3. As the distance between balls decreases, how does the force of gravitational attraction change?
FIRST click
on Electra.
THEN select
the red button.
WATCH how the black arrow
above each ball changes.
1. Yes, the greater the mass of either ball,
the greater the attraction between them.
2. NO, the force of attraction between balls
is equal. It's proportional to m1 X m2.
3. Increases: the closer the balls are together,
the greater the force of gravity.
16 Gravity and Distance 17 You will need: The Effects of Gravity book rubber duck If you were to drop the duck and the book at the same time,
which will hit the ground first, the duck or the book
TEST IT! Hold them
in the air
at exactly the
same height.
18 Both hit the ground
at exactly the same time!
Drop them at the same time,
from the same height.
2 3 Find two identical sheets of paper. Drop them at the same time,
from the same height.

What happens and WHY
Crumple one sheet into a tight ball. 4 5
19 What is another name for this famous tower The LEANING Tower of Pisa! Prior to Galileo’s experiments with this very question in the 1590s,
many people believed that
the larger object would land first.

Galileo insisted on TESTing IT!
That is exactly what you just did.

Galileo is said to have dropped his
test objects from the Tower of Pisa.
20 Balancing with Gravity Here is a balancing trick based on
an object's center of gravity.
Find the clay and
the clown in your kit.
Color the clown (optional). 1 2 Can you balance the clown on its nose
from the tip of your finger?
3 Cut out the clown. Find crayons and scissors, too. clay clown
21 4 5 Now balance the clown
on your finger!
Try the tip of your nose!
Here is a trick that makes it possible to balance the clown. The clay changed the distribution of the weight on the clown.
You have changed the clown’s CENTER of gravity.
Stick a piece of clay
on each hand
of the clown.
22 Here is one version of the famous
Balancing on the Tip of a Pencil Trick.
Balancing tricks often work by changing the center of gravity. You can balance a quarter
on the tip of a pencil
by lowering its
center of gravity
with two forks.
Find the four items. 1 2
23 There are many balancing tricks
based on the CENTER of GRAVITY.
Mr. Wizard Explains Let's measure the center of gravity of a truck!
24 The center of gravity
of a paper truck, you will need:
Unfold the paper clip. Tape
the string
to the coin.
1 2 from your kit Tie the string
to the paper clip.
the truck.
in your desk
or around
your home
25 4 5 Use the paper clip
to poke a hole near
a corner of the truck.

Hang the truck
from the paper clip.

Let the weighted
string come to rest.

Carefully mark
where the string
makes a line.
Choose another
corner on the truck.
Repeat step 5.
Use a ruler
and pencil
to draw the line
made by the string.
26 Use a ruler
and pencil
to mark the second line.
If the truck does not balance,
you may not have found the exact center.
Move the tack until you pinpoint the true center of gravity.
7 8 9 A point where the entire weight of an object is IMAGINED
to be concentrated. If supported at this point,
the body remains balanced.
Put a tack through the center of gravity and
the truck should balance.
The point where the
two lines cross is the
27 Measuring
the Center of Gravity
Measuring the Center of Gravity Mr Wizard - a coat hanger and the center of gravity A youtube video on seat belts
28 An object in motion tends to WHAT An object at rest
tends to WHAT
How does the distance
between objects affect
gravitational attraction
Does the mass of two
objects affect the force
of gravitational attraction
1 2 3 4 5 An object at rest stays at rest unless acted upon by The greater the MULTIPLE of the
two masses, the greater the force.
stay in MOTION stay at REST The closer they are together, the greater the gravitational attraction. an unbalanced force