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Vocab (5.5, 5.9) April 10th, 2007:
1.
2.
Weight: Fw ; the amount of
gravitational force that the earth
exerts on an object
Acceleration due to gravity: ag ; the
constant rate of acceleration when
an object falls; 9.8 m/s2
Acceleration due to Gravity
PS-5.5:
Explain how acceleration due
to gravity affects the velocity
of an object as it falls.
Gravity

All objects
accelerate as they
fall because Earth
continually exerts
a force
(gravitational
force) on them.
When we drop something…
It accelerates.
 The direction of the force is always
downward.
 The acceleration is in the direction of
the force so the direction of
acceleration is downward as well.

When we drop something…
It has an initial velocity of ZERO.
 It will accelerate at a constant rate of
9.8 m/s2.

– This means the object will speed up at a
constant rate of 9.8 m/s every second it
is falling (in the absence of air
resistance).
How velocity is affected…
vi
vf
1st sec
0.0
m/sec
9.8
m/sec
2nd sec
9.8
m/sec
19.6
m/sec
3rd sec
19.6
m/sec
29.4
m/sec
4th sec
29.4
m/sec
39.2
m/sec
5th sec
39.2
m/sec
49.0
m/sec
Conclusion
9.8 m/s2 IS the acceleration of
gravity.
 It’s symbol is ag.
 Since the object is accelerating
because of Earth’s gravity, the
velocity of the object continues to
increase in speed and continue in a
downward direction until it hits the
ground.

Motion Diagram for
a falling object
Practice Problems—
Show all your work!
1.
2.
3.
4.
Hallie chased the penguin for 5 meters. It took
her 15 seconds to catch it. What was her
speed?
Duncan slammed on brakes while he was
driving 50 m/s. It took him 25 seconds to stop.
What was his acceleration?
Daniela lifted her 10kg bookbag with a force of
20 N. With what acceleration did she move the
bookbag?
Robert grinds the railing of the steps with his
skateboard but missed it by a centimeter. The
board goes flying across the pavement, but
Robert magically lands on his feet. Which of
Newton’s laws does this demonstrate?
The Relationship
Between Mass and
Weight
PS-5.9 Explain the relationship
between mass and weight by using the
formula Fw = mag
The gravitational force keeps
the planets in orbit around the Sun.
The Rules of Gravity on Earth
Objects accelerate as they fall
 The only way that an object can
accelerate is for a net force to act on
it.
 What about objects in freefall?

– the net force acting on falling objects =
gravitational force exerted by Earth.
(neglecting air resistance)
– amount of force by Earth depends on the
object’s mass
Gravity on Earth: MASS
The greater the mass, the greater the
gravitational force.
 The amount of gravitational force that
the earth exerts on an object is called

weight.
Newton’s 2nd Law Rewritten
Fw = mag
Fw = Weight of object
 m = mass of the object (….kg)
 ag = acceleration of gravity for all
objects = 9.8m/s2


This formula is sometimes written as
w = mg
Examples
Austin measures the mass of an apple to
be 0.15 kg. What is it’s weight?
 What is the mass of Noell’s car if it
weighs 9800 N?
 A satellite with a mass of 230 kg
experiences a gravitational force of 1380
N. What is the acceleration due to gravity
at the altitude where the satellite is
orbiting?

Examples
A high school student has a mass of
60 kg. What will be the student’s
weight on the moon, where the
acceleration due to gravity is 1/6
that of the Earth’s gravity?
 How much gravitational force is
exerted on Joey’s bookbag, which
has a mass of 18 kg?

Calculating ag
Calculate the acceleration due to
gravity from your measurements of
Fw and mass, using the balance and
the spring scale.
 You must do this for 10 objects of
your choosing that are less than 0.4
kg (400g).
 Find an average value for ag.

Data Table
Object Description


Fw
m
ag
Organize your work in a nice summary that
includes: Question, Hypothesis, Data Table,
Answers to follow up questions, Conclusion.
You must attach a sheet which shows your
calculations in the correct format!
Follow Up Questions






Do objects accelerate as they fall?
What must act on an object for an object to
accelerate?
What is the relationship between mass and
weight?
What is the relationship between mass and
gravitational force?
If two objects have the same mass, will they
have the same weight? Why?
Suppose two identical objects were
subjected to different accelerations due to
gravity, would they still have the same
weight? Why?
Calculating Heights
• From careful measurements of time and
using ag = 9.8 m/s2, you will calculate the
height of the stadium bleachers.
• We will be going outside, so be on your best
behavior, or else you will be written up and
receive a ZERO for the lab exercise!
• You will calculate x - the height of the
bleachers. Everyone must do their own
work!
Instructions:
•
•
•
•
Copy the data table on the next slide.
Perform the procedure.
Record the results.
Knowing vi, calculate d - the height of
the bleachers. Everyone must do their
own work!
• Answer the follow up questions.
• Organize your work in a nice summary that
includes: Question, Hypothesis, Data Table,
Answers to follow up questions, Conclusion.
Data Table
ag (m/s2)
t (s)
vi (m/s)
x (m)
Follow Up Questions
• Draw a motion diagram of the ball falling, include
scaled arrows to indicate changes in velocity.
• How does the velocity and displacement of an object
vary with time as the object falls? Be specific.
• Explain what it means when we say that the
acceleration due to gravity is constant.
• Suppose the true height of the bleachers was 1 meter
taller than what you calculated, what could account
for this error?
• How could a skydiver calculate the height of the
airplane he jumped from?