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According to Newton’s
If the force is
held constant
the relationship between
mass and acceleration is
direct/inverse.
If the mass is
held constant
the relationship between
force and acceleration is
direct/inverse.
If the acceleration is
held constant
the relationship between
mass and force is
direct/inverse.
nd
2
Law
In this situation . . .
video
Mass
• Amount of matter in an
object
• Is always constant at any
place and time
• Is measured in kilograms
• Is measured using a balance
• Is an intrinsic property of an
object and is independent of
any external factor
Weight
• Force which a given mass
feels due to the gravity at its
place
• Depends on gravity at the
place
• Is measured in Newtons
• Is measured using a scale
• Depends on:
– Mass of the object
– Mass of the object that is
attracting it
To calculate weight:
When you know the mass use ΣF = ma
If standing on the Earth (no friction force),
acceleration due to gravity is 9.8 m/sec/sec
Mass = 65.0 kg
Weight on earth?
Weight on the moon?
Weight on Jupiter?
Free Fall
Falling without air resistance
∑F=ma
Gravity is the only force acting upon the
object causing the object to change it’s
velocity
 Acceleration (change in velocity) due to
gravity on earth is 9.8 m/sec/sec for all objects
That means . . . When an object is in free fall it
will be increasing its velocity 9.8 m/sec every
second
Videos – what will hit the
ground first; freefall on
moon
a = 9.8 m/sec/sec
Free Fall neglecting friction – which
object – the elephant or the featherwill hit the ground first
True or false:
 The elephant and the feather each have the same
force of gravity
 The elephant has the greatest acceleration
(change in velocity)
 On earth, all objects (whether an elephant or a
feather) have the same force of gravity
 The elephant and the feather have the same
acceleration (change in velocity) due to gravity
Free fall cont.
 In the absence of air resistance, the elephant
and the feather land at the same time.
 The force of gravity experienced by an object
is dependent upon the mass of that object.
RULE: all objects (regardless of their mass)
experience the same acceleration
(change in velocity) when in a
state of free fall
Free Fall
Draw a velocity vs. time graph
v
Draw an acceleration vs. time
graph
a
t
t
-9.8 m/s/s
Slope = -9.8 m/s/s
Acceleration of fall is less when
air resistance (drag) acts
Air resistance is the result of collisions of
the object's leading surface with air
∑F=ma
molecules.
Fluid friction (air drag) depends on two
things: surface area & speed
The greater the surface area, the greater the
friction
As an object falls, the friction increases as
the speed increases
Falling with Air Resistance
Terminal Velocity of Tennis Ball
Draw a velocity vs. time graph
v
Draw an acceleration vs. time
graph
a
t
t
Which object - the elephant or the
feather - will hit the ground first?
True or false:
 The elephant encounters a smaller force of air
resistance than the feather and therefore falls
faster.
 The elephant has a greater acceleration (change in
velocity) due to gravity than the feather and
therefore falls faster.
 Both elephant and feather have the same force
of gravity, yet the feather experiences a greater
air resistance.
 The feather has a smaller force of gravity so
friction equals the force of gravity sooner
causing a net force of zero (no acceleration);
whereas the elephant has a net force that is not
zero and will continue to speed up.
What is the acceleration for A?
What is the acceleration for B?
What is the acceleration for C?
What is the acceleration for D?
Lab 4 Air Resistance
• Purpose: Does changing the mass of a falling object affect
the velocities & accelerations observed on the motion graphs?
• Method: State independent, dependent and controlled
variables
• Materials: motion detector on ceiling; laptop; coffee filters
• Hypothesis:
• Procedure:
• Data: 5 different velocity vs. time graphs (copied into a word
document)
• Analysis: analyze motion on graphs and label with
descriptions of motion (ie where is the terminal velocity, etc)
• Conclusion: Write a concluding paragraph. Include a
discussion of the relationship between mass and terminal
velocity. Be sure to include data to back up your statements!
Which encounters the greater force of air
resistance—a falling elephant or a falling
feather?
Two smooth balls exactly the same size, one made of
wood and the other made of iron, are dropped from a
high building to the ground below. The ball to
encounter the greatest force of air resistance on the
way down is the:
a) wooden ball
b) iron ball
c) ... both the same
As she falls faster and faster through the air,
her acceleration (change in velocity)
a) increases
b) decreases
c) remains the same
What will be the acceleration (change in velocity)
of a rock thrown straight upward at the moment it
reaches the tippity-top of its trajectory (no air)?
End of Chapter Exercises
1.
2.
3.
4.
5.
6.
7.
What is the net force on a bright red Mercedes convertible traveling along
a straight road at a steady speed of 100 km/h?
On a long alley a bowling ball slows down as it rolls. Is any horizontal
force acting on the ball? How do you know?
Your empty hand is not hurt when it bangs lightly against a wall. Why is it
hurt if it does so while carrying a heavy load? Which of Newton's laws is
most applicable here?
When a junked car is crushed into a compact cube, does its mass change?
Its weight? Its volume? Explain.
What is the net force on a 1-N apple when you hold it at rest above your
head? What is the net force on it after you release it?
If it takes 1 N to push horizontally on your book to make it slide at
constant velocity, how much force of friction acts on the book?
A crate remains at rest on a factory floor while you push on it with a
horizontal force F. How big is the friction force exerted on the crate by
the floor?
1. Two basketballs are dropped from a high building through the
air. One ball is hollow and the other filled with rocks. Explain
in detail the fall of the two balls.
2. A parachutist, after opening the chute, finds herself gently
floating downward, no longer gaining speed. She feels the
upward pull of the harness, while gravity pulls her down.
Which of these two forces is greater? Or are they equal in
magnitude?
3. Why will a sheet of paper fall slower than one that is wadded
into a ball?
4. How does the gravitational force on a falling body compare
with the air resistance it encounters before it reaches terminal
velocity? After?
5. Why is it that a cat that accidentally falls from the top of a 50story building hits the ground no faster than if it falls from the
20th story?
Chapter 3 Review
1. A bear that weighs 4000 N grasps a vertical tree and
slides down at constant velocity. What is the friction
force that acts on the bear?
2. How does the force of gravity on a raindrop compare with
the air drag it encounters when it falls at constant
velocity?
3. Upon which will air resistance be greater; a sheet of
falling paper or the same paper wadded into a ball if they
have both reached terminal velocity? (Careful!)
4. Aristotle claimed the speed of a falling object depends on
its weight. We now know that objects in free fall,
whatever their weights, undergo the same gain in speed.
Why does weight not affect acceleration?