Chapter-04

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Transcript Chapter-04

Uniform Motion
The stretch or compression of a spring can be used to
measure a force. If the cart is at rest, a force is needed
to make the cart speed up.
If you stop pulling the cart, what will it do if there is no
friction between the cart and track?
Canonical Spaceship
Observation
If opposite thrusters fire, what is the net force?
Poll
Suppose that the spaceship is speeding up as it
moves to the right, when all of its thrusters turn off.
The spaceship will
1. immediately stop
2. slow down until it comes to a stop and then
remain stopped.
3. slow down until it stops, changes direction,
and speeds up while moving to the left.
4. continue to move to the right with the same
velocity as when the thrusters were turned
off.
Poll
In all possible cases that you could investigate, which
statement best describes the results of your
investigations?
1. If the net force on the spaceship is zero, it will
move in a straight line with a constant speed or it
will be at rest and will remain at rest.
2. If the net force on the spaceship is zero, then it
will remain at rest, or if it is moving, it will slow
down until it is at rest.
3. The net force on the spaceship is zero only
when it is at rest.
Poll
Moving objects left the traces shown
at left. The dots were laid down at
equal time intervals. For which
object(s) is the net force on the
object zero?
1) A
2) B
3) C
4) D
5) A and D
6) A and B
7) A, B, and D
Newton’s first law
A
In the absence of interactions, an object will remain at
rest or move in a straight line with a constant speed
(i.e. uniform motion).
2
1
Initial
direction
3
4
A spacecraft initially drifts in the direction shown. Exhaust
jets on the rocket (1, 2, 3, and 4) may be fired singly or in
combination to alter the rocket’s motion. If exhaust jet 2 is
turned on at instant t1 and turned off at instant t2, which
diagram most accurately shows the spacecraft’s path?
A.
B.
C.
D.
2
Initial
direction
3
1
4
A space module initially drifts in the direction shown.
Exhaust jets on the module (1, 2, 3, and 4) may be fired
singly or in combination to alter the module’s motion.
Starting with the original motion, what jet or combination
of jets would you have to turn on if you wish the module’s
velocity to end up directed toward the top of the screen?
A. 2
D. 2 and 4
B. 4
E. 3 and 4
C. 2 and 3
Interactions
A force is one side of an interaction. When two objects
interact, they exert forces on each other.
A
B
Newton’s third law says that these forces are equal in
magnitude and opposite in direction.
Demo
A
B
Force is a measure of the strength of the interaction.
What if they have different masses?
The carts exert equal and opposite forces on each
other, as is true in any interaction.
Newton’s third law does not depend on their masses
or motion. It just tells us about the “two-sided” nature
of an interaction.
Poll
A
B
If Cart A has twice the mass of Cart B, which cart
exerts a larger magnitude force on the other
during the collision?
1. Cart A
2. Cart B
3. neither, because they exert equal magnitude forces
on each other.
Poll
A
B
If Cart A is moving with twice the speed as Cart B
when they make a head-on collision, which cart
exerts a larger magnitude force on the other
during the collision?
1. Cart A
2. Cart B
3. neither, because they exert equal magnitude forces
on each other.
Poll
A certain ball is in free-fall. As it falls, Earth exerts
a constant downward force of 5 N on the ball.
What is the force of the ball on Earth?
1. zero; (the ball exerts a negligible, almost zero, force
on Earth)
2. a small, non-zero, upward force.
3. 5 N, upward
4. an upward force that is greater than 5 N.
Demo
A
B
Force is a measure of the strength of the interaction.
The carts exert equal and opposite forces on each
other, as is true in any interaction.
Which cart has a bigger acceleration as a result of the
collision?
Inertial Mass
A
B
m is inertial mass.
For a given interaction, a smaller mass will have a
larger acceleration.
Net Force
It’s the net interaction that determines motion.
A
B
A
Newton’s second law
It’s the net interaction that causes an object to change velocity.
A
B
A
Identifying forces
To identify all forces acting on an object, ask yourself the
following:
1. What objects are “touching” the given object?
2. What objects exert a force through “action at a
distance” such as a gravitational, electrostatic, or
magnetic force?
Gravitational Force (i.e. weight)
Near the surface of Earth:
Example
A girl pulls her red wagon in the +x direction with a
force of 50 N at an angle of 30 with respect to the
horizontal. List the forces that are exerted on the
wagon?
Example
A girl pulls her red wagon in the +x direction with a
force of 50 N at an angle of 30 with respect to the
horizontal. If the wagon moves at a constant velocity,
what is the net force on the wagon?
Summary
1. Newton’s first law: (equilibrium)
If the net force on an object is zero, then the object will
remain at rest or will move with a constant speed in a
straight line (uniform motion).
2. Newton’s second law: (accelerating motion)
The net force on an object will cause an object to
accelerate with an acceleration equal to the net force on
the object divided by its mass.
3. Newton’s third law: (interactions)
When two objects interact, they exert equal magnitude
forces on each other, in opposite directions. (This says
nothing about their motion or the results of these
forces.)
A swimmer pushes off
horizontally from a boat.
She then repeats the same
action, pushing off exactly as
hard as before, but meanwhile
a load with twice the mass of
the boat has been placed in the
boat.
a. The swimmer’s acceleration on the second try is
___________ her acceleration on the first.
A. less than
B. equal to
C. greater than
A swimmer pushes off
horizontally from a boat.
She then repeats the same
action, pushing off exactly as
hard as before, but meanwhile
a load with twice the mass of
the boat has been placed in the
boat.
b. The boat’s acceleration on the second try is __________
its acceleration on the first.
A. less than
B. equal to
C. greater than
The object or objects that exert forces on the book in the situation
shown here are ___________ .
A. the table and gravity
B. the table and the Earth
C. only gravity
D. only the Earth
E. the Earth and gravity
If the hand is causing blocks A and B to
accelerate, the force that A exerts on B is
_______ equal and opposite to the force that
B exerts on A.
A. always
B. sometimes
C. never
If the hand is causing blocks A and B to
accelerate, the force that A exerts on B is
_______ equal to the force that the hand
exerts on A.
A. always
B. sometimes
C. never