Uniform Motion

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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?
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Canonical Spaceship
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Observation
If opposite thrusters fire, what is the net force?
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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.

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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.

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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
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Newtons 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).
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1
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Initial direction
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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 rockets motion. If exhaust jet 2 is turned
on at instant t1 and turned off at instant t2,
which diagram most accurately shows the
spacecrafts path?
A. B. C.
D.
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2
1
3
Initial direction
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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 modules motion. Starting with the original
motion, what jet or combination of jets would you
have to turn on if you wish the modules velocity
to end up directed toward the top of the screen?
A. 2 B. 4 C. 2 and 3
D. 2 and 4 E. 3 and 4
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Interactions
A force is one side of an interaction. When two
objects interact, they exert forces on each other.
A
B
Newtons third law says that these forces are
equal in magnitude and opposite in direction.
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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. Newtons third law does not depend
on their masses or motion. It just tells us about
the two-sided nature of an interaction.
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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.

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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.

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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.

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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?
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Inertial Mass
A
B
m is inertial mass. For a given interaction, a
smaller mass will have a larger acceleration.
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Net Force
Its the net interaction that determines motion.
A
B
A
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Newtons second law
Its the net interaction that causes an object to
change velocity.
A
B
A
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Identifying forces

• To identify all forces acting on an object, ask
  yourself the following
• What objects are touching the given object?
• What objects exert a force through action at a
  distance such as a gravitational, electrostatic,
  or magnetic force?

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Gravitational Force (i.e. weight)
Near the surface of Earth
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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?
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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?
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Summary

1. Newtons 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. Newtons 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. Newtons 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.)

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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
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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
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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