Transcript PPT

Physics 211
Lecture 4
Today's Concepts: Newton’s Laws
a) Acceleration is caused by forces
b) Force changes momentum
c) Forces always come in pairs
d) Good reference frames
Newton’s 2nd Law
Acceleration is caused by force.
A bigger mass makes this harder
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Checkpoint
The net force on a box is in the positive x direction.
Which of the following statements best describes the
motion of the box :
A) Its velocity is parallel to the x axis
B) Its acceleration parallel to the x axis
C) Both its velocity and its acceleration are parallel to the x axis
D) Neither its velocity or its acceleration need be parallel to the x
axis
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Checkpoint
The net force on a box is in the positive x direction. Which of the following
statements best describes the motion of the box :
A) Its velocity is parallel to the x axis
B) Its acceleration is parallel to the x axis
C) Both its velocity and its acceleration are parallel to the x axis
D) Neither its velocity or its acceleration need be parallel to the x axis
B) Newton's second law states that the direction
of the net force on an object is the same as the
acceleration of that object. We don't know the
initial velocity of the box
C) If the force is parallel to the x-axis then the box
will accelerate in that same direction. Therefore, the
velocity will also be in that direction..
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ACT
A force F is applied to a small block, that pushes a larger
block. The two blocks accelerate to the right. Compare the
NET FORCE on the block with mass M, to the net force on

the block with mass 5M.

 F  ma
A) FM < F5M
B) FM = F5M
C) FM > F5M
Net Force
Same acceleration, so larger
mass has larger net force.
a
F
M
5M
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Checkpoint
You are driving a car with constant speed around a horizontal circular track. The
net force acting on your car
A) Points radically inward toward the center of the circular track
B) Points radically outward, away from the center of the circular track
C) Points forward in the same direction your car is moving
D) Points backward, opposite to the direction your car is moving
E) Is zero.
Since acceleration is directed towards the
center of motion, so is the force.
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Good Question!
Please explain why, even though the force due to
centripetal acceleration points inwards, the force we
feel pushes us towards the outside edge of the
circular path, and the cell phone and dice all move
away from the center of the circle of travel.
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Good Question!
Please explain why, even though the force due to
centripetal acceleration points inwards, the force we
feel pushes us towards the outside edge of the
circular path, and the cell phone and dice all move
away from the center of the circle of travel.
An observer inside the car is in a NONINERTIAL
reference frame. We are intuitively familiar with F = ma
and that is valid only in an INERTIAL reference frame.
In a NONINERTIAL reference frame, like a rotating
reference frame, unexpected “forces” show up. These
are usually called inertial forces; sometimes, even
fictitious forces.
We will only analyze or investigate motion as seen by
INERTIAL reference frames.
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Aside: Centripetal acceleration and force
1) Objects moving in a circle always have a component
of acceleration, called centripetal, which is toward
the center of the circle.*
2) Centripetal acceleration must be caused by a force:


Friction, gravity – whatever force keeps it moving in a circle.
This force is often called the “centripetal force”
3) There is no “new” kind of force here. (Important!)
4) There is no such thing as centrifugal force.
* They can have also have tangential acceleration if their speed is not constant
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Momentum & Force
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Checkpoint
You are driving a car with constant speed around a horizontal circular track. The
momentum of your car
A) Points radially inward toward the center of the circular track
B) Points radially outward, away from the center of the circular track
C) Points forward in the same direction your car is moving
D) Points backward, opposite to the direction your car is moving
E) Is zero.
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Checkpoint Responses
You are driving a car with constant speed around a horizontal circular track. The momentum of your
car
A) Points radially inward toward the center of the circular track
B) Points radially outward, away from the center of the circular track
C) Points forward in the same direction your car is moving
D) Points backward, opposite to the direction your car is moving
E) Is zero.
Remember,
Momentum is mass x velocity,
P=mv
So momentum always point the same as velocity.
A) The momentum is in the same direction as the force.
C) It points in the same direction as v, which is forward
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Checkpoint Responses
You are driving a car with constant speed around a horizontal circular track. The momentum of your
car
A) Points radially inward toward the center of the circular track
B) Points radially outward, away from the center of the circular track
C) Points forward in the same direction your car is moving
D) Points backward, opposite to the direction your car is moving
E) Is zero.
A) The momentum is in the same direction as the force.
C) It points in the same direction as v, which is forward
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Circular Motion

v

a
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Momentum and Force


p  mv


F  ma
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Newton’s 1st Law
Demos: dishes, bear
Ice-puck
Fake Forces: Coriolis
Inertial reference frames PLEASE
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Earth’s Rotation is important over
large
And makes hurricanes
rotatedistances
CCW (CW) in the Northern
(Southern) hemisphere.
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Newton’s 3rd Law
Forces come in pairs!
Demo:Firecart
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Newton’s 3rd Law Concerns
Remember, the forces in these “pairs” act on different
objects. That is very important but is easy to
overlook.
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ACT
A small guy and a large football player moving at the same
speed collide head-on. Which person experiences the
larger force during the collision?
A) The small guy.
B) The football player.
C) They experience the same force.
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ACT
A small guy and a large football player moving at the same
speed collide head-on. Which person experiences the
larger force during the collision?
A) The small guy.
B) The football player.
C) They experience the same force.
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ACT
A small guy and a large football player moving at the same
speed collide head-on. Which person experiences the
larger acceleration during the collision?
A) The small guy.
B) The football player.
C) The accelerations are the same.
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ACT
A small guy and a large football player moving at the same
speed collide head-on. Which person experiences the
larger acceleration during the collision?
I know everything has an opposite and equal
A) The small guy.
does a bug exert that much
B) The football player. reaction but howforce
on a car.
C) The accelerations are the same.
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ACT
A small guy moving at a high speed collides with a
stationary large football player. Now, which person
experiences the larger force during the collision?
A) The small guy experiences the larger force.
B) The football player experiences the larger force.
C) Both experience the same force.
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ACT
A small guy moving at a high speed collides with a
stationary large football player. Now, which person
experiences the larger force during the collision?
A) The small guy experiences the larger force.
B) The football player experiences the larger force.
C) Both experience the same force.
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