Classical Mechanics
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
Mechanics Lecture 4, Slide 1

First Midterm Exam on Friday
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 One-dimensional Kinematics
 Two-Dimensional Kinematics
 Relative and Circular Motion
 Unit conversion
 Trigonometry and Algebra
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

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Mechanics Lecture 4, Slide 2

Sir Isaac Newton and his 3 Laws
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Main Points
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Main Points
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Main Points
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Lecture Thoughts
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Let’s start with Newton’s 2 nd Law
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Newton’s 2 nd Law
Acceleration is caused by force.
A bigger mass makes this harder
Mechanics Lecture 4, Slide 9

Force is a Vector Quantity
Force is a vector. Acceleration vector is parallel to (same direction) as (Net) force vector.
Mechanics Lecture 4, Slide 10

Newton’s 2 nd Law
Acceleration is caused by force.
A bigger mass makes this harder
Mechanics Lecture 4, Slide 11

Pre-lecture Question 1 A.
B.
C.
D.
E.
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Pre-lecture Question 1
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Unit of Force
Kinematics units
Dynamics unit
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Momentum
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Spaceship example
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Newton’s First law
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Inertial Reference Frame
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Newton’s 3 rd Law
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Newton’s 3 rd Law
upon
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C.
D.
A.
B.
E.
Prelecture Question 2
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Prelecture Question 2
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Checkpoint 1
The net force on a box is in the positive x direction.
Which of the following statements best describes the motion of the box :
C.
D.
A.
B.
A) Its velocity is parallel to the x axis
B) Its acceleration parallel to the x axis
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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
Mechanics Lecture 4, Slide 23

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
B) Net force causes acceleration, but it does not necessarily say anything about the direction of the velocity.
Mechanics Lecture 4, Slide 24

Force on Two Masses
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 5 M .
A.
B.
C.
A) F
M
< F
5 M
B) F
M
C) F
M
= F
5 M
> F
5 M a
F
M
5 M
0% 0% 0%
Mechanics Lecture 4, Slide 25

Force on Two Masses
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 5 M .


F
 m
 a
A) F
M
< F
5 M
B) F
M
C) F
M
= F
5 M
> F
5 M
Net Force
Same acceleration, so larger mass has larger net force.
a
F
M
5 M
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Checkpoint A.
B.
C.
D.
You are driving a car with constant speed around a horizontal circular track. The net force acting on 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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Mechanics Lecture 4, Slide 29

CheckPoint
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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.
4) There is no such thing as centrifugal force.
*
They can have also have tangential acceleration if their speed is not constant
Mechanics Lecture 4, Slide 32

Momentum & Force
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Checkpoint Momementum
You are driving a car with constant speed around a horizontal circular track. The momentum of your car
C.
D.
A.
B.
E.
0% 0% 0% 0% 0%
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 v
Momentum & Centripetal Acceleration
 a
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Momentum & Centripetal Acceleration
 p
 m v

F

 m a

Mechanics Lecture 4, Slide 39

Newton’s 1 st Law
Ice-puck Fake Forces: Coriolis (YouTube)
Mechanics Lecture 4, Slide 40

And makes hurricanes rotate CCW (CW) in the
Northern (Southern) hemisphere.
This effect can make cannons miss their target if you don’t take it into account.
Hurricane Earl

Newton’s 3 rd Law
Forces come in pairs!
Fire-cart
Mechanics Lecture 4, Slide 42

Clicker Question
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?
33% 33% 33%
A.
B.
C.
A) The small guy.
B) The football player.
C) They experience the same force.
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Main Points
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Main Points
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Main Points
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