force

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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
Covers Units 1-3 plus “Math”
One-dimensional Kinematics
Two-Dimensional Kinematics
Relative and Circular Motion
Unit conversion
Trigonometry and Algebra
50 minute duration
Multiple choice
Review on Wednesday
Unit 4 will resume on Monday
Mechanics Lecture 4, Slide 2
Sir Isaac Newton and his 3 Laws
Mechanics Lecture 4, Slide 3
Main Points
Mechanics Lecture 4, Slide 4
Main Points
Mechanics Lecture 4, Slide 5
Main Points
Mechanics Lecture 4, Slide 6
Lecture Thoughts
Mechanics Lecture 4, Slide 7
Let’s start with Newton’s 2nd Law
Mechanics Lecture 4, Slide 8
Newton’s 2nd 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 2nd 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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Mechanics Lecture 4, Slide 12
Pre-lecture Question 1
Mechanics Lecture 4, Slide 13
Unit of Force
Kinematics units
Dynamics unit
Mechanics Lecture 4, Slide 14
Momentum
Mechanics Lecture 4, Slide 15
Spaceship example
Mechanics Lecture 4, Slide 16
Newton’s First law
Mechanics Lecture 4, Slide 17
Inertial Reference Frame
Mechanics Lecture 4, Slide 18
Newton’s 3rd Law
Mechanics Lecture 4, Slide 19
Newton’s 3rd Law
The motion of an
object is affected
by only the
forces acting
upon it.
Mechanics Lecture 4, Slide 20
Prelecture Question 2
A.
B.
C.
D.
E.
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Mechanics Lecture 4, Slide 21
Prelecture Question 2
Mechanics Lecture 4, Slide 22
Checkpoint 1
A.
B.
C.
D.
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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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.
B.
C.
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.
A) FM < F5M
B) FM = F5M
C) FM > F5M
a
F
M
5M
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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 5M.

F
A) FM < F5M
B) FM = F5M
C) FM > F5M
 ma
Net Force
Same acceleration, so larger
mass has larger net force.
a
F
M
5M
Mechanics Lecture 4, Slide 26
Mechanics Lecture 4, Slide 27
Mechanics Lecture 4, Slide 28
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
Mechanics Lecture 4, Slide 30
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
Mechanics Lecture 4, Slide 35
Checkpoint Momementum
You are driving a car with constant speed around a horizontal circular track. The momentum of your
car
A.
B.
C.
D.
E.
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Mechanics Lecture 4, Slide 36
Momentum & Centripetal Acceleration

v

a
Mechanics Lecture 4, Slide 38
Momentum & Centripetal Acceleration


p  mv


F  ma
Mechanics Lecture 4, Slide 39
Newton’s 1st Law
Ice-puck
Fake Forces: Coriolis (YouTube)
Mechanics Lecture 4, Slide 40
This effect can
make cannons
miss their target
if you don’t take
it into account.
Hurricane Earl
And makes hurricanes
rotate CCW (CW) in the
Northern (Southern)
hemisphere.
Mechanics
Lecture
Slide 41
41
Physics
211 Lecture
4,4,Slide
Newton’s 3rd Law
Forces come in pairs!
Fire-cart
Mechanics Lecture 4, Slide 42
Clicker Question
A.
B.
C.
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) The small guy.
B) The football player.
C) They experience the same force.
Mechanics Lecture 4, Slide 44
Mechanics Lecture 4, Slide 45
Main Points
Mechanics Lecture 4, Slide 46
Main Points
Mechanics Lecture 4, Slide 47
Main Points
Mechanics Lecture 4, Slide 48

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