prof. paul steinhardt - princeton universitysteinh/ph115/slides_10_3_19.pdflet’s exercise...
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“Physics for FUTURE LEADERS”Prof. Paul Steinhardt
PLEASE DON’T SIT IN THE LAST 6 ROWS
Physics 115
Princeton in the Nation’s ServiceWoodrow Wilson, 1896
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3. Force and Motion
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Why do things move?
Aristotle:All motion requires forces.Forces. Without forces, objects tend to stop.
Newton’s First Law:Only CHANGES from UNIFORM MOTION
(same speed & direction) require a net forceWithout forces, objects tend to keep moving
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Let’s exercise Newton’s First Law a bit:
I drive my car in a straight line, engines blasting,at 100 mph across rough terrain with a
strong wind in my face
A) There is a net force acting on the car
B) There is no net force acting on the car
C) More information is needed to decideif there is a net force acting on it
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Let’s exercise Newton’s First Law a bit:
I drive my car around theIndianapolis Motor Speedway
at a constant 200 mph
A) There is a net force acting on the car
B) There is no net force acting on the car
C) More information is needed to decideif there is a net force acting on it
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Let’s exercise Newton’s First Law a bit:
We have concluded that there is a net force actingon the block after I push it. Same with the cart.
What does Newton’s First Law tell me about the force?
A) It must be friction between the cart and floor
B) It must be air resistance
C) It must be air resistance, friction & gravity combined
D) It tells me nothing about the nature of the force
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Why do things move?
Aristotle:All motion requires force
Newton’s First Law:CHANGES from UNIFORM MOTION
(any change in speed or direction) require a net force
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Newton’s 2nd Law
F!
FORCE
= m !a
CHANGE FROMUNIFORM MOTION
Mass = resistance to change
vmt
D=
D
!
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“To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.”
1 2
Newton’s 3rd Law
F1 on 2 = – F2 on 1
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Demo: Skateboard and Cart
If I push on the cart, the cart pushes backSo, how can I ever move the cart?
My force
Opposite reaction
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Demo: Cart
Key Point: The action and reaction forces act on different objects
My force
Opposite reaction
Separate into two different objects
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Demo: Cart
Key Point: The action and reaction forces act on different objects
My force
Opposite reaction
So, now we see there is a net non-zero force on the cart and it moves,But what about me?
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Demo: Cart
Key Point: The action and reaction forces act on different objects
My force
Opposite reaction
friction
So, now we see there is NO net zero force on meBut what about the ground?
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Demo: Cart
Key Point: The action and reaction forces act on different objects
My force
Opposite reaction
friction
Force due to my feet pushing on the ground
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Princeton University, Physics Department and Council on Science and Technology, 2012. Adapted from Interactive Lecture Demonstrations by Sokoloff and Thornton
Through these demonstrations, we aim to practice: • using experiments to refine our understanding of motion. • using graphs to interpret and describe quantitative relationships and rates of change. • applying physical laws to make predictions.
Demonstration 1: Predict the position vs. time graph and the velocity vs. time graph for a cart with no net force on it. (The cart is given a small tap to set it in motion away from the motion detector and then released. Start your graph after the tap.) Use a dashed line for your predicted graph. After the experiment, sketch the observed graphs. Use a solid line or a different color pen/pencil. Demonstration 2: Predict the position vs. time graph and the velocity vs. time graph for a cart starting from rest and pushed away from the motion sensor by a constant force. (Start your graph immediately after the cart is released from rest.) Use a dashed line for your predicted graph. After the experiment, sketch the observed graphs. Use a solid line or a different color pen/pencil.
GRAPHING MOTION DUE TO A CONSTANT FORCE PHY 115
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Question: Is there a net force on the apple as it falls?How do you know?
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~ 10 m/s2
“Constant” vertical force: Near Earth Gravity
Fgravity = mapple
!g
|!g |
mapple (kg)× g (m / s2 ) = F (newtons)
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Question: What is the acceleration?
=mapple!aFgravity =mapple
!g
!g = !a