Physics 115
“Physics for FUTURE LEADERS”
Prof. Paul Steinhardt
Princeton in the Nation’s Service
Woodrow Wilson, 1896
3. Force and Motion
Why do things move?
Aristotle:
All motion requires net force.
Newton’s First Law:
Only CHANGES from UNIFORM MOTION
(same speed & direction) require a net force
Let’s exercise Newton’s First Law a bit:
I drive my car in a straight line, engines blasting,
at 100 mph across a dirt road 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 decide
if there is a net force acting on it
Let’s exercise Newton’s First Law a bit:
I drive my car around the
Indianapolis 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 decide
if there is a net force acting on it
Let’s exercise Newton’s First Law a bit:
We have concluded that there is a net force acting
on 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
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
Newton’s 2nd Law
!
F
FORCE
=
!
!
Δv
ma=m
Δt
CHANGE FROM
UNIFORM MOTION
Mass = resistance to change
Newton’s 3rd Law
“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.”
F1 on 2 = – F2 on 1
2
1
Demo: Skateboard and Cart
If I push on the cart, the cart pushes back
So, how can I ever move the cart?
My force
Opposite reaction
Demo: Cart
Key Point: The action and reaction forces
act on different objects
My force
Opposite reaction
Separate into two different objects
Demo: Cart
Key Point: The action and reaction forces
act on different objects
Opposite reaction
My force
So, now we see there is a net non-zero force on the cart and it moves,
But what about me?
Demo: Cart
Key Point: The action and reaction forces
act on different objects
Opposite reaction
My force
friction
So, now we see there is NO net zero force on me
But what about the ground?
Demo: Cart
Key Point: The action and reaction forces
act on different objects
Opposite reaction
My force
friction
Force due to my feet pushing on the ground
Newton’s 2nd Law
What do Newton’s Laws tell us about
!
F
FORCE
=
!
F
?
!
!
Δv
ma=m
Δt
CHANGE FROM
UNIFORM MOTION
Mass = resistance to change
GRAPHING MOTION DUE TO A CONSTANT FORCE
PHY 115
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.