“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

3. Force and Motion

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

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

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

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

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

= m !a

CHANGE FROMUNIFORM MOTION

Mass = resistance to change

vmt

D=

D

!

“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

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

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

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?

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?

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

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

Question: Is there a net force on the apple as it falls?How do you know?

~ 10 m/s2

“Constant” vertical force: Near Earth Gravity

Fgravity = mapple

!g

|!g |

mapple (kg)× g (m / s2 ) = F (newtons)

Question: What is the acceleration?

=mapple!aFgravity =mapple

!g

!g = !a