Newton’s First Law (1642-1727)

• “The Law of Inertia” • A body remains at

rest or moves in a straight line at a constant speed unless acted upon by a net force.– Objects do not

accelerate unless a net force is applied.

http://teachertech.rice.edu/Participants/louviere/Newton/newton.html; http://www.library.usyd.edu.au/libraries/rare/modernity/newton3.html

Objects at rest remain at rest and objects in

motion remain in motion, unless acted upon by an outside

force.

http://talesfromechocanyon.blogspot.com/2007_03_02_archive.html

Newton’s 1st Law of Motion

Newton’s 2nd law of Motion …mathematically

Net Force = (mass)(accel)

Fnet = ma

NEWTON'S 2nd LAW OF MOTION

Fa

or amF

F am

F am

m

F a

m

m

m

F a

F a

F aM

ma 1

M

M

When the acceleration is g we have Free Fall

m

F

gmF

2m

2F

gmF

22

Terminal Velocity

Acceleration = g

Acceleration < g

Acceleration << g

Acceleration = 0

Velocity = 0but motion is about to begin

v increasing downward

v still increasing downwardjust not as rapidly as before

Terminal velocity

mg

mg

mg

mg

F

F

F

Net Force

Falling with Air Resistance & Terminal Velocity

• When falling the force of air resistance becomes large enough to balance the force of gravity.

• At this instant in time, there is no net force — the object stops accelerating (see D below); terminal velocity has been reached.

www.physicsclassroom.com

Newton’s Third Law• Action-Reaction

• When one object exerts a force on another object, the second object exerts a force of equal strength in the opposite direction on the first object.

Example of Newton’s 3rd Law: http://bp3.blogger.com/_vQA3QRUnk3M/RrgyfrRniPI/AAAAAAAAAMs/DYlhM7pDeI8/s1600-h/newton

Newton’s Third Law of Motion

For every action, there is always a reaction of equal (magnitude) and opposite (direction) reaction.

“action” or “reaction” refers to force.Action/Reaction forces do

NOT act on the SAME object!

Action: tire pushes on road

Reaction: road pushes on tire

Action: rocket pushes on gases

Reaction: gases push on rocket

Action- Reaction Forces• Do Action-Reaction forces cancel

each other?• No, they are acting on different

objects. Forces can only be added together when they are acting on the same object.

http://quest.nasa.gov/space/teachers/rockets/principles.html

Newton’s 3rd Law of MotionAction Force: A guy is pushing a ball leftwards

Reaction Force: the ball is pushing the guy rightwards

Tug-a-war • If Fido and Rover play tug-a war, how do the “pulls” of the dogs compare?

• If each dog pulls with 50 N of force, what is the tension force in the middle of the rope (between the dogs)?

• While driving, Mrs. Ingle observed a bug striking the windshield of her car. The bug hits the windshield and the windshield hits the bug. Which of the two forces is greater: the force on the bug or the force on the windshield? EXPLAIN!

Putting Newton's Laws of Motion Together• An unbalanced force must be exerted for

a rocket to lift off from a launch pad or for a craft in space to change speed or direction (First Law).

• The amount of thrust (force) produced by a rocket engine will be determined by the rate at which the mass of the rocket fuel burns and the speed of the gas escaping the rocket (Second Law).

• The reaction, or motion, of the rocket is equal to and in the opposite direction of the action, or thrust, from the engine (Third Law).

http://quest.nasa.gov/space/teachers/rockets/principles.html

http://www.spacetoday.org/images/Rockets/ArianeRockets/Ariane5LaunchArianespace.jpg