Newton’s Laws of Motion

AP Physics Unit 2 Section 1Newton’s LawsLesson 1

Do Now:What is the acceleration on a F-150 (mass = 1500.0 Kg) if it starts from rest and reaches 25.0 m/s {about 55 mph} in 6.50 seconds?

What is the “Force” needed to attain this acceleration?

Objectives: 1st Law: Inertia 2nd Law: F = ma 3rd Law: Equal

and Opposite

Homework: TBA

Isaac Newton

Born 1642 Went to University of Cambridge in England as

a student and taught there as a professor after Never married Gave his attention mostly to physics and

mathematics, but he also gave his attention to religion and alchemy

Newton was the first to solve three mysteries that intrigued the scientists Laws of Motion Laws of Planetary Orbits Calculus

Three Laws of Motion

Newton’s Laws of Motion are laws discovered by Physicist and mathematician, Isaac Newton, that explains the objects’ motions depending on forces acted on them Newton’s First Law: Law of Inertia Newton’s Second Law: Law of Resultant

Force Newton’s Third Law: Law of Reciprocal

Action

Newton’s First Law: InertiaNO “net” acceleration An Object at rest remains at rest, and an

object in motion continues in motion with constant velocity (that is, constant speed in a straight line), unless it experiences a net external force.

The tendency to resist change in motion is called inertia People believed that all moving objects

would eventually stop before Newton came up with his laws

Newton’s First Law

When there is no force exerted on an object, the motion of the object remains the same like described in the diagram

Because the equation of Force is F=ma, the acceleration is 0m/s². So the equation is 0N=m*0m/s²

Therefore, force is not needed to keep the object in motion, when

The object is in equilibrium when it does not change its state of motion

Newton’s Second Law:Force“net” acceleration

The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass

anet = Fnet / mass or

F = maFnet

Acceleration

Force

Two types of forces Contact force

Force caused by physical contact

Field force Force caused by gravitational attraction

between two objects Force caused by electromagnetic

attraction between two objects

Friction

A force that causes resistance to motion Arises from contact between two

surfaces If the force applied is smaller than the

friction, then the object will not move If the object is not moving, then ffriction=Fapplied

The object eventually slips when the applied force is big enough

Friction

Friction was discovered by Galileo Galilee when he rolled a ball down a slope and observed that the ball rolls up the opposite slope to about the same height, and concluded that the difference between the initial height and the final height is caused by friction.

Galileo also noticed that the ball would roll almost forever on a flat surface so that the ball can elevate to the same height as where it started.

Two types of Friction

Static Friction Friction that exists

while the object is stationary

If the applied force on an object becomes greater than the maximum of static friction, then the object starts moving

fstatic≤μstaticn

Kinetic Friction The friction that exists

when an object is in motion

F-fkinetic produces acceleration to the direction the object is moving

If F=fkinetic, then the object moves at constant speed with no acceleration

fkinetic= μkineticn Kinetic friction and the

coefficient of kinetic friction are smaller than static friction and the static coefficient

The car is traveling rightward and crashes into a brick wall. The brick wall acts as an unbalanced force and stops the car.

The truck stops when it crashes into the red car.

But the ladder falls in front of the truck because the ladder was in motion with the truck but there is nothing stopping the ladder when the truck stops.

Unbalanced Force and Acceleration Force is equal to

acceleration multiplied by mass When an unbalanced

force acts on an object, there is always an acceleration

Acceleration differs depending on the net force

The acceleration is inversely related to the mass of the object

Net Force

Force is a vector Because it is a vector, the net force can be

determined by subtracting the force that resists motion from the force applied to the object.

If the force is applied at an angle, then trigonometry is used to find the force

Fnet

θR

R*cos θR*sin θ

θR

Gravitational Force

The force that exerts all objects toward the earth’s surface is called a gravitational force. The magnitude of the gravitational force is

called weight The acceleration due to gravity is

different in each location, but 9.80m/s² is most commonly used

Calculated with formula w=mg

Newton’s Third Law

If two objects interact, the force exerted on object 1 by object 2 is equal in magnitude but opposite in direction to the force exerted on object 2 by object 1

Forces always come in pair when two objects interact The forces are equal, but opposite in

direction Fg

Fn

As the man jumps off the boat, he exerts the force on the boat and the boat exerts the reaction force on the man.

The man leaps forward onto the pier, while the boat moves away from the pier.

Newton’s Third Law

Force exerted by the wheels

Force exerted by the road

Newton’s Third Law

Newton’s Third Law

Flow backward

Foil deflected down

Foil deflected up

Engine pushed forward

Flow pushed backward

Foil deflected down

Works Cited

Henderson, Tom. Physics. Course home page. 16 May 2008 http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/newtlaws/newtltoc.html>. Serway, Raymond A., and Jerry S. Faughn. "The Laws of Motion." College

Physics .      Fifth ed. 1999.

Benson, Tom. Newton’s Third Law applied to Aerodynamics 21 May 2008 http://www.grc.nasa.gov/WWW/K-12/airplane/newton3.html

Introduction to Rocket Performance. Newton’s Third Law. 12 March 2004 http://www.allstar.fiu.edu/aero/rocket1a.htm

Stern, David P. (16) Newton’s Laws of Physics. 1. Force and Inertia. 9 October 2004 http://www-istp.gsfc.nasa.gov/stargaze/Snewton.htm