PreClass Notes: Chapter 4, Sections 4.1-4 - U of T Physicsjharlow/teaching/phy131f15/cl05v... · PreClass Notes: Chapter 4, Sections 4.1-4.4 • From Essential University Physics

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© 2012 Pearson Education, Inc. Slide 1-1

PreClass Notes: Chapter 4, Sections 4.1-4.4

• From Essential University Physics 3rd Edition

• by Richard Wolfson, Middlebury College

• ©2016 by Pearson Education, Inc.

• Narration and extra little notes by Jason Harlow,

University of Toronto

• This video is meant for University of Toronto

students taking PHY131.


Outline

• 4.1 Newton’s First Law

• 4.2 Newton’s Second Law

• 4.3 The Fundamental

Forces

• 4.4 The Force of Gravity

“ ‘What keeps things

moving?’ is the wrong

question.” … “Galileo

declared that the question

needs no answer.” –

R.Wolfson

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Galileo

• “Galileo Galilei (1564-1642),

was an Italian astronomer,

physicist, engineer,

philosopher, and mathematician

who played a major role in the

scientific revolution during the

Renaissance.

• He is widely regarded as one of

the greatest scientists of all

time.

• Galileo has been called the ‘the

father of modern science’.”• - https://en.wikipedia.org/wiki/Galileo_Galilei


Galileo’s Concept of Inertia

• Balls rolling on downward-

sloping planes pick up speed.

• Balls rolling on upward-

sloping planes lose speed.

• So a ball on a horizontal

plane must maintain speed

forever.

• If the ball comes to rest, it is

not due to its “nature,” but

due to friction.This nice explanation is from “Conceptual Physics” 11th Edition by Paul G. Hewitt

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Net Force

Net force is the combination of all forces that change

an object’s state of motion.

Example: If you pull on a box with 5 N and a friend

pulls in the same direction with 5 N, the net force is 10 N in

the direction you are both pulling.

If you are both pulling in opposite directions with the same

force, the net force is zero.


Newton’s First Law of Motion

A body in uniform motion remains in uniform motion,

and a body at rest remains at rest, unless acted on by

a nonzero net force.

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Got it?

• On a horizontal tabletop is a curved barrier that

exerts a force on a ball, guiding its motion in a

circular path as shown. After the ball leaves the

barrier, which of the dashed paths shown does it

follow?


Momentum

• Momentum is a property of moving things.

• Momentum is defined as mass of an object

multiplied by its velocity:

Momentum = mass velocity

𝑝 = 𝑚 𝑣

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Newton’s Second Law of Motion

The rate at which a body’s momentum changes is

equal to the net force acting on the body:

(Newton’s 2nd Law) 𝐹net =𝑑 𝑝

𝑑𝑡


In another mathematically equivalent form:


net forceAcceleration

mass

Examples:

If net force acting on object is doubled

object’s acceleration will be doubled.

If mass of object is doubled

object’s acceleration will be halved.

𝑎 = 𝐹net

𝑚(Newton’s 2nd Law)

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Inertial Reference Frames

A physics student

cruises at a constant

velocity in an airplane

A ball placed on the

floor stays at rest relative

to the airplane

Newton’s laws are only valid in reference frames that

are not accelerating.

A reference frame that is not accelerating is called an

inertial reference frame.

This airplane is an inertial reference frame.

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Inertial Reference Frames

A physics student is

standing up in an airplane

during takeoff

A ball placed on the

floor rolls toward the

back of the plane

There are no horizontal forces on the ball, and yet the

ball accelerates in the plane’s reference frame

Newton’s first law is violated, therefore this airplane is

not an inertial reference frame.


Got it?

• If an object moves with constant velocity in a

straight line, which of the following statements is

true?

A. There are no forces acting on the object.

B. The net force on the object is zero.

C. There is a constant force in the direction of

motion.

D. There is a constant force in the direction

opposite of motion.

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What is a force?

Contact forces are forces that act on an object by touching it at a point of contact

The bat must touch the ball to hit it

Long-range forces are forces that act on an object without physical contact

A coffee cup released from your hand is pulled to the earth by the long-range force of gravity


The Fundamental Forces

• Physicists now recognize three

fundamental forces:

– Gravity

– The strong force

– The electroweak force

• All common forces fall under these

three categories.

– Nearly all everyday forces, except

gravity, are electromagnetic

forces, which is one aspect of the

electroweak force.

• A goal of physics is to unify all

forces in a “Theory of Everything.”

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Mass, Weight, and Gravity

• Weight is the force of gravity on an object:

– Mass doesn’t depend on the presence or strength of

gravity.

– Weight depends on gravity, so varies with location:

Weight is different on different planets.

Near Earth’s surface, 𝑔 has magnitude 9.8 m/s2 or

9.8 N/kg, and is directed downward.

w mg


Got it?

• If you were to move to the Moon,

A. your mass would change, but your weight would

not.

B. your weight would change, but your mass would

not.

C. both your weight and mass would change.

D. neither your weight nor your mass would change.

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PreClass Notes: Chapter 4, Sections 4.1-4 - U of T Physicsjharlow/teaching/phy131f15/cl05v... · PreClass Notes: Chapter 4, Sections 4.1-4.4 • From Essential University Physics