- Homework 1.2 due on Thursday, Jan. 28.

- Web page for class is:

http://www.wfu.edu/~gutholdm/Physics110/phy110.htm

-Bring i-clicker to class

-You are allowed 30 missed points in the i-clicker total score (~ 160 points)

-Last day to add class: Jan. 27

-Homework solutions are posted on web page (will be password protected)

Announcements:

PHY110 TUTOR SESSIONS

Tutor: Jillian Bjerke & Maggie Baldwin

Session 1: Mo, 4-6 pm (Jill)Session 2:   We, 4-6 pm (Jill)Session 3: Th, 5-7 pm (Maggie)

All tutorial session will be in Olin 101 (class room).

The tutor sessions in semesters past were very successful and received high marks from many students. All students are encouraged to take advantage of this opportunity. 

There are also private tutors available, contact Judy Swicegood in the Physics office (Olin 100)

Chapter 1: The laws of motion, Part IFirst two chapters: Introduce the “language of physics”

Subsequent chapters: Explore objects and underlying physical concepts

- Reading assignment for today: Chapter 1.3

- Reading assignment for next class: Chapter 2.1

- Homework 1.3 (Calli Nguyen):

(due Tuesday, Feb. 2, in class):

Exercises: 20, 23, 24, 27, 34, 35, 39

Problem: 8, 9, 10, 13, 14, 15, 16, 22

Chapter 1.3 Newton III, energy,work, ramps

- Tug of war- Lifting stuff- Carrying stuff- Using a ramp

- Newton’s third law: every action has an equal and opposite reaction- Net force- Work and energy- Kinetic energy- Gravitational potential energy- Ramps

Demos and Objects Concepts

i-clicker question-1:

An apple is sitting on your desk. Which statements are true.

A.Only the force of gravity acts on the apple.B.At least one more force acts on the apple.C.Not enough information.D.It is not possible that more than one force acts on an object.E.A & D.

Type of Force

On ball:

• Weight (gravity): down

• Support force: up– Prevents something from penetrating a surface– Points directly away from that surface

Ball resting on table:

What kind of forces can we see?

The net force on the apple is zero

Physics Concept

• Net Force– The sum of all forces on an object.– Determines object’s acceleration.

Tug-of-war

Newton’s Third Law

For every force that one object exerts on a second object, there is an equal but oppositely directed force that the second object exerts on the first object.

F12 = -F21

If you push on a friend (on ice, no friction), how will the force you exert on your friend compare to the force your friend exerts on you?

A. You push harderB. Your friend pushes

harderC. The forces are equal in

magnitude

i-clicker question-2:

Forces Present:Ball resting on table (revisited):

What kind of forces can we see?

1. On earth due to gravity from the ball

2. On ball due to gravity from the earth (weight)

3. On ball due to support from table

4. On table due to support from ballPair

Pair

• Since the ball doesn’t accelerate, 2 and 3 must cancel perfectly

Two Crucial Notes:

• While the forces two objects exert on one another must be equal and opposite, the net force on each object can be anything.

• Each force within an equal-but-opposite pair is exerted on a different object, so they don’t cancel directly.

If the force of the cart on the donkey is the same (but oppositely directed) as the force of the donkey on the cart, why does it move?

Why does the Donkey Move?

F=ma so adonkey = F(on donkey)/mdonkey

Force of cart on donkey

Force of the ground on donkey

Net Forceon donkey = Fground on donkey+Fcart on donkey

Force of gravity

+Fgravity

Why does the Cart AND Donkey Move?

F=ma so acart+donkey = F(on cart+donkey)/mcart+donkey

Force of the ground on donkey

Net Forceon donkey = Fground on donkey+Fground on cart

Force of gravity

+Fgravity

Force of the ground on cart

i-clicker question-3:

The diagram shows a top view of three people pulling on a donkey (disk) of mass 100 kg. Ignoring other forces (friction, etc), what is the acceleration of the donkey?

A. 0.5 m/s2 up; 1.0 m/s2 leftB. 1.0 m/s2 up; 2.0 m/s2 leftC. 0 m/s2 up; 1.0 m/s2

leftD. 50.0 m/s2 up; 100 m/s2 leftE. 0 m/s2 up; 0 left

100 N

50 N

100 N

Sum of Floor Forces

Force Due to Gravity= m g

Ft = Ff + Fg

So Ft= m a = 0and a = 0!

Force due to gravity = weight

Component into ramp

Force from ramp

Net Force = m a

Net force on piano

small F

medium F

large F

Component into ramp

Force from ramp

Weight= mg

Net Force

Net downwards force on piano depends on angle

2000N!

Challenge: Get the piano up to the second floor!Ramp or Straight Lift?

RAMP

Straight Lift

Observations About Ramps

• Lifting an object straight up is often difficult

• Pushing the object up a ramp is usually easier

• The ease depends on the ramp’s steepness

• Shallow ramps require only gentle pushes

• You seem to get something for nothing

• How does distance figure in to the picture?

Physical Quantities: Energy and Work

• Energy– A conserved quantity– The capacity to do work

• Work– The mechanical means of transferring energy.

– work = force · distance (where force and distance are in the same direction)

Unit of energy & work: 1 N·m = 1J (Joule) = 0.238 cal

Energy and WorkEnergy: the capacity to make things happenWork: is the transference of energy

Forms of Energy

Kinetic Energy – Energy of motion

PotentialChemical Energy

GravitationalNuclear Energy

Thermal Energy

Energy and WorkEnergy: the capacity to make things happenWork: is the transference of energy

F=mg=2000N

h

Work = Force x DistanceW = F d

W = Fh = mghWork is the transfer of energyWhere did energy of lifters go?into potential energy of piano!

Gravitational potential energy = m g h

Component into ramp

Force from ramp

Weight= mg

Net Force

h

= mg hWW = F d W = F d

W = F d

Work on piano = change in energy of piano = same!

A man loads a refrigerator onto a truck using a ramp.

He claims he would be doing less work if the length of the ramp would be longer. Is this true?

A.Yes

B.No

C.Not enough information

i-clicker question -4

Work Lifting a Piano

• Going straight up:

work = force · distance

• Going up ramp:

work = force · distance• The work is the same, either way!

Black board example 1.3.1:

2000N!RAMPStraight Lift

1. Superman is lifting a piano (mass 100 kg) straight up onto a 1 m high platform. How much work is he doing?

2. You are pushing the same piano along a 10 m long (frictionless) ramp onto the 1m platform. How much work are you doing?

3. How much force do you apply to the piano?

4. How much energy did the piano gain by being lifted

5. Where did that energy come from?

i-clicker-4

A. 100 JB. 200 JC. 98 JD. 1 JE. 980 J

How much work is done when just holding up an object?

W

dFW

What is the work done when lifting?

W

dFW

Strongest man lifting up 140 kg boulder by 1 m.

Black board example 1.3.2

Angus is pulling a 10,000 kg truck with all his might (2000N) on a frictionless surface for 10.0 m.

How much work is the man doing?

What kind of energy does the truck gain?

What is the speed of the truck if he pulls for ten seconds?