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- 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
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
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