Transcript Chapter1_3

Announcements:
- 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)
PHY110 TUTOR SESSIONS
Tutor:
Jillian Bjerke & Maggie Baldwin
Session 1:
Session 2:
Session 3:
Mo, 4-6 pm
We, 4-6 pm
Th, 5-7 pm
(Jill)
(Jill)
(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 I
First 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
Concepts
Demos and Objects
-
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
i-clicker question-1:
An apple is sitting on your desk. Which statements are true.
A.
B.
C.
D.
Only the force of gravity acts on the apple.
At least one more force acts on the apple.
Not enough information.
It is not possible that more than one force acts on an
object.
E. A & D.
Type of Force
Ball resting on table:
What kind of forces can we see?
On ball:
• Weight (gravity): down
• Support force: up
– Prevents something from penetrating a surface
– Points directly away from that surface
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
i-clicker question-2:
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 harder
B. Your friend pushes
harder
C. The forces are equal in
magnitude
Ball resting on table (revisited):
Forces Present:
1.
2.
3.
4.
What kind of forces can we see?
On earth due to gravity from the ball
On ball due to gravity from the earth (weight)
On ball due to support from table
Pair
On table due to support from ball
•
Since the ball doesn’t accelerate, 2 and 3
must cancel perfectly
Pair
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 gravity
Force of
cart on
donkey
Force of the ground
on donkey
Net Forceon donkey = Fground on donkey+Fcart on
donkey
+Fgravity
Why does the Cart AND Donkey Move?
F=ma so acart+donkey = F(on cart+donkey)/mcart+donkey
Force of the
ground on
cart
Force of the ground
on donkey
Net Forceon donkey = Fground on donkey+Fground on cart +Fgravity
Force of gravity
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.
B.
C.
D.
E.
0.5 m/s2 up;
1.0 m/s2 up;
0 m/s2 up;
50.0 m/s2 up;
0 m/s2 up;
1.0 m/s2 left
2.0 m/s2 left
1.0 m/s2 left
100 m/s2 left
0 left
100 N
100 N
50 N
Ft = Ff + Fg
So Ft= m a
=0
and a = 0!
Sum of Floor Forces
Force Due to Gravity
=mg
Net force on piano
Force from ramp
Net Force = m a
Component into ramp
Force due to gravity = weight
Force from
ramp
Net Force
Component into
ramp
Weight
= mg
large F
medium F
small F
Net downwards force on piano depends on angle
Challenge: Get the piano up to the second floor!
Ramp or Straight Lift?
RAMP
2000N!
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 Work
Energy: the capacity to make things happen
Work: is the transference of energy
Forms of Energy
Kinetic Energy – Energy of motion
Potential
Chemical Energy
Gravitational
Nuclear Energy
Thermal Energy
Energy and Work
Energy: the capacity to make things happen
Work: is the transference of energy
Work = Force x Distance
W=F d
F=mg=2000N
W = Fh = mgh
Work is the transfer of energy
Where did energy of lifters go?
into potential energy of piano!
h
Gravitational potential energy = m g h
Force from
ramp
Net Force
Component into
ramp
h
Weight
= mg
W = FW
d = W
Fd =
F
d W = mg h
Work on piano = change in energy of piano = same!
i-clicker question -4
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
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:
RAMP
1.
2000N!
Straight Lift
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.
B.
C.
D.
E.
100 J
200 J
98 J
1J
980 J
How much work is done
when just holding up an
object?
W  F d
W
What is the work
done when lifting?
W  F d
W
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?