Transcript Physics 102 Introduction to Physics

Physics 102-002
Announcements
• Clickers – should have by now
• WebAssign – must have Account Codes
by tomorrow (Tues, Feb 6)
• Exam #1 Wednesday
– Covers Chapters 1-4
Class Schedule
1/17
Chapter 1 Introduction, About Science
1/22
Chapter 2 Newton’s First Law
1/24
Chapter 3 Linear Motion
1/29
Chapter 4 Newton’s Second Law
1/31
Midterm Exam 1
2/5
Chapter 5 Newton’s Third Law
2/7
Chapter 6 Momentum
Chapter 5
Newton’s Third Law
• Forces and Interactions
• Newton’s Third Law
– Defining your system
– Action and reaction on different masses
• Summary of Newton’s Three Laws
• Vectors
Force and Interactions
Force: A push or pull exerted on an object
Examples:
Hand against a wall.
Book on a table.
Prize fighter and tissue paper
Earth and you.
Interacting objects exert forces on each other.
You can’t exert a force on
something unless the other
objects exerts and equal
and opposite force on you!
Force of
skater B on
skater A
FBA
Force of
skater A on
skater B
FAB
Question 1
While driving down the road, an unfortunate bug strikes
the windshield of a bus. Quite obviously, this is a case
of Newton's third law of motion. The bug hit the bus and
the windshield hit the bus. Which of the two forces is
greater:
a.
the force on the bug?
b.
the force on the bus?
Question 2
A gun recoils when it is fired. The recoil is the result of actionreaction force pairs. As the gases from the gunpowder explosion
expand, the gun pushes the bullet forwards and the bullet pushes
the gun backwards. The acceleration of the recoiling gun is ...
a. greater than the acceleration of the bullet.
b. smaller than the acceleration of the bullet.
c. the same size as the acceleration of the bullet.
Newton’s Third Law
To every action there is an equal and opposite reaction.
ON impact, the interaction
forces between the blue ball
and the yellow ball STOP the
blue ball and MOVE the
yellow ball.
Defining the “System”
System: Gives the boundaries beyond which forces are “external”
Example in the book: The orange and the apple.
Here, the orange is the system: an external force is acting on
the orange, causing it to accelerate.
The apple provides the external force to accelerate the system
to the right. Notice that the force exerted by the apple on the
orange is still equal and opposite to the force of the orange
on the apple.
If the system is the apple AND the orange: an external force is
still acting on the system to accelerate it to the right. The force
is provided by the friction between the apple’s “feet” and the
ground.
Action and Reaction on Different
Masses
The action and reaction forces are equal and opposite, no matter
whether the masses are the same or not!
The earth pulls “downward” on the rock.
The falling rock pulls “upward” on the earth
(with the exact same amount of force).
The force exerted on the
cannonball is the same as the
force exerted on the cannon.
But the cannonball accerates
more because of its much
lower inertia (or mass).
F
a
m
Cannonball:
(Smaller mass,
Larger acceleration)
Cannon:
F
a
M
(Larger mass,
Smaller acceleration)
The force of
attraction
between the 2
“planets” is
equal and
opposite
regardless of
the mass
difference.
(Both planets
attract each
other)
Newton’s 3 Laws:
Summary
• Newton’s First Law
An object at rest tends to stay at rest and an object in
motion tends to stay in motion at constant speed and
in the same direction unless acted on by an
unbalanced force.
• Newton’s Second Law
F = ma
• Newton’s Third Law
To every action there is an equal and opposite
reaction.
Vectors
Vectors have magnitude and direction. They add or subtract
depending on their directions.
Parallel vectors are pretty simple:
+
50 N
50 N
=
50 N
=
50 N
What if the vectors are NOT parallel:
+
100 N
0N
12 km North
Example: What if I walked 16 km East and 12 km North
The result is a NET movement of 20 km Northeast
Resultant Vector
16 km East
Component Vectors
Vectors continued
All vectors (force, velocity, acceleration, etc) add the same way.
The 30N and 40N forces add to get a resultant force of 50N.
The DIRECTION of the resultant 50N force is given by the
diagonal of the “parallelagram” (The Parallelagram Rule)
Nellie illustrates the parallelagram
rule.
Note that the 2 tensions add to
more than Nellie’s weight!
Support Force Interactive Figure
Resultants of vectors applet
http://www.walter-fendt.de/ph14e/resultant.htm
Vector components applet
http://id.mind.net/~zona/mstm/physics/mechanics/vectors/
components/vectorComponents.html
Vectors continued
Velocity vectors add: The crosswind of 60 km/h adds
to the planes velocity of 80 km/h to blow the plane
offcourse at 100 km/h.
Some example resultant velocities.
When you throw a rock, it’s velocity
has a horizontal and vertical
component
Question 3
The 3 boats shown are crossing a river.
Which of the boats will follow the shortest
path to the opposite shore?
Velocity of the boat
Velocity of the river