Physics 102-002
Announcements
• Clickers (should be in Mon or Tues)
• WebAssign – must have Account Codes in
2 weeks (from yesterday)
– You can use without a code till then
– Homework opens on the day of class, closes
1 week later before class.
• 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
2/5
Midterm Exam 1
Chapter 5 Newton’s Third Law
Chapter 4
Newton’s Second Law
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Force Causes Acceleration
Friction
Mass and Weight
Mass Resists Acceleration
Newton’s Second Law
Free Fall – When Acceleration = g
Nonfree Fall – When Acceleration < g
Force Causes Acceleration
According to Newton’s First Law:
- An object can’t accelerate unless a force acts on it
Examples:
Stationary object .. Won’t move unless pushed
Cart on Air Track .. Moves at constant velocity unless stopped or pushed
- Acceleration depends on the NET force on an object.
- Double the net force; double the acceleration
- Triple the net force; triple the acceleration
… in other words …
Acceleration ~ Net Force
(means … “is proportional to”)
The Physics Place … video “Force Causes Acceleration”
Friction
Friction occurs when surfaces slide against each other.
The force of friction always opposes the direction of motion
Static Friction: Resists STARTING something in motion.
Sliding Friction: Resists motion once it has started.
Force of friction doesn’t depend on the AREA in contact … it only
depends on the mass of the object.
Friction Applet: Inclined Plane
Friction Applet: Traffic Jam
Friction in air = air resistance (or air drag)
Mass and Weight
Mass: a measure of the inertia of an object.
Weight: The force exerted on an object by gravity.
Weight = Mass x acceleration of gravity
W = mg
Units of mass = kg
English Units of weight = pounds (lb)
A brick with a mass of 1kg weighs 2.2 lb
In metric units, weight is expressed in Newtons (N)
The acceleration of gravity is g = 9.8 m/s2 (or about 10 m/s2)
A brick with a mass of 1kg weighs 9.8 N (or about 10 N)
Problem: What is the weight of a ball with a mass of 2 kg?
W = (2 kg)(10 m/s2) = 20 N
Question 1
What is the weight of a 1 kg brick?
a.
1 kg.
b.
2.2 lb.
c.
11 N.
d.
100 carats.
Mass Resists Acceleration
Heavier objects are harder to accelerate!
So the amount of acceleration you can impart to an object depends on
the mass of the object.
The less mass, the more acceleration.
The more mass, the less acceleration.
or
Acceleration ~
1
mass
Newton’s Second Law
The acceleration of an object as produced by a net force is directly
proportional to the magnitude of the net force, in the same direction as the
net force, and inversely proportional to the mass of the object.
In other words ….
F = ma
Remember
this
equation!!!
Question 2
Which of the following is a statement of Newton’s Second Law?
a.
E = mc2
b.
What goes around comes around.
c.
F = ma
d.
Haste makes waste.
Total Acceleration = g … Free Fall
When the force of gravity is the only force acting on an object.
No supporting force.
No air resistance.
Net force = the object’s weight (Weight = mg)
Force = ma
The acceleration of an object in free fall is g
Example: The net force on a 10N object in free fall is 10N
Free fall animation – elephant vs. feather
Total Acceleration < g … non-Free Fall
In the presence of air resistance, the net force acting on a
falling object is less than the object’s weight!
Net Force =
1000N Down
Net Force =
600N Down
a=
Net Force =
200N Down
Net Force
=
mass
Net Force = 0
Weight – air resistance
mass
Non-free fall animation – elephant vs. feather