Download Dynamics - Polson 7-8

Chapter 4: Forces and
The Laws of Motion
dynamics: the study of forces and motion
4.1 Objectives
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Understand the concept of a force.
Know common types of forces.
Be able to draw a “free-body diagram.”
Be able to determine the net force acting on
an object.
Forces
force: a push or a pull; can change motion
• measured in Newtons (N = kgm/s2) with a scale
• vector quantity
4 fundamental forces: electromagnetic, gravity,
nuclear strong, nuclear weak
Common Forces
weight (FW)
normal (FN)
friction (FF)
applied (FA)
air resistance (FR) tension (FT)
Free Body Diagrams
• show all forces acting on an object (concurrent
forces)
• arrows point away from center
• net force (SF): resultant of all forces
FN = 12 N
FA = 9 N
FF = 4 N
FW = 12 N
SF = 5 N
4.2 Objectives
• Understand the concept of inertia.
• Understand and apply Newton’s first law of
motion.
• Understand the concept of equilibrium.
Inertia
• inertia: the tendency of objects to resist a change in
motion (essentially mass)
• DEMO: inertia ball
Newton’s First Law
Newton’s 1st Law: an object will maintain its current
velocity (at rest or in motion) unless an unbalanced
force (SF) acts on it—it has “inertia”
constant
velocity
FN
constant
velocity
changing velocity
(acceleration)
NO FORCES
UNBALANCED
FORCE
FW
BALANCED
FORCES
FW
Spacecraft launched in the 1970s have inertia
and are still traveling due to the first law.
Equilibrium
equilibrium: if no net force, no change in velocity
(no acceleration!)
FN = 12 N
FN = 5 N
FF = 6 N
FW = 5 N
SF = 0 N
FA = 6 N
FW = 12 N
SF = 0 N
Net Force--Acceleration
FN = 12 N
SF = 4 N (to the left)
FF = 4 N
FW = 12 N
SF = 3 N (down)
FW = 3 N
net forces present so
objects must accelerate
Equilibrium Problem
• A 24.6 N weight hangs stationary from a wire,
producing a 162o angle pointing upward in the wire.
What is the tension in wire (on either side—it will be
the same)?
4.3 Objectives
• Understand the difference between mass and
weight.
• Be able to calculate mass and weight.
• Understand and apply Newton’s second law of
motion.
• Understand and apply Newton’s third law of
motion.
Mass and Weight
mass: amount of matter (kg on a balance)
weight: force of gravity (N on a scale)
Fw  m  g
What is the weight of a 320 g (0.320 kg) apple?
Newton’s Second Law
No net force, no change in velocity = uniform motion
Net force present, change in velocity = acceleration
How does SF affect a? As SF ↑, a ↑ a ~ SF
How does m affect a? As m ↑, a ↓ a ~ 1 / m
F
a~
m
F
a
m
F  m a
Newton’s second Law
Newton’s Second Law Problems
• A car has a mass of 1538 kg. If the car accelerates at
-1.49 m/s2 during braking, what is the magnitude of
the net force applied by the brakes?
• A toy rocket weighing 5.3 N is pushed horizontally
by a 12.4 N force. What is the acceleration of the
rocket?
• A man with a mass of 74 kg slides down a metal pole.
If his acceleration is 0.38 m/s2 downward, what is
the magnitude of the upward force exerted by
friction? Gravity is the only other force to consider.
Objectives
• Understand Newton’s third law of motion.
• Solve problems involving Newton’s third law.
Newton’s Third Law
Why does it hurt your hand when you hit a wall?
Because the wall “hits” back!
Newton’s third law: for
every force (action) there
is an equal and opposite
force (reaction).
Important: action-reaction pairs act on different
objects, so they do not “cancel out.”
Newton’s Third Law
½ kg
FW = earth pulls on apple (action)
FR = apple pulls on earth (reaction)
Why doesn’t the earth accelerate upward at g?
on apple: SF = m · a 4.9 N = (0.5 kg)(9.8 m/s2)
on earth: SF = m · a 4.9 N = (6x1024 kg)(8.2 x 10-25 m/s2)
Newton’s Third Law
Identify the action-reaction pairs:
• A person takes a step
• A snowball hits you in the face
• You catch a softball.
When a bug splatters on a windshield of a fast-moving
truck, which experiences a larger force of impact?
Which accelerates more?
4.4 Objectives
• Be able to identify various “everyday forces.”
• Make calculations involving friction and the
coefficient of friction.
• Calculate forces acting on inclines.
Friction
friction: force resisting motion between two surfaces
What affects friction?
(1) the surfaces in contact (m ~ coefficient of friction)
(2) the normal force acting on the surfaces
FF = m FN
There are two kinds of friction: static and kinetic
mS > mK
bonding is weaker when there is motion
Measuring Friction and m
• Example: What is the force of friction between a
brick and a table and how much is the coefficient of
kinetic friction?
Friction and Acceleration
• Example: p. 147, #1
Braking vs. Skidding
If your brakes lock, you will skid. How does this
affect the coefficient of friction and the distance?
braking: FS
skidding: FFK
mS > mK
What happens if you try
braking on smooth ice
without studded tires (m ~ 0)?
Newton’s first law—you just keep moving!
Forces on Inclines
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normal force is perpendicular to the surface
weight can be separated into FII and F^
FII = Fwsinq
F^ = Fwcosq
FN = F ^
Forces on Inclines
• Example: p. 147, #3 (do with a = 0, then a =
3.60)
Terminal Velocity
FW
SF = FW
a=g
SF
FR
SF
FW
SF < FW
a<g
terminal velocity: occurs when FR = FW
the object falls at a constant velocity
FR
FW
SF = 0
a=0