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DEVIL PHYSICS
THE BADDEST CLASS ON CAMPUS
IB PHYSICS
PRE-IB PHYSICS
TSOKOS LESSON 2-5
NEWTON’S SECOND AND THIRD
LAWS
Reading Activity Questions?
Assessment Statements
2.2.8. State Newton’s second law of motion.
2.2.9. Solve problems involving Newton’s
second law.
2.2.14. State Newton’s third law of motion.
2.2.15. Discuss examples of Newton’s third law.
Objectives
 Recognize situations of equilibrium, i.e.
situations where the net force and hence the
acceleration are zero.
 Draw the forces on the body of interest and
apply Newton’s second law on that body, .
 Recognize that the net force on a body is in
the same direction as the acceleration of that
body.
 Identify pairs of forces that come from
Newton’s Third Law.
Video: Newton’s Second Law
of Motion
Newton’s Second Law
 The net force on a body is proportional to
that body’s acceleration and is in the same
direction as the acceleration


F  ma
where m is mass, the constant of
proportionality
Newton’s Second Law


F  ma
 The unit of force is the Newton (N)
 1N = 1kg•m/s2
 ma = (kg)(m/s2)
 Weight is a force caused by gravitational
attraction


F  ma  mg
ag
Newton’s Second Law


F  ma
 When you jump from the emergency exit
after the fire alarm gets hit by a volleyball,
how do you minimize the force on your body?
Newton’s Second Law


F  ma
 When you jump from the emergency exit
after the fire alarm gets hit by a volleyball,
how do you minimize the force on your body?
 You bend your knees to decrease the
acceleration and execute a parachute
landing fall to translate vertical acceleration
into rotational acceleration.
Newton’s Second Law


F  ma
 How do car manufacturers try to minimize
the forces absorbed by passengers during a
collision?
Newton’s Second Law


F  ma
 How do car manufacturers try to minimize
the forces absorbed by passengers during a
collision?
 Air bags and crumple zones.
Newton’s Second Law
Sample Problem
 A 200-kg hot air balloon is
held to the ground by two
wires that make a 60-degree
angle to the ground. When
the wires are released, the
balloon accelerates upward
at 3 m/s2. What is the
tension in each cable?


F  ma
Newton’s Second Law
Sample Problem


F  ma
 A 200-kg hot air balloon is
held to the ground by two
wires that make a 60-degree
angle to the ground. When
the wires are released, the
balloon accelerates upward
at 3 m/s2. What is the
tension in each cable?
FL
FTy
Fg
Newton’s Second Law


Sample Problem
 F  ma
 A 200-kg hot air balloon is
held to the ground by two
wires that make a 60-degree
angle to the ground. When
the wires are released, the
balloon accelerates upward
at 3 m/s2. What is the
tension in each cable?
FL  Fg  FTy  0
FTy  FL  Fg
Fg  mg


 F  ma
FL  Fg  ma
FL  ma  mg
Newton’s Second Law
Sample Problem
 A 200-kg hot air balloon is
held to the ground by two
wires that make a 60-degree
angle to the ground. When
the wires are released, the
balloon accelerates upward
at 3 m/s2. What is the
tension in each cable?
FTy  FL  Fg
Fg  mg
FL  ma  mg
FTy  ma  mg  mg
FTy  ma  600 N
Newton’s Second Law
Sample Problem
 A 200-kg hot air balloon is
held to the ground by two
wires that make a 60-degree
angle to the ground. When
the wires are released, the
balloon accelerates upward
at 3 m/s2. What is the
tension in each cable?
FTy  600 N
Sin 60 

FTy
2 FT
600 N
FT 
2 Sin 60
FT  346 N
Terminal Velocity
 When a body moves through a fluid (gas or
liquid) it experiences an opposing force,
similar to friction, called drag
 This force is dependent on velocity
 For lower velocities, drag is proportional to
velocity
 For high velocities, drag is proportional to velocity
squared
Terminal Velocity
 In horizontal movement
through a fluid, when drag
equals the propulsion force,
velocity is constant.
 In a falling body, terminal
velocity occurs when the
force of aerodynamic drag
equals the force of gravity.
Fg  mg
FD  kv
Fg  FD @ vT
mg  kvT
mg
vT 
k
Inclined Plane Problem
 A 150kg mass is placed on an plane inclined
at a 17° angle. The coefficient of static
friction is 0.30 and the coefficient of
dynamic friction is 0.25. What happens?
Fuzzy Dice Problem
 A pair of fuzzy dice hang from the rearview
mirror of a car. The car accelerates from a
stoplight at 2m/s2. What happens?
Video: Newton’s Third Law of
Motion
Newton’s Third Law of Motion
 If Body A exerts a force F on Body B, then
Body B exerts an equal but opposite force F
on Body A.
Newton’s Third Law of Motion
Examples
 Pushing against a wall while wearing
rollerskates.
 Stepping off a boat onto a dock.
 A helicopter hovering.
 A book sitting on a table.
Summary Review
 Can you recognize situations of equilibrium,
i.e. situations where the net force and hence
the acceleration are zero?
 Can you draw the forces on the body of
interest and apply Newton’s second law on
that body?
 Can you recognize that the net force on a
body is in the same direction as the
acceleration of that body?
 Can you identify pairs of forces that come
from Newton’s Third Law?
Assessment Statements
2.2.8. State Newton’s second law of motion.
2.2.9. Solve problems involving Newton’s
second law.
2.2.14. State Newton’s third law of motion.
2.2.15. Discuss examples of Newton’s third law.
QUESTIONS?
Homework
#1-25