Download Notes - Newton`s 3rd Law

Newton’s 3rd
Law of Motion
notes
Monday, 9/26 and Tuesday,
9/27
Newton’s 3rd Law of Motion
 Known
as the law of interaction.
 Forces
always occur in pairs; either playing the
action or reaction part of interaction.
 When
a force is exerted on an object (action)
the object exerts a force on the object causing
the force (reaction).
 The
reaction force is equal (in magnitude) and
opposite in direction to the action force.
Force Pair Demo
Write your observations below.
Force Pair example
Kent is pulling upon a rope that is attached to an elephant.
Fkent = 500 N, left
(Reaction)
Felephant = 500 N, right
(Action)
Image source: http://www.physicsclassroom.com/class/newtlaws/Lesson-4/Newton-s-Third-Law
Newton’s 3rd Law – analyzing systems
A
system consists of two or more objects
interacting with each other.
Image source: http://4.bp.blogspot.com/-kIWrZZBgsUw/U9CYf4QFBYI/AAAAAAAAEhg/dMduMORi4R4/s1600/Newton's+Third+Law.png
Example: A 100 kg basketball player throws a 20 kg medicine
ball. The player accelerates to the left at 1 m/s2. Calculate
the acceleration of the ball.
Fplayer, ball,
Fplayer, ball = (100 kg)(1 m/s2)
Fplayer, ball = 100 N
Fball, player,
a=
Fball, player
mball
a=
100 N
20 kg
a=
5 m/s2 right
Analyzing Systems of Objects
1. Newton’s 3rd Law explains that
the tension on M1 is the same
as the tension on M2.
2. When objects are connected
They have the same acceleration.
System; M1 and M2
Image source: http://bama.ua.edu/~jharrell/PH105-S03/exercises/coef_frict_lab_files/image002.gif
Example: connected objects
A 3 kg block (M1)is at rest on a table. It is
attached to a 2 kg block(M2) by a string
that passes over a pulley.
1. Calculate the acceleration of
the system.
2. Calculate the tension in the string.
Steps
1.
2.
3.
4.
5.
6.
Draw a FBD
Write net force equations
Apply Newton’s 2nd Law of Motion
(a = F/m)
Solve the system of equations.
Calculate acceleration
Calculate tension using a net force
equation.
Step 1: Draw a FBD
A 3 kg block (M1)is at rest on a table. It is
attached to a 2 kg block(M2) by a string
that passes over a pulley.
Step 2: Write the net force equations
A 3 kg block (M1)is at rest on a table. It is
attached to a 2 kg block(M2) by a string
that passes over a pulley.
Step 3: Apply Newton’s 2nd Law (a = F/m)
A 3 kg block (M1)is at rest on a table. It is
attached to a 2 kg block(M2) by a string
that passes over a pulley.
Step 4: Solve the system of equations to
find acceleration.
A 3 kg block (M1)is at rest on a table. It is
attached to a 2 kg block(M2) by a string
that passes over a pulley.
Step 5: Find tension using net force equation.
A 3 kg block (M1)is at rest on a table. It is
attached to a 2 kg block(M2) by a string
that passes over a pulley.
Takeaways….
 The
acceleration of the system is equal to
the net force of the system divided by the
mass of the system.
 Tension force between two objects in a
system is call the internal force.