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AP Physics
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Notes: Third Law to Friction
4.4 Newton’s Third Law

If object 1 and object 2 interact, the force exerted by object 1 on object 2 is equal in magnitude but
opposite in direction to the force exerted by object 2 on object 1.
F12  F21
o Equivalent to saying a single isolated force
cannot exist

F12 may be called the action force and F21 the reaction
force
o Actually, either force can be the action or the
reaction force

'
Thenaction
and reaction forces act on different objects
Some Action-Reaction Pairs

n and n '
o n is the normal force, the force the table exerts on
the TV

o
n is always perpendicular to the surface
o
n ' is the reaction – the TV on the table
o
n  n '
Fg and Fg'
o
Fg
is the force the Earth exerts on the
object
o
Fg'
is the force the object exerts on the
earth
o
Fg  Fg'
Forces Acting on an Object


Newton’s Law uses the forces acting on an object
n and Fg are acting on the object
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
'
g
n ' and F
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are acting on other objects
Quick Quiz 4.5 A small sports car collides head on with a massive truck. The greater impact force (in
magnitude) acts on
a. the car
b. the truck
c. neither, the forc is the same on both
Which vehicle undergoes the greater magnitude acceleration?
a. the car
b. the truck
c. the accelerations are the same
4.5 Applications of Newton’s Laws

Assumptions

Objects behave as particles
o can ignore rotational motion (for now)

Masses of strings or ropes are negligible

Interested only in the forces acting on the object
o can neglect reaction forces
Free Body Diagram

Must identify all the forces acting on the object of interest

Choose an appropriate coordinate system

If the free body diagram is incorrect, the solution will likely be
incorrect





The force T is the tension acting on the box
The tension is the same at all points along the rope


n and Fg are the forces exerted by the earth and the ground
Only forces acting directly on the object are included in the free
body diagram

Reaction forces act on other objects and so are not included

The reaction forces do not directly influence the object’s motion
Solving Newton’s Second Law Problems

Read the problem at least once

Draw a picture of the system
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
Identify the object of primary interest

Indicate forces with arrows

Label each force

Use labels that bring to mind the physical quantity involved

Draw a free body diagram

If additional objects are involved, draw separate free body diagrams for each object

Choose a convenient coordinate system for each object

Apply Newton’s Second Law

The x- and y-components should be taken from the vector equation and written separately

Solve for the unknown(s)
Equilibrium

An object either at rest or moving with a constant velocity is said to be in equilibrium

The net force acting on the object is zero (since the acceleration is zero)
F  0

Easier to work with the equation in terms of its components:
F
x

 0 and
F
y
This could be extended to three dimensions
Quick Quiz 4.6: Consider the two situations shown in Figure 4.11, in
which there is no acceleration. In both cases, the men pull with a force
of magnitude F. Is the reading on the scale in part i of the figure
a. greater than the reading in part ii
b. less than the reading in part ii
c. equal to the reading in part ii
0
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Example 4.5 A traffic light weighing 1.00 x 102 N hangs from a vertical cable tied to two other cables
that are fastened to a support, as in the diagram. The upper cables make angles of 37.0° and 53.0 ° with
the horizontal. Find the tension in each of the three cables.
Example 4.6: A child holds a sled at rest on a frictionless, snow-covered hill, as shown in the diagram.
If the sled weighs 77.0 N, find the force exerted by the rope on the sled and the magnitude of the force

n exerted by the hill on the sled
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Example 4.7: The combined weight of the crate and dolly in the diagram is 3.00 x 102 N. If the man
pulls on the rope with a constant force of 20.0 N, what is the acceleration of ther system (crate plus
dolly), and how far will it move in 2.00 s? Assume that the system starts
from rest and that there are no friction forces opposing the motion.
Example 4.8: A car of mass m is on an icy driveway inclined at an angle θ = 20.0°, as in the diagram
a. Determine the acceleration of the car, assuming that the incline is frictionless.
b. If the length of the driveway is 25.0 m and the car starts from rest at the top, how long does it
take to travel to the bottom?
c. What is the car’s speed at the bottom?
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Example 4.9: A man wighs a fish with a spring scale attached to the ceiling of an elevator, as shown in
the diagram. While the elevator is at rest, he measures a weight of 40.0 N
a. What weight does the scale read if the elevator accelerates upward at 2.00 m/s2 ?
b. What does the scale read if the elevator accelerates downward at 2.00 m/s2 ?
c. If the elevator cable breaks, what does the scale read?