Transcript Newton`s Third Law Action-Reaction

Newton’s Third of
Motion
Newton’s Third Law
• Action-Reaction
• Whenever one body
exerts a force on a
second body…
• …the second body
exerts an equal and
opposite force on the
first body.
• Figure 5.7
Newton’s 3rd law of motion
• Source of force - other
objects
• 3rd law - relates forces
between objects
• “Whenever two objects
interact, the force exerted
on one object is equal in
size and opposite in
direction to the force
exerted on the other
object.”
FA due to B =FB due to A
• You push on an box sitting on the floor
horizontally with a force of 15 Newtons
and the box does not move. The force
of friction on the box is
– A) 0 Newtons
– B) 15 Newtons in the direction of your
push
– C) 15 Newtons opposite to your push
– D) less that 15 Newtons.
Questions from Page 78
and 80
• We know that the Earth pulls on the
moon. Does this mean that the moon
also pulls on the Earth?
– A. Yes
– B. No
Question
• An archer shoots an arrow. Consider
the action force to be the bowstring
against the arrow. The reaction force
is the…
– (a) weight of the arrow.
– (b) air resistance against the bow.
– (c) friction of the ground against the
archer’s feet
– (d) grip of the archer’s had on the bow
– (e) arrow’s push against the bowstring.
Questions from Page 78 and
80
• A high speed bus and a bug have a
head-on collision.
• The force of impact splatters the bug.
– Is the corresponding force that the bug
exerts against the windshield greater,
less, or the same?
Questions from Page 78 and
80
• A high speed bus and a bug have a head-on
collision.
• The force of impact splatters the bug.
– Is the corresponding force that the bug exerts
against the windshield greater, less, or the
same?
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•
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•
•
A. Greater
B. Less
C. Same
D. It depends on the speed of impact.
E. All of these
Definitions
• Vector quantity - a quantity that has both
magnitude and direction
• Vector - an arrow drawn to scale used to
represent a vector quantity
• Scalar quantity - a quantity that has magnitude
but not direction
Vector or Scalar?
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•
•
•
•
•
Speed………..
Velocity……...
Acceleration..
Time………….
Distance……..
Force…………
scalar
vector
vector
scalar
scalar
vector
The Components of a Vector
Even though you know how far and in which
direction the library is, you may not be able
to walk there in a straight line:
The Components of a Vector
Can resolve vector into perpendicular
components using a two-dimensional
coordinate system:
The Components of a Vector
Length, angle, and components can be
calculated from each other using
trigonometry:
The Components of a Vector
Signs of vector components:
Addition of Vectors
• The sum of two or more vectors is
called their resultant.
• To find the resultant of two vectors that
are at angles to each other, we use the
tip-to-tail method.
Adding and Subtracting Vectors
Adding vectors graphically: Place the tail of the
second at the head of the first. The sum points
from the tail of the first to the head of the last.
Adding and Subtracting Vectors
Adding Vectors Using Components:
1. Find the components of each vector to be
added.
2. Add the x- and y-components separately.
3. Find the resultant vector.
Adding and Subtracting Vectors
Adding and Subtracting Vectors
Subtracting Vectors: The negative of a vector is
a vector of the same magnitude pointing in the
opposite direction. Here, D = A – B.
Position, Displacement, Velocity, and
Acceleration Vectors
Position vector r points from the origin to the
location in question.
The displacement vector
Δr points from the original
position to the final
position.
Relative Motion
The speed of the passenger with respect to
the ground depends on the relative directions
of the passenger’s and train’s speeds:
Relative Motion
This also works in two dimensions:
Motion is Relative
• Suppose that an airplane is traveling North
at 120 km/h relative to the air.
• (a) If the wind is blowing 20 km/h toward the
North, how fast will the plane travel relative
to the ground?
• (b) What if the wind is blowing South?
• (c) East?
• Consider a boat that normally travels 10km/h in still
water.
• If the boat travels in a river that flows also at a rate
of 10km/h, what will it velocity relative to the shore
when it heads directly upstream?
• When it heads directly downstream?
• When it heads across the river?