Transcript force vectors and free body diagrams notes

A force is something that is
capable of changing an object’s
state of motion, that is, changing
its velocity
1)
2)
A ski jumper competing for an Olympic gold
medal wants to jump a horizontal distance of 135
meters. The takeoff point of the ski jump is at a
height of 25 meters. With what horizontal speed
must he leave the jump?
Challenge! A marshmallow is dropped from a 5meter high pedestrian bridge and 0.83 seconds
later, it lands right on the head of an unsuspecting
person walking underneath. How tall is the
person with the marshmallow on his head?
Learning
Targets
2.1
2.2
2.3
2.4
2.5
2.6
1.4
Description of Learning Target
Resolve a vector into it’s x and y components.
Express a vector in magnitude-angle or component
notation.
Describe the relationship between horizontal and
vertical motion.
Calculate the velocity, range, or flight time of a
projectile where the initial y-velocity is zero.
Calculate the velocity, range, or flight time of a
projectile where the initial y-velocity is NOT
equal to zero.
Describe relative velocity for a moving object
from various perspectives.
Report measurements with appropriate metric
units and significant figures.
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Force ( F ) is the cause of acceleration, or
change in velocity
Force is a vector quantity
There must be a net, unbalanced force
acting on an object for the object to
change its velocity
10 N
30 N
Net Force = 20 N
This figure illustrates
what happens in the
presence of zero and
nonzero net force.
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Net Force,  F i or Fnet is the vector
sum, the resultant, or the unbalanced
force acting on an object. The symbol, F,
means the vector sum of forces.
The unit of force in SI is a derived unit
called newton, N. 1 N = 1 kg*m/s2
1 N is the net force required to accelerate
a mass of 1 kg with an acceleration of 1
m/s2
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1.
2.
To analyze the force acting on an object,
you should draw a free-body diagram.
Isolate the object of interest (this should
be the only object in the diagram)
Draw all the forces acting on the object as
vectors, including directions
1.
2.
3.
First isolate the book and analyze the forces
acting on the book. There are two forces
acting on the book, the gravitational force, or
weight, and the supporting force on the book
by the desk.
First, the gravitational force is always present
if we are dealing with objects on the Earth.
Second, whenever an object makes a physical
contact with another object, a force results.
Here the book makes a contact with the desk,
so there is a supporting force. In most cases,
these contact forces are perpendicular to the
contact surface and therefore are called
normal forces, (N).
N
w
A mass at the origin is pulled by four perpendicular
ropes
ˆ , (8N ) x
ˆ , (15 N )  x
ˆ , (18 N )  y
ˆ
(10 N ) y
Find the magnitude of the resultant force.
2. Find the angle at which the force is acting.
Now each of the ropes is pulled at 30o
counterclockwise from the given axes.
3. Draw a free-body diagram and find the horizontal
and vertical components of the new resultant
force.
4. At what angle is the resultant force acting with
respect to the x-axis?
1.
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1.
We distinguish two types of forces:
A contact force, such as a push or pull,
friction, tension from a rope or string, and
so on.
 What
2.
are the 5 types of contact forces?
A force that acts at a distance, such as
gravity, the magnetic force, or the electric
force.
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A net force is not required to keep an
object at rest. If the sum of all the forces
is zero, the object is considered “static”
(not moving)
A net force is not required to keep an
object in motion. Where there is no
friction, objects will continue moving
with constant velocity. Therefore, THE
NET FORCE ACTING ON AN OBJECT
WITH CONSTANT VELOCITY IS ZERO.
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Objects in Equilibrium
 Nellie Newton Sketches (p.1)
Objects in Motion
 Vectors and the Parallelogram Rule (p. 23)
The mass of the object is too large to be moved by
the vertical component of the rope’s force, so it is
not in motion.  Fy = 0, where
 Fy = FN – mg + Fy
Fy = FN – (500kg)(9.8 m/s2) + (150N)(sin45o)
Free-body diagrams for four situations
are shown below. The net force is known
for each situation. However, the
magnitudes of a few of the individual
forces are not known. Analyze each
situation individually and determine the
magnitude of the unknown forces.
BONUS: What is the 5th type of contact force?
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An object at rest will remain at rest and
an object in motion will keep moving
with constant velocity if the net force on
the object is zero.
Often called the law of inertia. Inertia is
the natural tendency of an object to resist
a change in motion, measured
quantitatively by its mass.
What happens if the net force is not zero?
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Acceleration depends on the net force and on the
mass of the object
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 Fnet

a
or Fnet  ma
m
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Newton’s law is a vector equation.
Acceleration is in the direction of the net force,
not necessarily the direction of the velocity.
Include all forces acting on an object to determine
the net force.
Do not include balanced forces (such as weight
and normal force) because they cancel each other
The units of force are called newtons.
1 Newton = 1N = 1 kg . m/s2.
A 5.0 N horizontal force pulls a 20 kg box
on a horizontal surface. A 3.0N friction
force slows the motion. What is the
acceleration of the object?
1. draw a free-body diagram of the box
2. write the given information
3. Apply the equation F = ma to solve the
problem
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Find the magnitude of the weight of a
3.50 kg object on the surface of the earth
where
w = mg
Newton’s second law applies separately to
each component of the force.
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If object 1 exerts force on object 2, then
object 2 exerts an equal and opposite
force on object 1.
The two forces are acting on two
different objects, therefore even though
the two forces are equal and opposite,
they do not necessarily cancel each other.
Often called the law of action and
reaction.
For every force (action), there is an equal and
opposite force (reaction).
Note that the action and reaction forces act
on different objects.
This image shows how
a block exerts a
downward force on a
table; the table exerts
an equal and opposite
force on the block,
called the normal force
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A large truck collides head-on with a
small car and causes a lot of damage to
the small car. Explain why there is more
damage to the small car than to the large
truck.
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P. 131 -132; 2, 3, 5, 6, 13, 14, 15, 16, 19