Download How can we

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Do Now:
1. What is the formula to calculate weight?
2. A. Draw the resultant force and calculate it.
7N
B. Calculate the angle.
Tan 
 1.4
5N
  Tan (1.4)
1
R = 8.6N
7N
7N
Θ=54.5°
5N
5N
  54.5
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Aim: How can we calculate forces
acting on an object?
Free-Body diagrams
FN
Fapp
Ff
Fg
Free-body diagrams – is a sketch that shows all of the
forces acting on an object.
 Example1: Draw a free-body diagram of the forces acting
on a text book sitting on a table.
Normal force – contact force exerted by the surface on the
object. (always perpendicular to the surface)
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FN
Fg
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Ex2: An .1 kilogram egg is free-falling from
a nest in a tree. Neglect air resistance.
Fg
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Ex3: A 50 newton object is suspended
motionless from the ceiling by two ropes.
FT
FT
Fg
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A rightward force is applied to a book in order to
move it across a desk. Draw the free-body
diagram.
FN
Ff
Fapp
Fg
Aim: How can we describe
objects in equilibrium?
Explain equilibrium in one word.
 Determine the normal force of a 35 newton
object at rest on a table.
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How can we define equilibrium?
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Forces are equal in a specific direction. (xdirection & Y-direction)
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Resultant force equals zero. (net force = 0N)
Ex: A box has a mass of 10 kg. Calculate the
magnitude of the normal force.
FN
Fg
Equilibrium
At rest
At constant velocity
Fnet = 0 Newtons
A student pushes a 15 newton box across
the table with 20 newtons of force at
constant velocity.
 ***Constant velocity the net force equals
zero***
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What does “a 15 newton box mean?”
 Sketch a free body diagram.
 Determine the frictional force.
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A flying bird is gliding (no wing flaps) from a tree
to the ground at constant velocity. Consider air
resistance. Draw a free-body diagram.
Fair fric
Fg
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A student holds a 4.5 kilogram object from
a string. The object is motionless.
Calculate the force of tension in the string.
FT
Fg
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Do Now:
1. List three ways an object can have
acceleration.
A 10 Newton force to the left is applied to a box.
Also, a 10 Newton force to the right is applied to
the same box. The box must be
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A) at rest
B) in motion
C) either in motion or at rest
How can we describe Newton’s
1st law?
Introduction to Newton’s laws of
Motion
Dynamics – is the branch of mechanics
that deals with how the forces acting on an
object affect its motion.
 An object in dynamic equilibrium moves
with constant velocity. That means No
NET Force! Net force equals zero.
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How can we apply Newton’s first
law of motion?
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A .5 kilogram mass is hanging from a
spring. How can we determine the net
force?
How can we describe the two states of
equilibrium?
Equilibrium
Static Equilibrium
Net force zero, object at rest
Dynamic Equilibrium
Net force zero, object moving
with constant velocity
How can we describe inertia?
Inertia is the tendency of an object to
resist a change in state of motion.
 Mass is used to measure inertia.
(kilograms)
 Inertia and mass are directly proportional.
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Example: The more mass an object has the
more inertia the object has.
Newton’s first law of motion – An object at
rest will remain at rest and an object in
motion (constant velocity) will remain in
motion unless acted on by an unbalanced
force.
 Have you ever experienced inertia in an
automobile while it is braking to a stop?
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Describe the following situation using Newton’s
first law.
While riding a skateboard (or bicycle), you fly
forward off the board when hitting a curb which
abruptly halts the motion of the skateboard.
To dislodge ketchup from the bottom of a
ketchup bottle, it is often turned upside down
and thrusted downward at high speeds and then
abruptly halted.
How can we apply Newton’s 2nd
law?
A 6 kilogram box is being pushed with a
25 newton force to the left on a table. The
box is moving with constant velocity.
 Sketch free body diagram and label all of
the forces.
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How can we define Newton’s
2nd law?
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Newton’s second law states when an
unbalanced force acts on an object, the object
is accelerated in the same direction of the
force.
The net force is NOT zero. NO Equilibrium.
Formula:
net
Fnet (sum of forces), m (mass), a
(acceleration).
F
 ma
Describe the mathematical relationship
between acceleration and net force.
 Its acceleration is directly proportional to the
net force
 Describe the mathematical relationship
between acceleration and mass.
 Its acceleration is inversely proportional to
the object’s mass
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A 5 kilogram box is pushed with a 20
newton force to the right on a frictionless
surface. Determine the acceleration.
 A 2 kilogram box at rest accelerates to a
speed of 10 meters per second in 2
seconds. Calculate the net force.
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How can we determine the net
force?
Determine net force using a picture
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Example 1: A 2kg block is sitting on a table. A 10 N
force is applied to the right and a 45 N force is applied
to the left.
A) Determine the net force acting on the block?
B) Calculate the acceleration.
F net in the x direction is the
difference between the two forces
acting in the x direction.
Fnet  30 N  10 N
Fnet  20 N to the left
Refer to example 1:
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The net force acting on the box is 20 Newtons
to the right.
Calculate the acceleration.
Fnet  ma
20 N  (2kg)a
20 N
a
2kg
a  10m / s 2 to the left
A 20 newton force is applied to a 4
kilogram object to the right accelerating it
at 4.5 meters per second2.
 A) Determine the net force.
 B) Calculate the frictional force.
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A person pushes a 7.0 kg box on a frictionless
surface. The box accelerates 5 m/s2. Calculate
the applied force on the box.
Fnet  ma
Fnet  (7.0kg)(5m / s 2 )
Fnet  35N
to the right
How can we use Newton’s 2nd law
to solve motion problems?
Define Newton’s 1st law.
 Sketch the equilibrium diagram.
 Determine the net force on an object when
it is moving with constant velocity.
 A 3 kilogram chair is being push with a 10newton force to the right. Sketch a free
body diagram.
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Cart demonstration
As mass decreases, acceleration
_____________.
 How would you sketch a graph of
acceleration vs. mass?
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A 50 newton force is required to
accelerate a 3.5 kilogram object at 8.6m/s2
over a level floor.
A) Sketch a free body diagram.
 B) Determine the net force
 C) Calculate the magnitude of the frictional
force acting on the object
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A 6.5 kilogram object is accelerating at 8.0
meters per second2. The magnitude of the
frictional force is 10 newtons.
 A) Sketch a free body diagram.
 B) Determine the net force.
 C) Calculate the applied force.
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**A constant 3,000 newton frictional force
is acting on a 1200 kilogram car. The car
is brought to rest in 20 seconds.
 A) Identify the net force.
 B) Calculate the deceleration.
 C) Calculate the initial velocity of the car.
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