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State Newton’s First Law
Provide an example of his first law
Define Inertia
An object in motion wants to stay in
motion at the same speed in the
same direction, and an object at rest
wants to stay at rest.
Headrests are placed in cars to prevent
whiplash injuries during rear-end
collisions. Your body wants to continue
the forward movement unless there is
something to stop it.
The resistance an object has to
change in motion. The more mass
an object has the more inertia it
has (In other words it is harder to
change its motion)
If you were careful pulling the Jenga
pieces out of the tower, the remaining
pieces stayed in place. If you applied a
force which disturbed the pieces
above, then they all fell (motion
change)
What terms can be used to describe
motion when the forces are balanced?
Provide an example in the form of a
free body diagram.
An outside force is required to
change the motion of an object.
What is the net force of balanced
forces?
The net force of balanced forces is
always zero.
Constant Motion
At equilibrium
At Rest
Arrows should be equal in
magnitude.
What is the net force of unbalanced
forces?
What terms can be used to describe
motion when the forces are
unbalanced?
Provide an example in the form of a
free body diagram.
If the net force is anything but zero,
then the forces are not balanced.
Acceleration
One arrow has a greater magnitude
then the others.
Fnet > Zero
What formula is used to describe
FW?
Fw = mg
Fw = Force of Weight
m = mass
g = gravity
Fw is the result of acceleration due
to gravity, so you multiply the mass
of the object times the g value for
the location. (Earth -9.8m/s/s)
On the moon your mass is ___the
same__ to your mass on earth.
What forces oppose the force of
gravity?
Air Resistance
Lift Force
Normal Force (If the object is at rest on
a horizontal surface)
On earth your weight is __greater___
than on the moon.
If there are two arrows pointing
upward and you have a Fw of
240N, what is the upward force on
each arrow if the net force is zero?
Each arrow must stand for 120 N,
unless other values are stated in
the problem.
Calculate the mass of an object
with a force of 12 N on Earth, then
use that calculated mass to find the
force of the object on the moon. ( g
= -1.63 m/s2)
Fw = mg
20N = m(-9.8m/s/s)
20/9.8 = m
2.04g = m
Fw = mg
Fw = 2.04g (-1.63m/s/s)
Fw = 3.3N on the moon
You must know that mass stays the
same no matter your location to
solve this problem. (Weight
changes from location to location,
on the moon is less than on Earth)
What is basic definition for an
applied force?
What force opposes this force?
A push or pull
On any type of surface the force of
friction (Ff) opposed motion. You must
overcome the force of friction to get an
object to move. If there is no applied
force then the object will eventually
stop.
Compare and Contrast Fair resistance and
Fnormal
Both are vertical forces, however air
resistance is felt as an object moves
through the air and the normal force
is applied when an object is located
on a surface.
(Since the normal force is
perpendicular to a surface, it is not
just simply a vertical surface if the
object is on an incline plane.)
Draw a free body diagram for an
object moving to the right at a
constant velocity.
If the force applied to an object is
53 N, what force is needed to
change the motion of that object?
A force greater than 53 N must be
applied, because only 53 N would
keep motion the same.
Draw a free body diagram for an object
at rest.
Draw a free body diagram for an
object under the influence of only
gravity.
Draw a free body diagram for an object
moving with a rightward acceleration.
Draw a free body diagram for an
object coasting to the right.
State Newton’s second law and
provide an example:
F = ma
The rate of acceleration is directly
proportional to the force applied to
the object.
F = ma
What units does the F stand for?
Units of Force is Newtons = (kg)(m/s2)
What units does the m stand for?
Units for mass = kg
What units does the a stand for?
Units for acceleration = m/s2
If you increase the force applied
what happens to the acceleration?
It will increase
Jenny is pushing a box to the right with
a force of 5 N, Julie is pulling the same
box to the left with a force of 6 N.
Draw the free body diagram and find
the net force. Is the motion changing
or staying the same?
The net force is 1N to the left, since it is
not a net force of zero, the motion will
change.
State Newton’s Third Law:
For every action there is an equal
but opposite reaction.
The rate of acceleration is inversely
proportional to the mass of the
object. (The greater the mass the
less acceleration you can expect for
the object)
Ex) If you double the force applied
to an object, then the acceleration
doubles.
If you double the mass of the object
then the acceleration is cut in ½.
1 kg of feathers v. 1 kg of nails,
which has the greatest weight?
Since their mass is the same and you
multiply them be the same
acceleration of gravity (-9.8), the
have the same weight.
6N
If you increase the mass you are
applying the force to, what
happens to the acceleration?
The acceleration will not be as
great due to the larger mass having
more inertia.
This means forces happen in pairs
and there is some type of reaction
force for each action.
Ex) You push on the wall, the wall
pushes back.
5N
More Practice Problems
What is the weight of an object (on
earth) that has a mass of 45 kg?
Fw = mg
45kg (-9.8m/s/s) = 441 N
The value of gravitational pull on the
moon is 1.6 m/s/s. What is the weight
of a 75 kg astronaut on the moon?
Fw = mg = 75 (1.6) = 120N
What is the force on a 1 kg ball that
is falling freely due to the pull of
gravity?
Fw = mg = 1 kg (-9.8m/s/s) = 9.8N
What is the mass of a person who
weighs 500 N?
Fw = mg
500N = (m)(-9.8m/s/s)
500N/(-9.8) = 51kg
How much force is needed to
accelerate a 66 kg skier at 2 m/sec2?
F= ma = 66kg(2m/s2) = 132N
What is the force on a 1000 kg
elevator that is falling freely at 9.8
m/sec2?
F= ma = (1000kg)( 9.8 m/sec2) =
9800N
What is the acceleration of a 50 kg
object pushed with a force of 500
newtons?
F/m = a
500N/50kg = 100m/s2
The mass of a large car is 1000 kg. How
much force would be required to
accelerate the car at a rate of 3
m/sec2?
F= ma = (1000)(3) = 3000N
A 50 kg skater pushed by a friend
accelerates 5 m/sec2. How much
force did the friend apply?
F= ma = 50(5) = 250N
Remember to study all your notes and practice pages Ex) free body diagrams and force problems