Download Net Force Problems

Net Force Problems
1. Find the net force for the following problems:
a.
= ???
a.
a.
a.
= ???
= ???
= ???
Net Force Problems
1. Find the net force for the following problems:
a. 5N Left - 1N Right = ???
a. 10NLeft - 5N Right = ???
a. 34N Right - 20N Left = ???
a. I 40N Left - 20 N Right I = ???
a. I 100N Left - 200N Right I = ???
Newton’s First Law of Motion
First we need to define the word
FORCE:
• The cause of motion (what causes objects
to move)
• Two types of forces
– Pushes
– Pulls
Slide from www.science-class.net
Forces may be balanced or
unbalanced
• Balanced forces – all forces acting on an
object are equal
– There is NO MOTION
• Unbalanced forces – one or more forces
acting on an object are stronger than others
– There is MOTION
• A NET FORCE
Objects at Rest
• Objects at rest tend to
stay at rest unless acted
upon by a force. [push
or pull]
• Newton described this
tendency as inertia.
• Inertia can be
described as the
tendency of an object to
keep doing whatever’s
it’s doing.
Difference between Weight and
Mass???
– Difference Between Mass and Weight???
•Mass is a measure of how much matter
an object has.
•Weight is a measure of how strongly
gravity pulls on matter. The MORE
MASS an object has, the MORE
INERTIA the object has.
Mass & Inertia
Which vehicle has more
inertia?
• Mass is the amount of
matter in an object.
• The more MASS an
object has, the more
INERTIA the object
has.
• Bigger objects are
harder to start & stop
Slide from www.science-class.net
What about objects that are
already in motion?
• Newton stated that
objects in motion tend
to stay in motion until
acted upon by a force
(or hits it.)
Newton’s 1st Law
(also known as the law of inertia)
• A moving object moves in a straight line with
constant speed unless a force acts on it.
• The tendency of an object at rest to remain
at rest and an object in motion to remain in
motion unless acted upon by an unbalanced
force.
• Objects do not change their motion unless
a force acts on them
The truck is in motion. What is the force
that causes it to stop?
The push of the stopped car.
The car is at rest. What is the force that
causes it to move?
The push of the truck.
Slide from www.science-class.net
Newton’s Second Law of Motion
How fast does it go?
Acceleration
• An unbalanced force causes something to
accelerate.
Acceleration
• Acceleration is directly related to the size of
the force and the direction of the force.
• It accelerates in the direction you push or pull
it.
In other words….
Large Force
= Large Acceleration
F
a
In other words….
Small Force
= Small Acceleration
F
a
So….if you push twice as hard, it accelerates twice as much.
But there is a twist….
• Acceleration is INVERSELY related to the mass
of the object.
In other words…..using the same
amount of force….
F
Small acceleration
Large Mass
a
Large acceleration
F
Small Mass
a
Newton’s Second Law
• Newton, that brilliant genius, observed those
“rules” of acceleration and came up with his
second law of motion. It is both a formula & a
law.
Newton’s Second Law
• The acceleration of an object is directly
proportional to the net force & inversely
proportional to it’s mass.
•Force = Mass x Acceleration
• F = ma
Okay then…
• First, you need to know the units of Force,
Mass & Acceleration.
– The units used for force are Newtons (N)
– The units used for mass are kilograms (kg)
– The acceleration units are meters per second
squared (m/sec2).
Next, what is speed?
Average Speed
• Comparison of time and distance
– Distance traveled per unit time
Next, what is acceleration?
Acceleration
• A change in velocity
– Speeding up
• Positive acceleration
– Slowing down
• Negative acceleration
• Deceleration
– Changing direction
Calculating Speed
Given Distance & Time
D
S
Speed =
T
Distance
Time
Speed = Distance ÷ Time
Let’s Practice
• Riley chunks a water balloon at a guy.
Running for his life, he travels 100m in
9.83s. What was his average speed?
S= Distance/time
S= d/t
S= 100m/9.83s
S=10.17 mps (meters per second)
Calculating Distance
Given Speed & Time
D
S
Distance = Speed • Time
T
Multiply Speed and Time
Distance = Speed X Time
Speed X Time = Distance
Let’s Practice
1. If you ran 15 km/h for 20 h, how much
distance would you cover?
Distance = Speed x time
D=ST
D=15 km/h x 20 h
D = 300 km
Calculating Time
Given Distance and Speed
D
S
Time =
T
Distance
Speed
Divide Distance by Time
Distance ÷ Speed = Time
Time = Distance ÷ Speed
Let’s Practice
•
Marcy doesn't want to be late for class so
she rushes to McDonald’s at lunch. How
much time would it take Marcy to walk 2 km
to McDonalds for a Big Mac if she walked at
a rate of 4.5 km/h?
Time = distance/speed
T=DS
T = 2km / 4.5 km/h
T= .44 hours or 26.4 minutes
Calculating Acceleration
• Acceleration Equation
(final speed (in m/s) – initial speed (in m/s))
Acceleration =
Time (in seconds)
A=
(sf-si)
t
Let’s practice…
•
Rob is really bored one Saturday night and
goes outside to study the nocturnal habits of
mice in the hayfield. He sees a mouse
sniffing along at 0.1 m/s. but it hears and
starts to scurry for safety. In just 3.7 s it
accelerates to 0.9 m/s. Find its acceleration.
Let’s Practice
(final speed (in m/s) – initial speed (in m/s))
Acceleration =
A=
A=
(sf-si)
t
0.9m/s – 0.1 m/s
3.7s
A=
0.8m/s
3.7s
A= 0.22 m/s2
Time (in seconds)
Let’s Practice
•
A roach moves down the hall at 1.2 m/s.
When he sees the janitor coming down the
hall, he begins to run. After 3.2 s, he is
moving at 3.6 m/s. What is his acceleration?
Let’s Practice
•
While waiting for his Mom to come out of the
hairdresser's, Sean accidentally puts the car
in gear and it begins to roll forward. How far
would the vehicle travel if it moved at 34 m/s
for 2.5 s?
Let’s Practice
•
While showing off for some girls at the skate
park, Josh D crashes.. After the crash he
tumbled 30 m in 4.2 s, what was his speed
in m/s
Let’s Practice
•
Colin skateboards down the sidewalk in front
of the school, traveling at 24 km/h. How
much time would it take him to travel 6.0
km?
Newton’s 3rd Law
For every action…..
Newton’s Third Law
3
Action and Reaction
• Newton’s third law describes something else
that happens when one object exerts a force
on another object.
• According to Newton’s third law of motion,
forces always act in equal but opposite pairs.
Newton’s Third Law
3
Action and Reaction
• Another way of saying this is for every
action, there is an equal but opposite
reaction.
• This means that when you push on a wall,
the wall pushes back on you with a force
equal in strength to the force you exerted.
Newton’s Third Law
3
Action and Reaction
Forces Don’t Cancel
• The forces exerted by two objects on each
other are often called and action-reaction
force pair.
• Either force can be considered the action
force or the reaction force.
• Action and reaction force pairs don’t cancel
because they act on different objects.
Newton’s Third Law
3
Action and Reaction
Forces Don’t Cancel
• You constantly use actionreaction force pairs as you
move about.
• When you jump, you push
down on the ground.
• The ground then pushes up
on you. It is this upward
force that pushes you into
the air.
Newton’s Third Law
3
Action and Reaction
Forces Don’t Cancel
• When a bird flies, its wings push in a
downward and a backward direction.
• This pushes air downward and backward.
• By Newton’s third law, the air pushes back
on the bird in the opposite directions—
upward and forward.
• This force keeps a bird in the air and
propels it forward.
Newton’s Third Law
3
Large and Small Objects
• When you walk
forward, you
push backward
on the ground.
• Your shoe pushes
Earth backward,
and Earth pushes
your shoe forward.
Newton’s Third Law
3
Large and Small Objects
• Earth has so much mass compared to you
that it does not move noticeably when you
push it.
• If you step on something that has less
mass than you do, like a skateboard, you
can see it being pushed back.
Newton’s Third Law
3
A Rocket Launch
• When the rocket fuel is ignited, a hot gas
is produced.
• As the gas
molecules collide
with the inside
engine walls, the
walls exert a force
that pushes them
out of the bottom
of the engine.
Newton’s Third Law
3
A Rocket Launch
• This downward push is the action force.
• The reaction force is the upward push on
the rocket engine by the gas molecules.
• This is the thrust that propels the rocket
upward.
Newton’s Third Law
3
Weightlessness
• You might have seen pictures of astronauts
floating inside a space shuttle as it orbits
Earth.
• The astronauts are said to be weightless.
• Yet the force of gravity on the shuttle is
almost 90 percent as large as at Earth’s
surface.
• Newton’s laws of motion can explain why
the astronauts float as if there were no
forces acting on them.
Newton’s Third Law
3
Measuring Weight
• When you stand on a
scale, your weight
pushes down on the
scale.
• This causes the scale
pointer to point to
your weight.
Newton’s Third Law
3
Measuring Weight
• At the same time, by Newton’s third law the
scale pushes up on you with a force equal to
your weight.
• This fore balances the downward pull of
gravity on you.
Newton’s Third Law
3
Free Fall and Weightlessness
• Now suppose you were
standing on a scale in an
elevator that is falling.
• A falling object is in free
fall when the only force
acting on the force is
gravity.
• You and the scale are
both in free fall.
Newton’s Third Law
3
Free Fall and Weightlessness
• Because the only force
acting on you is gravity,
the scale no longer is
pushing up on you.
• According to Newton’s
third law, you no longer
push down on the scale.
Newton’s Third Law
3
Free Fall and Weightlessness
• So the scale pointer stays
at zero and you seem to be
weightless.
• Weightlessness is the
condition that occurs in
free fall when the weight
of an object seems to be
zero.