Transcript Force, Work, and Power

Force, Work, and Power
Net Force Practice Problems
1. A 1200kg truck that is traveling at 50 m/s2 has a headon collision with a 400kg car that is traveling at 30 m/s2.
What is the net force of the collision and which direction
will the crash go?
48,000
2. Josh and I are pushing a T.V. cart down the hallway…
Josh pushes with a force of 25N and I push with a force
of 15N. What is the net force?
40N of force
3. I am playing tug of war with my dog… she is pulling on
the rope with a force of 13N to the right and I am pulling
with a force of 23N to the left. What is the net force
and which direction is the rope going?
10N
Newton’s First LAW
LAW 1 (Law of Inertia): An object at
rest will remain at rest and an object in
motion remains in motion unless acted
upon an unbalanced force.
Inertia: The property of an object to
resist changes in motion unless acted
upon by an unbalanced force.
Unbalanced force: A net force that is
not equal to zero.
Ex. When the car suddenly stops and
your body snaps forward.
Newton’s Second LAW
LAW 2: The unbalanced force acting on
an object equals the object’s mass
times its’ acceleration.
To accelerate the mass of an object you
must apply a force!
F= m x a
Ex. Any time you move an object
Newton’s Third LAW
LAW 3:For every action force, there is
an equal and opposite reaction force.
Ex. Hitting a homerun off of a pitcher
in baseball.
Examples of Newton’s Laws
If you are pushing an empty grocery
cart with 20N of force… will the
grocery cart have the same acceleration
if you continue to push it at the same
force of 20N when it is full.
Which Law and why?
A: 2nd law. NO…the acceleration will be
less once the cart is full b/c you have
added mass, and a = F/m
Examples of Newton’s Laws
What would happen if you were standing
on a skateboard and threw a really
heavy ball?
Which law and why?
A 3rd law. : You will go backwards on
the skateboard b/c the ball is pushing
you with an equal and opposite force the
other way. For every action there is an
equal and opposite reaction.
Examples of Newton’s Laws
If you are riding on the bus and your
books are on the seat next to you… if
the bus driver stomps on the
brakes…where do your books go???
Which law and why?
A: 1st law. onto the floor… An object in
motion stays in motion until acted on by
an unbalanced force.
WORK
The Transfer of
Energy
How does the previous investigation help us to understand how
forces transfer energy?
The Transfer of Energy is
called WORK
Work measures the effects of a force
acting over a distance.
Work = F*d so…
Work= energy transferred
The units are N * m = Joules (J)
1 Joule= 1kg x m2/s2
(F)
* (d) = Work
(1kg x m/s2 x m)= 1kg x m2/s2
Work and Power Warm Up
1. Who does more work - a man who
lifts a large box from the ground up
into the back of a truck, or a man
who puts the same box on a trolley
and wheels it up a ramp into the
truck?
A the man who lifts it
B the man with the trolley
C they do the same work
Work and Power Warm Up
1. Who does more work - a man who
lifts a large box from the ground up
into the back of a truck, or a man
who puts the same box on a trolley
and wheels it up a ramp into the
truck?
A the man who lifts it
B the man with the trolley
C they do the same work
Work and Power Warm Up
2. Energy and work use the same
unit of the Joule because:
A that is the unit chosen by the scientist
named Pascal who studied heat
B energy is required to do work
C both measure the speed at which power is
used
Work and Power Warm Up
2. Energy and work use the same
unit of the Joule because:
A that is the unit chosen by the scientist
named Pascal who studied heat
B energy is required to do work
C both measure the speed at which power is
used
Work Practice Problems Pg. 285
*Challenge*
F = m*g
5.) d = .5m
(g = 9.8m/s2)
m = 1200kg
F=?N
= (1200kg)*(9.8m/s2) =
Force= 11,760N
F = 11,760N
W = F*d
d = .5m
W = ?J
= (11,760N)*(.5m) = 5880J
Spectrum Text Book pg 697-698 (81 and 83)
WORK
Q: If you try to move a lab table with
12,000N of force and it doesn’t
move…How much work have you done?
A: NONE!!! Because…
W = 0J b/c the distance was 0m and W =
12,000N* 0m = 0J
POWER
What’s Power?
Power = how much work is done in a given amount of
time.
Does running require more work than walking the
same distance?
No! So what is the difference between running and
walking?
A: The time it takes!!!
So we need to account for the time it takes to do
work… the equation that measures that is
P = W (Units for Power are Joules = Watts)
t
sec
Think about light bulbs!
POWER
in DUFAS
1. While running track, Drew’s legs do 5780J
of work in 183sec. What is his power output?
31.58 Watts
2. The chain that is pulling a rollercoaster up
the first hill does 24652J of work over a
79sec time interval. What is the power
output of the chain?
312.05 Watts
3. It takes Ms. Webb 20s to apply 23N of
force to lift a box 5m. What was her power
output?
5.75 Watts
Methods of
Energy
TRANSFER
Energy TRANSFER:
Energy transfer - is how the same type
of energy is passed from one object to
another object.
There are only THREE ways that energy
can be transferred between objects…
Energy TRANSFER:
Method 1: Conduction
The transfer of energy through matter
by direct contact of particles.
Examples: a pot on a stove, a metal spoon
in a bowl of hot soup, wires carrying
electricity, friction/air resistance
transforms energy to heat.
Energy TRANSFER:
Method 2: Radiation
The transfer of energy in the form of a
wave. (mechanical or electromagnetic
waves)
Examples: How Earth gets it’s energy
from the sun, a microwave oven cooking
food, sound.
Energy TRANSFER:
Method 3: Convection
The transfer of energy by the movement
of gases or liquids. (fluid movement only!!!)
Hot fluids will rise while cold fluids will
sink… this creates a swirling motion.
Examples: Hurricanes, pasta or rice swirling
in boiling water, a hot air balloon.