Download Chapter 12 Notes - Brookville Local Schools

Chapter 12
Warm Up 11/20
1. Give an example of a unit for time
2. Give an example of a unit for speed
3. Give an example of a unit for velocity
4. Give an example of a unit for acceleration.
Forces
Part 1
A force is a PUSH or a PULL on an object
Causes things to:
Move
Accelerate
Change direction
2
Unit: Newton or kg*m/s
Balanced Forces
When opposing forces are EQUAL, they are BALANCED
and nothing happens
Ex: Tug of war with equal teams
Arm wrestling with 2 people of equal strength
Unbalanced Forces
When opposing forces are UNEQUAL, the LARGER one wins
Ex: Tug of war with twice as many people on one side
Miss J arm wrestling with Thor
Friction
When objects touch and try to MOVE past each other,
the force of FRICTION opposes that
-Friction DECREASES as
something begins to
move FASTER
-can happen with any
of the states of matter
4 Types of Friction:
Static
Sliding
Rolling
Fluid
Static Friction
• Acts on objects that aren’t moving
Ex: Trying to make a wall move
Sliding Friction
Friction that opposes the motion of the object
-smaller than static, use less force to keep an
object moving
Ex: Pushing a box
Sliding a paper
Rolling Friction
• Acts on rolling objects
• A lot less than sliding and static friction
Ex: Pushing a cart
Rollerblading
Fluid Friction
• Opposes motion through fluid
Fluids= liquids as well as gas mixtures
Ex: Running in Water
Airplane flying
Sailing
Which type of does each object have working on it?
How can you reduce friction?
• Lubricate with something like OIL
• ROLLING vs. SLIDING
• SMOOTHING surfaces
• Decrease FORCES pressing
surfaces together
Mythbusters: Separating Phone Books
• https://youtu.be/AX_lCOjLCTo
• https://youtu.be/QMW_uYWwHWQ
Warm Up
1. What are the 4 types of friction?
2. Which type of friction happens only with balanced forces?
3. What is the equation to find velocity?
Gravity
Force between any TWO masses pulling them together
• Objects do not need to be in CONTACT and it does not
act over LARGE distances
• Gravity pulls down toward the center of the OBJECT but
there is an equal FORCE pushing back
• The amount of force depends on the MASS and
DISTANCE between two objects
Falling Motion
• Things fall at the same ACCELERATION and move CONTINUOSLY
unless something slows them down
• As things fall through the air, AIR RESISTANCE slows them down
• Gravity acts DOWNWARD, air resistance acts UPWARDS and slows acceleration
EX:
• When objects fall, they ACCELERATE
• The faster they fall, the MORE the air resistance
• Eventually, they fall no faster (TERMINAL VELOCITY)
• Net force = ZERO, the object moves at a constant
velocity
Equations
•
Our equations for falling motion are different than horizontal motion.
Constant acceleration for gravity = 9.8 m/s2
To calculate velocity, we multiply the acceleration due to GRAVITY and the time of the fall
Velocity equation:
v = at
= (9.8 m/s2)( t )
To calculate distance for falling objects, you will multiply ½, acceleration due
to gravity, and time squared.
Distance equation:
d=½
2
at
= (.5)(9.8 m/s2 )( t2)
CALCULATING THE ACCELERATION OF A
FALLING OBJECT
• Constant acceleration for gravity = 9.8 m/s2
v = at
= (9.8 m/s2)( t )
d=½
2
at
= (.5)(9.8 m/s2 )( t2)
•
When you throw a ball, it follows a CURVED path. When
something falls after it is given a forward velocity, it is called a
PROJECTILE
The only forces acting on projectiles are AIR RESISTANCE and GRAVITY
• Once you release a projectile, you no longer have any influence on it.
•
Has CONSTANT HORIZONTAL VELOCITY(we ignore air resistance)
• DOWNWARD velocity is accelerated due to GRAVITY
•
MYTHBUSTERS: BULLET FIRED VS. SHOT
• https://youtu.be/2tiOmp1BE8Y?list=PLqCHwhfQ685Qm4WXfkWKUpe_j75wIgFv
Warm Up 11/24
What variable are we solving for in the following questions?
1. How
fast?
2. How far?
3. How high?
4. How many seconds?
5. Average velocity?
An object starts at rest and falls freely for 8 seconds.
a.
What is its velocity?
b.
What is its average velocity?
c.
What is the distance it falls?
A car slowed down from a velocity of 20 m/sec and
stopped in 2 seconds.
a.
What was its deceleration?
b.
What was its average velocity?
c.
How far did it go while slowing down?
A cliff diver jumped from a cliff and waited 2
seconds before opening his parachute.
a. How fast was he going when he released it?
b. How far did he fall during the 2 seconds?
Warm Up 12.1
Read Section 12.2 on Newton’s 1st and 2nd Laws
On a separate sheet of paper:
1. Define Each law
2. Give an example of each law
12.2 Newton’s Laws of Motion
Sir Isaac Newton
Newton’s
st
1
Law
Unless a force acts on it, an object will stay at rest and
object in motion stays in motion with same speed and
direction
AKA: Law of Inertia
Inertia=tendency of object to resist change in its motion
• An object at rest stays at rest
• An object in motion stays in motion with
the same direction and speed
You experience inertia when you ride in a car.
When someone slams on the brakes, you lean
forward – continue in path of initial force
Acceleration of ball is PROPORTIONAL to force
acting on it.
-If you DOUBLE ACCELERATION, you will
double FORCE
Swinging ball on string
TableCloth
Hammer and Mass
https://youtu.be/SgAoyHfRd4s
Quarter Stack
Egg Drop
https://youtu.be/6gzCeXDhUAA
Raw egg vs. Boiled egg
Warm Up 12/1
1. Explain how throwing a baseball follows Newton’s
1st law.
2. How can you increase the acceleration of a baseball
in the air?
Some things to keep in mind:
• How do unbalanced forces affect an object’s velocity and
acceleration?
•
Changes them
• If you throw a ball, it accelerates. What happens if you use more
force and throw harder?
•
It accelerates more.
• Acceleration of ball is directly proportional to force acting on it.
•
If you double the force, you will double acceleration.
Newton’s
nd
2
Law
Acceleration of object is equal to force
acting on it divided by mass
•
Doubling the mass cuts the acceleration in half
The larger the force acting on the object, the greater
the acceleration of the object.
The larger an object’s mass, the greater the force needed to
give it the same acceleration.
• Also works if force acts in direction opposite
object’s motion like in seatbelts.
• Seatbelt applies force that opposes or restricts
motion and decelerates (negative acceleration)
passenger.
• Newton’s 2nd Law:
a = _F_ OR F = ma
m
a = acceleration
F = force
m = mass
N = kg*m
s2
Weight and Mass
• Would your weight be the same on the moon?
• No.
• Would your mass be the same on the moon?
• Yes.
• Mass: amount of matter in an object; units: g
or kg, etc.
Weight and Mass – mass does NOT equal weight!
-Weight: force of GRAVITY acting on an object
-Mass and weight are PROPORTIONAL
(Doubling the mass doubles the weight.)
Formula: 𝑤 = 𝑚 𝑋 𝑔
w = weight
m = mass
g = acceleration due to gravity (9.8 m/s2)
Math Practice
• A boy pushes forward a cart of groceries with a
total mass of 40.0 kg. What is the acceleration
of the cart if the net force on the cart is 60.0N?
• a = F = 60 N = 1.5 m/s2
m 40 kg
Math Practice
• What is the upward acceleration of a helicopter
with a mass of 5000 kg if a force of 10,000N
acts on it in an upward direction?
• a = F = 10,000 N = 2 m/s2
m 5000 kg
Math Practice
• An automobile with a mass of 1200 kg accelerates at
a rate of 3.0 m/s2 in the forward direction. What is
the net force acting on the automobile? (Hint: Solve
the formula for force.)
• F = ma = 1200 kg * 3.0m/s2 = 3600 N
Math Practice
• A 25 N force accelerates a boy in a wheelchair at .5
m/s2. What is the mass of the boy and the
wheelchair? (Hint: Solve formula for mass.)
• m = F = 25 N
a .5m/s 2
= 50 kg
Warm Up 12/2
A 25 N force accelerates a boy in a wheelchair at
.5 m/s2. What is the mass of the boy and the
wheelchair?
Newton’s
rd
3
Law
Forces exist in pairs; whenever 1 object exerts
force on 2nd object, 2nd object exerts = and
opposite force on 1st object
• Bumper cars are examples of Newton’s 3rd Law in action.
• Also known as Action and Reaction Forces
• Ex.: Bumper Cars: Action Force: force of your bumper
Reaction Force: force of other car on me
• They are equal in size and opposite in direction.
• Describe hammer and nail:
Action: When hammer contacts nail
• Reaction: From nail - Equal and opposite to hammer and
ends hammer’s motion
•
Newton’s 3rd Law
• Action-Reaction Forces and Motion
•
Swimming
Action Forces: arms push against water
• Reaction Forces: by water on swimmer and pushes her
forward
•
•
Not all action-reaction forces lead to motion.
Newton’s 3rd Law
• Action-Reaction Forces do NOT cancel for the
swimmer
•
Why?
•
•
don’t act on same object (water, person)
When would they cancel each other?
•
If equal and opposite on SAME object
• 2 people pushing on a cart with the same force but in
opposite directions
Newton’s 3rd Law
• Ex: Rocket Propulsion
How?
• 1st: Fuel burns making hot gases.
• 2nd: Hot gases expand and push
against inside of the rocket and
escape to bottom of rocket (action force).
• 3rd: Rocket accelerates in
opposite direction (reaction force).
•
• https://youtu.be/gDIL9SdN-ls
http://science.nasa.gov/headlines/y2003/28jan_envirorocket.htm
Momentum
• Definition: product of object’s mass and velocity
• Difficult to stop object with large momentum.
•
•
Ex.: Can you stop a speeding bullet?
Ex.: Can you stop a speeding bus?
• Momentum is large if product of mass and velocity is large.
Momentum
• At rest, momentum = 0!!! It doesn’t matter how large the object is.
•momentum = m * v
•p = m * v
•
•
p = momentum; m = mass; v = velocity
unit = kg * m/s
Momentum
• The momentum of a ball dropped from a bridge is 1.5 kg *
m/s. Its mass is .25 kg. Find velocity.
• p = m * v  v = p/m
• v = 1.5 kg * m/s = 6 m/s
.25 kg
Conservation of Momentum
• means momentum doesn’t increase or decrease.
• Law of Conservation of Momentum: if no net force acts
on system then total momentum of system doesn’t
change
Momentum is neither created nor destroyed. It is transferred
between objects.
• Momentum can change only if mass, velocity, or both change.
•
• Key Concept: Loss of momentum of one = gain in
momentum of another object.
•
Momentum is conserved.
Metal Balls
• https://youtu.be/r8E5dUnLmh4 -car crash
• https://youtu.be/OuA-znVMY3I
-massive
• https://youtu.be/cP0Bb3WXJ_k?list=PLlwib77IOrGGqlNBnl7FstlxnF7QUkYO -explanation
• https://youtu.be/sDfsdtRo2PE?list=PLlwib77IOrGGqlNBnl7FstlxnF7QUkYO -pool shots
Centripetal Force
• center-directed force that continuously changes direction
of object to make it move in circle
•
•
•
•
•
Means “toward the center”
Ex: Sunglasses slide across dashboard away from driver as make
sharp left.
Ex: Car rounding curve at proper speed – follows curve easily
Ex: Car rounding curve at high speed – Inertia greater and car
leaves road at curve
Ex: Highways and racetracks are banked.
• Satellites in Orbit – need centripetal force provided by
gravity and its inertia to stay in orbit
https://youtu.be/yyDRI6iQ9Fw
Warm Up 12/4
1. What is the equation for momentum?
2. What variable has the unit m/s
3. What variable has the unit kg*m/s
4. What unit does mass need to be in to calculate momentum?
Review Problems
What is the force required to push a
wheelbarrow with a mass 25 kg, and
holding 10kg of dirt, at an acceleration of
1.5 m/sec2 ?
What is the acceleration of a bowling ball,
with a mass of 11,000 grams, if a force of 50
Newton’s is applied?
What is the weight on Earth of a girl with a mass
of 30kg?
A 5 g marble travels with a velocity of 5 m/s,
what is the momentum of the marble?
The mass of a gerbil is 430 grams and its momentum is
5 kg *m/s forward, what is the velocity of the gerbil?