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Chapter 11: Forces
- Newton’s 3 laws of motion
- Sir Isaac Newton
- attempt to explain behavior of moving object
Newton’s First Law (Law of ______)
***an object at rest remains at ______and an object in
motion maintains its velocity unless it experiences an
unbalanced ______***
- an object will continue to travel at the same speed
forever unless some force stops it
- an object not moving will continue to not move until
a force moves it
- Ex.- seat belts
- you travel same speed as car
- when car stops, you continue to travel forward
- most have unbalanced force to stop your
motion. . . seat belt
- if no seat belt. . . .windshield, pavement, etc.
- Ex.- throw a ball
- What stops it?
- gravity?
- no, gravity accelerates it downward
- ______stops it
- air resistance slow it
- friction with ______stops it
- Ex.- pop can on desk
- will stay in place until some force moves it
- ______– the tendency of an object to resist being moved
or, if the object is moving to resist a change in
speed or direction until an outside force acts on
the object
- amount of ______determines the amount of inertia
- small mass; little ______
- large mass; lots of ______
- Ex.- kicking a soccer ball vs. bowling ball
- easier to get soccer ball to 20 m/hr than
bowling ball
- Which will be easier to stop?
- soccer ball
*______mass makes it easier to start and stop moving*
Newton’s ______Law
- describes the effect of an unbalanced force on the motion
of an object
***the unbalanced force action on an object ______the
object’s mass time its acceleration***
force = mass x acceleration
f = (m)(a), m= f/a, a= f/m
- Ex.- pushing empty cart vs. full one
- takes more force to move heavy cart
if both carts at the same speed
- if both carts are traveling at same speed, takes
more force to stop heavier cart
- force is measured in ______(N)
- 1 N equals the force necessary to accelerate a 1 ____
mass at 1 ______
1 N = 1 Kg x 1 m/s²
N = Kg x m
s²
- English measurement system
- pound (lb.)
1 N = .225 lb.
1 lb. = 4.448 N
Practice Problems
1. Zookeepers lift a stretcher that holds a sedated lion. The
total mass of the lion and stretcher is 157 kg, and the
lion’s upward acceleration is .657 m/s2. What is the
unbalanced force necessary to produce this acceleration
of the lion and the stretcher?
2. What is the net force necessary for a 1.6 x 103 kg
automobile to accelerate forward at 2 m/s2?
3. A baseball accelerates downward at 9.8 m/s2. If the
gravitational force acting on the baseball and is 1.4 N,
what is the baseball’s mass?
4. A sailboat and its crew have a combined mass of 655
kg. Ignoring frictional forces, if the sailboat experiences
a net force of 895 N pushing it forward, what is the
sailboat’s acceleration?
Answers
1.
2.
3.
4.
- every day example:
- stopping distance of a loaded car vs. an empty car
Law of Universal ______
- Sir Isaac Newton
- all objects are attracted to one another
- force of attraction is gravity
2 Parts
1) size (______) of objects
- small mass, less gravitational force
- large mass, more gravitational force
- more massive you are, the more attractive
you are!!!!
- Ex.- stem of apple breaks, apple falls to Earth
- force of attraction is greater between apple
and Earth than between apple and tree
- Earth’s mass is greater than trees
- Ex.- elephant vs. cat
- elephant has more mass than cat
- greater gravitational force between
elephant and Earth than cat and Earth
- easier to pick up cat
- gravitational force between cat and
elephant is small because mass is small
compared to mass of Earth
2) ______between objects
- as distance increases, force ______
- if distance doubles, force decreases to ¼
original valve
- if distance is tripled, force decreases to 1/9
original valve
F = (G) m1 m2
___
Q. If all objects are attracted to each other why don’t I
have stuff stuck to me? Why, when I drop a pen does it
go to the center of Earth and not the center of me?
1)Earth’s ______is huge
2) even though the distance between center of me and
pen is less than pen to center of Earth, difference in mass more than makes up for it
- ______– the motion of a body when only the force of
gravity is acting on the body
- near Earth’s surface; g = ______
- If you drop a penny and a feather at same time
which one will hit the ground first?
- penny
- Why?
- in the absence of air resistance they would hit at the same
time
- Newton’s 2nd law says a heavier object experiences a
greater gravitational force than a lighter object
- however, a heavier object takes longer to
accelerate because it has more mass
- ______– force on an object due to gravity
F = ma
weight = (mass)(gravity)
- apparent ______– when 2 objects are in free fall
and neither exerts a force on
the other
- Ex.- astronauts in space shuttle
- mass is the measure of amount of matter than object
- weight is the gravitational force exerted on the mass
- moon has 1/6 Earth’s gravity
- mass is same
- weight will be less
- ______affects ______
- land organisms – must have strong ______structure
- Ex.- skeletons, trunks
- water organisms – force of gravity is balanced by
______force of water
- Ex.- jellyfish
- graceful in water
- collapses on land
- ______velocity – the constant velocity of a falling
object when the force of air resistance
is equal in magnitude & opposite in
direction to the force of gravity
- air resistance = ______
- Ex.- skydivers
- jump out of plane – accelerate
- as they accelerate they must
move more air molecules out of
their way to continue
accelerating
- eventually they can’t move
particles fast enough
- continue to fall, do not speed up
- Ex.- penny off of empire state building
- ______– only occurs if gravity is the only force acting on
an object
- must be no air
- otherwise there is air resistance
- Ex.- astronaut & space craft
- both are falling at same rate
- orbiting is a result of motion in ___ directions
- object orbiting is being pulled toward Earth by
______
- object is also moving ______
- result is ______
- ______motion – curved path on object follows when
thrown, launched, or otherwise
projected near the ______of the Earth
2 components
1. ______
2. ______
- both are independent of each other
- Ex.- no matter how hard you throw a ball,
gravity accelerates it downward
- combination of both produced ______path
______Component
- motion ______(90°) to surface of Earth
- Ex.- throw a ball
- once you release the ball, no more forces acting on
ball (except air resistance)
______Component
- caused by ______
- absence of ______; acceleration = 9.8 m/s²
- pull of gravity is equal on items ______and items in
projectile ______
- application
- to hit a target you must aim ____ so gravity can
pull it down to hit object
Newton’s ______experiment
- imagine a cannon on top of a tall mountain
- cannon fires . . . what happens?
- cannonball has projectile motion
- ______
- do it again with greater speed on ball
- travels ______before hitting ground
- What happens when speed gets high enough?
- fall’s around Earth . . . . ______
Newton’s ______Law
***For every action ______there is an equal and opposite ______force***
- law of action and reaction
- all forces act in ______
- whenever a force is exerted, another force occurs that
is ______in size and ______in direction
- still occurs when no motion
- Ex.- sit on chair
- you push on chair (action force)
- chair pushes back on you (reaction force)
- pairs of forces ______act on the same object
- Ex.- swimming
- hand and foot push on water
- water pushes on hands and feet
- Ex.- walking
- you push on Earth
- Earth pushes on you
- you and Earth move
- your mass is much smaller then the Earth’s so
you experience more acceleration than the Earth (Newton’s 2nd Law)
Momentum
- ______– a quantity defined as the product of the
mass and velocity of an object
- ____ unit is (kg)(m)
s
momentum = mass x velocity
p = mv, m = p/v, v = p/m
- ______of objects in motion
- affected by mass and velocity
- larger mass and larger velocity increases
momentum
- no ______, no momentum
- has direction
Practice Problems
1) Calculate the momentum of a 6.00 kg bowling ball
moving at 10.0 m/s down the alley toward the pins.
2) Calculate the momentum of a 75 kg speed skater moving
forward at 16 m/s.
3) Calculate the momentum of a 135 kg ostrich running
north at 16.2 m/s.
4) Calculate the momentum of a 5.0 kg baby on a train
moving eastward at 72 m/s.
5) Calculate the momentum of a seated 48.5 kg passenger
on a train that is stopped.
6) Calculate the velocity of a 0.8 kg kitten with a
momentum of 5 kg · m/s forward.
Answers
1) (6 kg)(10m/s)= 60 kg m/s down the alley
2) (75 kg)(16 m/s)=1200kg·m/s forward
3) (135 kg)(16.2 m/s) = 2187 kg·m/s north
4) (5.0 kg)(72 m/s)= 360 kg·m/s eastward
5) (48.5 kg)(0 m/s) = 0 kg·m/s
6) 5 kg·m/s = 6.3 m/s forward
0.8 kg
- must apply ______to stop an objects ______
- if you extend ______that the force is applied, less force
will have to be applied at once
- Ex.- egg toss
- catching egg stiff handed vs. moving with egg
- move indirection of egg, longer time to apply
force, less force needs to be applied, egg stays
intact
- Ex.- boxer “rolling with the punches”
- Law of Conservation of ______
- the total amount of momentum in an isolated system
is ______
- momentum cannot be created or destroyed, it is
______from 1 object to the next
- Ex.- bowling ball
- momentum of ball before contact =
momentum after + momentum of all pins
- Ex.- kick of guns
- momentum of bullet = momentum of gun
- opposite directions
- difference in masses
- when a moving object hits a second object, some or
all of its ______is transferred to the second
- whatever isn’t transferred stays with the _____
- Ex.- rockets
- rocket pushes gases backward
- burning chamber push rocket forward
* ______need atmosphere*