Transcript Motion and Forcex - MrsHarrisPhysicalScience

Motion and Forces
Physical Science Chap 2 and 3
Describing Motion
2.1
What is your current speed?
 Using the sun as a reference point, you are
moving about ______
30 km/s.
Motion
 Motion occurs when an object changes its
position relative to a reference point.
 You are not moving relative to your desk
or your school building, but you are
moving relative to the other planets in the
solar system and the Sun.
The Tortoise and the Hare
 Who wins the race?
 Why?
 What is speed?
 What is average speed?
 What are some units used to describe speed?
Describing Motion
 Distance – how far something has moved
 In science, we use meters ( m , km , cm , mm )
Describing Motion
 Speed – how fast something moves
 distance/unit of time (such as m/s)
Describing Motion
 Average Speed – the total distance traveled
divided by the total time of travel.
 If the total distance traveled was 5 km
and the total time was 1/4 h, or 0.25 h.
The average speed was:
total distance
avg. speed 
total time
Video clip
Distance vs. displacement
 Pg. 39 – Figure 2
 How far did the runner run?
 80 meters (don’t forget units)
 What is her distance from the starting line?
 20 meters
 Can you determine the runners speed?
 Why not?
 Time is not given
Speed vs. Velocity
 Question: A storm is 10 km away and is
moving at a speed of 60 km/h. Should you
be worried?
 Answer: It depends on the storm’s
direction…
Describing Motion
 Velocity – includes the speed of an object
and the direction of its motion
 The speed of a car might be constant
while its velocity changes.
Who skates faster?
Your neighbor skates at a speed of 4 m/s.
You can skate 100 m in 20 s.
GIVEN:
WORK:

d = 100 m
t = 20 s
v=?
v=d÷t
d
v = 5 m/s
v t
v = (100 m) ÷ (20 s)
You skate faster!

Sound travels 330 m/s. If a lightning bolt
strikes the ground 1 km away from you,
how long will it take for you to hear it?
GIVEN:
WORK:
v = 330 m/s
t=d÷v
d = 1km = 1000m
t = (1000 m) ÷ (330 m/s)
tD.=Calculations
?
t
=
3.03
s
d
v t
Distance-time Graph
A
 Who started out faster?
 A (steeper slope)
 Who had a constant speed?
 A
B
 Describe B from 10-20 min.
 B stopped moving
 Find their average speeds.
 A = (2400m) ÷ (30min)
A = 80 m/min
 B = (1200m) ÷ (30min)
B = 40 m/min
Speed-time Graph
Specify the time period when
the object was...
3
• slowing down
• 5 to 10 seconds
Speed (m/s)
2
• speeding up
• 0 to 3 seconds
• moving at a constant speed
• 3 to 5 seconds
1
0
0
2
4
6
Time (s)
8
10
• not moving
• 0 & 10 seconds
Motion over the years – gradual
 Moving Plates
 Cause mountains,
earthquakes, etc.
 Pacific ocean is
getting smaller
 From 1 cm/yr (in
CA) to 17 cm/yr
(Australia)
Self-Check 2.1
d
v t
1. At 8:00 AM you leave home and walk 0.5
km to a friend's house. At 11:30 AM you
return home, then travel by car to the mall,
which is 10 km away, and arrive at 11:45
AM. What is the total distance traveled?
2. If a student leaves home at 7:30 AM to walk
to his school 2 km away, stops at a
neighbor's house for 15 min, then arrives at
school at 8:00 AM, what is the student's
average speed for the trip to school?
Acceleration
2.2
Acceleration
 Acceleration – the rate of change of velocity
(occurs with change in speed or direction)
 Positive acceleration - if the
acceleration is in the same
direction as the velocity
 Negative acceleration – if the
acceleration is in the opposite
direction from the velocity
Calculating Acceleration
vf - vi
a t
Application: Roller coasters
 Engineers use the laws of physics to
design amusement park rides that are
thrilling, but harmless.

A roller coaster starts down a hill at 10 m/s.
Three seconds later, its speed is 32 m/s.
What is the roller coaster’s acceleration?
GIVEN:
WORK:
vi = 10 m/s
t=3s
vf = 32 m/s
vf - v i
a=?
a = (vf - vi) ÷ t
a t
a = (32m/s - 10m/s) ÷
(3s)
a = 22 m/s ÷ 3 s

How long will it take a car traveling 30 m/s
to come to a stop if its acceleration is
-3 m/s2?
GIVEN:
WORK:
t=?
vi = 30 m/s
vf = 0 m/s
a = -3 m/s2 vf - vi
t = (vf - vi) ÷ a
a t
t = (0m/s-30m/s)÷(3m/s2)
t = -30 m/s ÷ -3m/s2
Airliner acceleration
 Suppose a jet airliner starts at rest at the end of a
runway and reaches a speed of 80 m/s in 20 s.
Stop Skating?
 Now imagine that a skateboarder is
moving in a straight line at a constant
speed of 3 m/s and comes to a stop in 2 s.
 a = (0 m/s – 3 m/s) / 2 s = –1.5 m/s2
Videos and animations
 http://www.brainpop.com/science/motionsf
orcesandtime/acceleration/zoom.weml
 http://www.glenbrook.k12.il.us/gbssci/Phy
s/mmedia/kinema/acceln.html
Self-Check 2.2
1. If a car is traveling 100 km/h and comes
to a stop in 3 min, what is the car's
acceleration?
2. If a runner maintains a constant speed of
12 km/h, how long will it take to
complete a marathon race of 26.2 miles?
3. List 3 ways you can accelerate while
walking.
Motion and Forces
2.3
Force
 A force is a push or pull.
 The force of the moving ball causes a ball at
rest to move in the direction of the force.
Net Force
 Net force – the combination of two or more
forces acting on an object at the same time
Inertia
 Inertia - the tendency of an object to resist
any change in its motion
 An object will keep moving at the same
speed and in the same direction unless
an unbalanced force acts on it.
Newton’s First Law of Motion
 Newton's first law of motion states that an
object moving at a constant velocity keeps
moving at that velocity unless an
unbalanced net force acts on it.
 If an object is at rest, it stays at rest
unless an unbalanced net force acts on it.
 This law is sometimes called the law of
inertia.
Inertia in a car crash
 When a car traveling about 50 km/h
collides head-on with something solid, the
car crumples, slows down, and stops
within approximately 0.1 s.
 Any passenger not wearing a safety belt
continues to move forward at the same
speed the car was traveling.
 http://www.physicsclassroom.com/mme
dia/newtlaws/cci.cfm
Videos
 http://www.brainpop.com/science/motionsf
orcesandtime/force/
Self-Check 2.3
1. Can there be forces acting on an object if
the object is at rest?
2. What is the net force on a refrigerator if
you push on it and it doesn’t move?
3. Two students push on a box in the same
direction, and one pushes in the opposite
direction. What is the net force on the
box if each pushes with a force of 50 N ?
Newton’s Second Law
3.1
Force and acceleration
 What is different about throwing
a ball horizontally as hard as you
can and tossing it gently?
 A hard-thrown ball has a
greater acceleration than a
gently tossed ball.
Mass and acceleration
 If you throw a softball and a baseball as
hard as you can, why don’t they have the
same speed?
 The softball would have
less acceleration because it
has a greater mass.
Newton’s Second Law
F
m a
 The acceleration of an object is directly
proportional to the net force acting on it
and inversely proportional to its mass.
 The acceleration is in the same direction as
the net force on the object.
Force to accelerate
 What force would be required to
accelerate a 40 kg mass by 4 m/s2?
Friction
 Friction is the force that opposes the
sliding motion of two surfaces that are
touching each other.
 Depends on the kind of surface and the
force pressing the surfaces together
Pinewood Derby Cars
 What are some sources of friction in the
cars that may hinder movement?
 List ways to reduce friction.
Air Resistance
 Air resistance is a friction-like force that
opposes the motion of objects through air
• Depends on speed, size, and
shape of object
• Note: Air resistance, not the
object’s mass, is why feathers,
leaves, and pieces of paper fall
more slowly than pennies,
acorns, and apples.
Motion vs. Acceleration?
 Could this block be moving horizontally?
 Yes
 How?
 If the forces acting on an object are
balanced and the object is in motion,
then it will continue in motion with the
same velocity.
 Remember: Forces do not cause motion.
Forces cause accelerations.
Videos and Animations
 http://www.glenbrook.k12.il.us/gbssci/phy
s/mmedia/newtlaws/efff.html
Self-check 3.1
1. What 3 variables does Newton’s second
law of motion connect?
Gravity
3.2
Gravity
 Gravity is an attractive force between any
two objects that depends on the masses of
the objects and the distance between them.
Newton
 Isaac Newton formulated the law of
universal gravitation, which he published
in 1687.
Range of Gravity
 According to the law of universal
gravitation, the gravitational force between
two masses decreases rapidly as the
distance between the masses increases.
 No matter how far apart two objects are,
the gravitational force between them never
completely goes to zero.
Earth’s Gravitational Acceleration
 Close to Earth’s surface, the acceleration
of a falling object in free fall is about 9.8
2
m/s .
 This acceleration is given the symbol g
and is sometimes called the acceleration
of gravity.
Weight
 The gravitational force exerted on an
object is called the object’s weight.
Weight and Mass Relationship
Projectile Motion
 If you’ve tossed a ball to someone, you’ve
probably noticed that thrown objects don’t
always travel in straight lines. They curve
downward.
 Earth’s gravity causes projectiles to
follow a curved path.
Horizontal and Vertical Distance
 If you were to throw a
ball as hard as you could
from shoulder height in a
perfectly horizontal
direction, would it take
longer to reach the ground
than if you dropped a ball
from the same height?
Click image
Centripetal Acceleration
 Centripetal acceleration is acceleration toward
the center of a curved or circular path.
 When a ball enters a curve, even if its speed
does not change, it is accelerating because its
direction is changing.
Centripetal Force
 According to the second law of motion,
when a ball has centripetal acceleration,
the direction of the net force on the ball
also must be toward the center of the
curved path.
 Centripetal force is the net force exerted
toward the center of a curved path
Gravity can be a centripetal force
 In the same way, Earth’s gravity exerts a
centripetal force on the Moon that keeps it
moving in a nearly circular orbit.
Videos
 http://www.brainpop.com/science/motionsf
orcesandtime/gravity/
Self-check 3.2
1. How do weight and mass differ?
Newton’s Third Law
3.3
Newton’s Third Law
 Newton’s third law of motion: When one
object exerts a force on a second object,
the second one exerts a force on the first
that is equal in strength and opposite in
direction.
Action and Reaction
 When a force is applied in nature, a reaction
force occurs at the same time.
 When you jump on a trampoline you exert a
downward force on the trampoline. The
trampoline exerts an equal force upward,
sending you high into the air.
 Even though the forces are equal, they are not
balanced because they act on different objects.
Action / Reaction Forces Don’t Cancel
 A swimmer “acts” on the water, the
“reaction” of the water pushes the
swimmer forward.
 In a rocket, the hot gases are forced
out the back of the rocket therefore
pushing it forward.
Momentum
p
m v
 A moving object has a property called
momentum that is related to how much
force is needed to change its motion.
Notice that momentum has a direction because velocity has a direction.
Force and Changing Momentum
 Recall that acceleration is the difference
between the initial and final velocity,
divided by the time.
 Also, from Newton’s second law, the net
force on an object equals its mass times its
acceleration.
 By combining these two relationships:
Law of Conservation of Momentum
 The momentum of an object doesn’t change
unless its mass, velocity, or both change.
 Momentum, however, can be transferred
from one object to another.
 The law of conservation of momentum states
that if a group of objects exerts forces only
on each other, their total momentum doesn’t
change.
When Objects Collide
 The results of a collision depend on
the momentum of each object.
 When the first puck hits the
second puck from behind, it gives
the second puck momentum in
the same direction.
 If the pucks are speeding toward
each other with the same speed,
the total momentum is zero.
Bumper Car Momentum
 Find the momentum of a bumper car if it
has a total mass of 280 kg and a velocity
of 3.2 m/s.
Bumper Car Velocity
 The momentum of a second bumper car
is 675 kg·m/s. What is its velocity if its
total mass is 300 kg?
Videos
 http://www.brainpop.com/science/motionsf
orcesandtime/newtonslawsofmotion/zoom.
weml
Self-Check 3.3
1. The momentum of an object is the
product of its ________ and ________.
2. When two objects collide, what happens
to their momentum?