Transcript Force

MOTION
When is an object in motion?
An object is in motion when its distance
from another object is changing.
How do you determine when
an object is in motion?
By using a reference point.
A reference point is a place or object used
for comparison to determine if something
is in motion.
Are distance and displacement
of an object the same?
NO!!!!!!!
Distance and Displacement
Distance is a scalar quantity that refers to
"how much ground an object has covered"
during its motion.
Displacement is a vector quantity that
refers to "how far out of place an object
is"; it is the object's overall change in
position.
Distance and Displacement
A physics teacher walks 4 meters East, 2 meters
South, 4 meters West, and finally 2 meters
North.
Distance Traveled?
Displacement?
12 meters
0 meters
The diagram below shows the position of a cross-country
skier at various times. At each of the indicated times,
the skier turns around and reverses the direction of
travel. The skier moves from A to B to C to D.
Determine the resulting displacement and the distance
traveled by the skier during these 3 minutes.
Distance Traveled?
420 meters
Displacement?
140 m, rightward or east
A football coach paces back and forth along the
sidelines. The diagram below shows several of the
coach’s positions at various times. At each marked
position, the coach makes a “U-turn” and moves in the
opposite direction. The coach moves from A to B to C to
D.
Distance Traveled?
95 yards
Displacement?
55 yards, leftward or west.
Check Your Understanding
What is the
displacement of the
cross-country team if
they begin at the
school, run 10 miles
and finish back at the
school?
0 miles
What is the distance
and the displacement of
the race car drivers in
the Indy 500?
Displacement =
0 miles
Distance =
500 miles
Scalars and Vectors
To understand the distinction between distance
and displacement, you must understand vector
and scalar quantities.
Scalars are quantities, which are fully described
by magnitude (or numerical value) alone.
Vectors are quantities, which are fully described
by both a magnitude and a direction.
Scalar or Vector?
1. 5 meters
2. 30 m/sec, East
3. 5 mi., North
4. 20 degrees Celsius
5. 256 bytes
6. 4000 calories
Scalar
Vector
Vector
Scalar
Scalar
Scalar
How can motion be
described?
By using a reference
point – A (usually)
stationary object that
helps you detect the
motion of an object.
(Ex. tree, house etc.)
Once the reference
point is determined you
can define motion by
speed, position, and
direction.
What is speed?
Speed refers to how fast
an object travels. (Rate at
which an object moves.)
Depends on:
Scalar
–Distance
Quantity
–Time
Various forms of speed:
–Instantaneous– speed at a
given moment (speedometer)
–Constant– speed that doesn’t
change (cruise control)
–Changing– speed that varies
(running a mile)
Calculating Speed
–Formula for objects
moving at a constant rate.
Speed = distance
time
Scalar
Quantity
–Average – speed is the distance
per time ratio.
Scalar
–Formula:
Quantity
Average Speed
= Total distance traveled
Total time
Ex. If you ran the distance
of a football field (91 meters)
in 30s, would you run the
same speed from start to
finish?
No, you find the average
speed you ran.
Average Speed
= Total distance traveled
Total time
= 91m
30s
= 3.03 m/s
Two cars traveled at 50
km/h on the same
highway. After 2 hours
the cars are 200 km away
from each other. How is
this possible?
THEY HAVE DIFFERENT
VELOCITIES
What is velocity?
Would you rather know the
hurricane’s speed or
velocity?
Velocity –speed of an object in a
particular direction.
Velocity changes when speed or
direction of an object changes.
Vector
Quantity
Average Velocity – the
displacement or position change
per time ratio.
Average Velocity =
Vector
Quantity
displacement
time
Can you have a constant
speed but a changing velocity?
Yes, the Ferris wheel can
be moving at a constant
speed but its constantly
changing direction!
Check Your Understanding
While on vacation, Trevor Mobley traveled
a total distance of 440 miles. His trip took
8 hours. What was his average speed?
Average Speed = Total distance/Total time
Average Speed = 440 mi/8hr
= 55 mi/hr
Check Your Understanding
A physics teacher walks 4 meters East, 2 meters
South, 4 meters West, and finally 2 meters
North. The entire motion lasted 24 seconds.
Average Speed?
Average Velocity?
12 m/24s = .50m/s
0 m/s
The diagram below shows the position of a cross-country
skier at various times. At each of the indicated times,
the skier turns around and reverses the direction of
travel. The skier moves from A to B to C to D.
Determine the average speed and the average velocity
of the skier during these 3 minutes.
Average Speed?
420 m/3 min = 140m/min
Average Velocity?
140 m, east/3min = 46.7 m/min, east
A football coach paces back and forth along the
sidelines. The diagram below shows several of the
coach’s positions at various times. At each marked
position, the coach makes a “U-turn” and moves in the
opposite direction. The coach moves from A to B to C to
D.
Average Speed?
95yd/10 min = 9.5 yd/min
Average Velocity?
55 yd, left/10min = 5.5 yd/min, left
In conclusion
Speed, being a scalar
quantity, is the rate at
which an object
covers distance.
Velocity is the rate at
which the position
changes (vector
quantity).
The average speed is
the distance (scalar)
per time ratio.
The average velocity
is the displacement or
position change
(vector quantity) per
time ratio.
How can you graph speed?
How is acceleration
related to velocity?
Acceleration – rate of change
of speed (velocity)
Occurs when an object
changes:
1.Increase speed
2.Decrease speed
3.Direction
What are the types
of acceleration?
Positive Acceleration –
speeding up
Negative Acceleration
– slowing down
How do you find
acceleration?
Formula:
a= vf –vi
t
What are velocity/time
graphs?
Shows change in
velocity over time
(acceleration).
Moving at a constant velocity
Moving at a positive
acceleration – velocity changing
at constant rate.
Moving at a negative
acceleration – velocity changing
at constant rate.
What is centripetal
acceleration?
Acceleration that occurs
in a circular motion;
object traveling in a circle
is constantly changing
directions; so acceleration
occurs continuously
FORCES
S8P3. Students will
investigate relationship
between force, mass, and the
motion of objects.
b. Demonstrate the effect of
balanced and unbalanced
forces on an object in terms of
gravity, inertia, and friction.
What is a force?
A force is a push or
pull that one body
exerts on another.
Ex. Friction, gravity,
kicking a football.
Unit: Newton (N)
What is a net force?
The sum of the
forces acting on
an object.
What is a
balanced force?
When the forces acting on
the object are equal in size
and opposite in direction.
Net force = 0 (forces
cancel each other out)
No change in motion
occurs!!
What is an unbalanced force?
Occurs when forces
acting on the object are
unequal.
Net Force is in the
direction of the larger
force. (object will move in
that direction)
What is the difference between
balanced and unbalanced
forces?
You Try It: Net Force
10N
20N
4N
16N
0N
0N
What is
Newton’s
st
1 Law of
Motion
state?
An object in motion will
stay in motion unless an
outside force acts upon it.
An object at rest will stay
at rest unless an outside
force acts upon it.
What is another name
st
for the 1 Law?
The Law of Inertia
What is Inertia?
The tendency of an
object to resist
change.
Which has a greater
inertia?
Elephant – inertia
depends on mass!!
What does Newton’s
Law state?
nd
2
Acceleration
depends on the
objects mass and on
the net force acting
on the object.
What is the relationship
between force, mass, and
acceleration?
Force and Acceleration
The greater the force
applied to an object
the greater the
acceleration will be.
Force and Mass
Acceleration will be less
on a object that is heavier.
–Ex. Softball v. Baseball
An object will accelerate
in the direction of the net
force.
F = m x a, a = F/m,
m = F/a
What is the unit for
force?
Newton
kg x m/s/s
Types of Forces
What is a force that opposes
motion between 2 moving
objects?
FRICTION
What does the amount
of friction depend on?
The strength of the force of
friction depends on two
factors:
1. how hard the surfaces
push together and
2. the types of surfaces
involved.
What is static friction?
Friction between
two surfaces that
are not moving
past one another.
What is sliding
friction?
Force that opposes
motion of 2 objects
sliding past one
another.
What is Rolling
Friction?
Opposing motion
between two objects
rolling past each
other.
What is Fluid
Friction?
Friction that occurs as
an object moves
through fluid.
How is friction helpful?
Without friction the tires
could not push against
the ground to move the
car forward and brakes
couldn’t stop the car.
How can you reduce
friction?
Lubricants – are used to
reduce friction.
–Ex. Motor oil, wax, grease
Switching from sliding to
rolling friction
–Ex. Ball bearings between
wheel & axles
What is Gravity?
A downward force
Any 2 masses exert an
attractive force on each other.
Strength of gravitational force
depend on:
1.) Mass
2.) Distance
What is the law of universal
gravitation?
All objects are attracted to each
other by gravitational force.
–The larger the object – The
greater the force
–The closer together the object
– the greater the force.
Is weight a force?
Weight is considered
to be a gravitational
force that decreases
as you move from
Earth.
What is the
difference between
mass and weight?
Mass – how much matter
makes up an object
–(Doesn’t change as you move
from the Earth to the moon.)
Weight – considered to be a
gravitational force
–(Decreases as you move from
the Earth to the moon.)
Free Fall
• Free fall occurs when the only force
acting on an object is gravity.
• An object in free fall is accelerating.
• In free fall, the force of gravity is an
unbalanced force, which causes an
object to accelerate.
What is acceleration
due to gravity?
Anything that falls
through the air falls at an
acceleration of 9.8m/s/s.
This means that for
every second an object
is falling, its velocity
increases by 9.8 m/s.
What is Air
Resistance?
Effects anything that is
in Earth’s atmosphere.
Upward force that
opposes gravity of
anything falling through
the air.
Air Resistance
Depends on 3 things:
1. SPEED
2. SIZE
3. SHAPE
of an object.
If there were
no air
resistance a
hammer and
feather would
fall at the
same time.
What is a projectile?
Anything that
is thrown or shot
through the air.
Why do projectiles
follow a curved
path?
They follow a curved
path due to pull of
gravity.
–Ex: An arrow, Bullet,
Basketball
What is Newton’s 3rd Law
of Motion?
When one object exerts a
force on a second object,
the second one exerts a
force that is equal and in
size and opposite in
direction.
“Action/Reaction Occurs
What is Momentum?
Momentum is related to how
much force is needed to
change an object’s motion.
Momentum = mass x
velocity
p= m x v
What is the law of
conservation of momentum?
Momentum can be transferred
from one object to another
The total momentum of any group
of objects remains the same, or is
conserved unless outside forces
act on the objects.
What is the relationship
between mass and velocity?
If two trucks have same
velocity, but different
masses then . . .
the truck with the larger
mass has more momentum