Transcript Acceleration

Motion
Session 2
Acceleration and Force
Learning Objectives
• TLW know concepts of force and motion evident in
everyday life (TEKS 4)
• TLW be able to assess the relationship between force,
mass, and acceleration, noting the relationship is
independent of the nature of the force (TEKS 4.D)
• TLW demonstrate relationships of force, mass, and
acceleration using moving toys, falling objects, etc.
(TEKS 4.D)
• TLW perform calculations to determine force, mass,
acceleration of everyday objects (TEKS 4.D)
I. Acceleration
A. Acceleration - any change in
velocity; speed or direction
1. acceleration is NOT just speed
but also direction
2. In a car, when a driver steps
on the gas pedal, the car
goes faster or accelerates
3. But, when the brakes are
applied – the car slows down
**this is also acceleration =
because there is a change in
velocity
4. When a car makes a turn, even
if the speed does not change, it
is accelerating
B. Acceleration information
1. If acceleration is small =
speed increases gradually
2. If acceleration is large =
speed increases rapidly
3. Positive acceleration =
velocity increases or
speeds up
4. Negative acceleration =
velocity decreases or
slows down (deceleration)
5. Acceleration of an object is
zero if velocity is constant
~ NO CHANGE ~
in speed or direction =
**** NO ACCELERATION ****
6. Formula for calculating
acceleration
acceleration =
(final velocity – initial velocity)
time
a = v/t
final velocity = the speed at which
the object ends
initial velocity = the beginning
speed of the object
7. Ex. A water balloon falls off a
second-story windowsill. The water
balloon starts from rest and hits the
sidewalk 1.5 s later with a velocity of
14.7m/s. What is the average
acceleration of the water balloon?
a = (final velocity – initial velocity)
time
a = (14.7 m/s – 0 m/s) / 1.5 s
a = 9.8 m/s2
8. The units for acceleration is
meters per second squared
2
or m/s .
9. Ex. A bike rider was traveling north
13 m/s and after 3 seconds his speed
was 3.25 m/s.
What is the rider’s acceleration?
a = (final velocity – initial velocity)/t
a = (3.25 m/s – 13 m/s) / 3 s
a = (-9.75 m/s) / 3s
a = -3.25 m/s2
* a negative # (-3.25) means
acceleration is slowing down
Group Practice
4.Marisa’s car accelerates at an average
rate of 2.6 m/s2. Calculate how long it
takes her car to accelerate from 24.6 m/s
to 26.8 m/s. Use this formula: time = (final
v – initial v)/ acceleration.
t = unknown
initial v = 24.6 m/s
final v = 26.8 m/s
acceleration = 2.6 m/s2
Group Practice
5. A cyclist travels at a constant velocity of
4.5 m/s westward, then speeds up with a
steady acceleration of 2.3 m/s2. Calculate
the cyclist speed after accelerating for 5.0
s.
Use this formula: final v = initial v X a X t
final v = unknown
a = 2.3 m/s2
initial v = 4.5 m/s
t = 5.0 s
Group Practice
1. Natalie accelerates her skateboard along
a straight path from 0 m/s to 4.0 m/s in
2.5 s. Find her average acceleration.
Group Practice
2. A turtle swimming in a straight line toward
shore has a speed of 0.50 m/s. After 4.0
s, its speed is 0.80 m/s. What is the turtle’s
average acceleration?
Group Practice
3. Find the average acceleration of a
northbound subway train that slows down
from 12 m/s to 9.6 m/s in 0.8 s.
Independent Practice
• Acceleration - 1
II. Force
A. Force – the cause of
acceleration or change in an
object’s velocity.
Force equals mass times acceleration
F=mxa
On earth to calculate F multiply mass (kg) by
acceleration due to gravity (9.8 m/sec2)
Force is measured in kg m / sec2 which
equals a Newton (N)
Example – if a student weighs 50 kg what is
the force he exerts?
50 kg x 9.8 m/sec2 = 490 kg m / sec2 or
490 N
Group Practice - #1
• What is the force exerted by a 4 kg rock
on the earth? (g = 9.8 m/s2)
Group Practice - #2
• What is the acceleration of a 10 kg cat
exerting a force of 50 N?
Group Practice - #3
• A car accelerates from a stop to 50 m/s in
5 s. At this point it is exerting a force of
10,000 N. What is the mass of the car?
B. Types of forces
1. Net force – the combination
of all forces on an object
a. An object accelerates in
the direction of the net
force
b. It won’t accelerate if the
net force is zero
2. Balanced forces –
forces on an object that creates
a zero net force
a. An object doesn’t move with
balanced forces
b. Balanced forces cancel each
other
c. Ex. Tug-of-War between
equal sized teams
3. Unbalanced forces – action on
an object that creates a
positive net force
a. An object moves if forces
are unbalanced
b. The object moves in the
direction of the greater
force
C. Forces that affect motion
1. Friction – a force that acts
against the direction of
motion
a. Because of friction – a
constant force must be
applied to an object to
keep it moving
b. Ex. a car - no gas - will
stop
c. Amount of friction depends on
the surfaces
1. Both surfaces rough –
greater friction
2. Both surfaces smooth –
less friction
What’s easier to run on – a beach
or a paved road?
2. Air Resistance – a form of
friction
a. Amount of air resistance
depends on objects:
1. Speed
2. Shape
3. Size
b. The more aerodynamic –
the less air resistance
3. Gravity - the attraction between
two particles of matter due to
their masses
a. Gravity is a force
b. Every object exerts a
gravitational force
c. All objects are pulled towards
the center of Earth due to
the force of gravity
Calculations
• Independent Practice Set – velocity,
acceleration, force
Lab
• One, some, or all of the following labs will be performed
from IPC Manual
– How is the speed of the car changing? (p. 14 & 15)
– What is the relationship between force, mass, and
acceleration? (p. 16 – 19)
– How does increasing the mass of the car affect its
acceleration? (p. 20 – 21)
• Periodic Groups will
– Read lab procedures together
– Set up labs according to scientific method
– Identify potential hazards, precautions, and PPE (if
needed)