Physics I Honors
Mechanics IIAcceleration
Acceleration
Physics I Honors
Acceleration• is a vector quantity defined as the
rate at which an object changes its velocity. An object is accelerating if it is changing its velocity.
Physics I Honors
Acceleration
• The increase or decrease of velocity per unit time is called acceleration.
• It is the rate of change of velocity.• Acceleration may be
– A change in magnitude
Or
– A change in direction
Or
– Both
• Its units are meters per second squared (m/s2).
Physics I Honors
…and some notes…
• Acceleration is a vector.
• It can be positive or negative.
• At rest means initial time is zero.
• When velocity is constant, acceleration is zero.
• When velocity increases at a constant rate, acceleration is uniform.
Physics I Honors
Uniformly Accelerated Motion Along a Straight Line
• In this case…• acceleration is a constant
• and the acceleration vector lies in the line of the displacement vector.
Physics I Honors
Accelerating Objects
Physics I Honors
Acceleration
Average acceleration is the change in velocity divided by the change in time.
Acceleration = change in velocity
change in time
tv
tt
vva
if
if
av
Physics I Honors
The 5 Acceleration Equations!
tvdav
2
) (t v vd
i f
atvvif
221 attvd
i
advvif
222
Physics I Honors
S-t, v-t, a-t graphs for constant acceleration
Physics I Honors
Definitions
• Instantaneous Velocity– the slope of the displacement versus time graph
• Instantaneous Acceleration– the slope of the velocity versus time graph
Physics I Honors
SlopesD
ispl
acem
ent
Time
A
B
Physics I Honors
Six Cases of Acceleration
1 - speed up in positive direction = positive accel.
a
+ direction
vf = 8 m/svi = 5 m/s
t = 3 seconds
fin
al
init
ial
Calculate average acceleration!
Physics I Honors
Six Cases of Acceleration
2 - slow down in positive direction = negative accel.
a
+ direction
vi = 8 m/s vf = 5 m/s
t = 3 seconds
fin
al
init
ial
Calculate average acceleration!
Physics I Honors
Six Cases of Acceleration
3 - speed up in negative direction = negative accel.
+ direction
vf = -8 m/s vi = -5 m/s
a
t = 3 seconds
fin
al
init
ial
Calculate average acceleration!What is happening to speed?, velocity?
Physics I Honors
Six Cases of Acceleration4 - slow down in negative direction = positive accel.
fin
al
init
ial+ direction
vf = -5 m/s vi = -8 m/s
t = 3 seconds
Calculate average acceleration!What is happening to speed?, velocity?
Physics I Honors
Six Cases of Acceleration
5 - reverse directions from pos to neg = negative accel.
vf = -1 m/svi = +1 m/s
a
+ direction t = 3 seconds
init
ial
fin
al
Calculate average acceleration!
Physics I Honors
Six Cases of Acceleration
6 - reverse directions from neg to pos = positive accel.
vf = -1 m/svi = +1 m/s
a
+ direction t = 3 seconds
init
ial
fin
al
Calculate average acceleration!
Physics I Honors
Horizontal AccelerationHorizontal Acceleration
Consider an airplane taking off. As it goes down the runway it increases its speed until it is going fast enough to “lift off” the ground.
Physics I Honors
……but wait a minute…but wait a minute…
What if it’s vertical straight line motion…say, like a rocket going up at Cape Canaveral?!?
When you are traveling in a vertical direction, acceleration is always the same.
It is the acceleration of gravity, g, which always has the same value.
For vertical motion problems, simply substitute g for a in any of the straight-line motion equations.
2/80.9 smg
Physics I Honors
Airborne motion isUNIFORMLY ACCELERATED MOTION
the change in velocity over time is linear so we say the change in velocity is constant
This constant acceleration is = -9.8 m/s2
This is the rate at which any airborne object will accelerate.
Physics I Honors
Free Fall
• The force of gravity points downward– Acceleration of gravity near the surface of Earth is
called g = 9.8 m/s2 = 32.1 ft/s2
• Air resistance ignored
• We have then the conditions of one-dimensional kinematics – straight line motion with constant acceleration.
Physics I Honors
• 6-9 Gs: "Increased chest pain and pressure; breathing difficult, with shallow respiration from position of nearly full inspiration; further reduction in peripheral vision, increased blurring, occasional tunneling, great concentration to maintain focus; occasional lacrimation; body, legs, and arms cannot be lifted at 8 G; head cannot be lifted at 9 G."
• 9-12 Gs: "Breathing difficulty severe; increased chest pain; marked fatigue; loss of peripheral vision, diminution of central acuity, lacrimation."
• 15 Gs: "Extreme difficulty in breathing and speaking; severe vise-like chest pain; loss of tactile sensation; recurrent complete loss of vision.
Human Response to Sustained g’s
Data primarily from: Bioastronautics Data Book, second edition, 1973, NASA)
In certain activities people experience + & - accelerations. By standardizing these accelerations to the normal acceleration on earth (-9.8 m/s/s) you get an idea of how much force they are experiencing
Physics I Honors
Problem Solution
1. Draw a picture.
2. List values for any parameters that are given.
3. Find equations in which all of the variables are known except the one that you are trying to find.
4. Isolate
5. Substitute values for variable and solve.
Top Related