Bellwork
Sept. 8, 2016
What can you tell me about the motion of the ball rolling down the ramp from the picture below? Explain.
Definitions
Acceleration is the rate at which an object changes its velocity.
An object is accelerating if it is changing its velocity.
Remember, velocity is a vector, and therefore so is acceleration.
Since velocity is a vector, a change in magnitude OR direction
constitute a change in velocity.
Example
Since acceleration is a vector, we need to describe the direction of the acceleration vector to fully describe it.
As a general rule of thumb, if an object is moving in a straight line and slowing down, then the direction of the acceleration is opposite the direction the object is moving.
If the object is speeding up, the acceleration direction is the same as the direction of motion.
Read the following statements and indicate the direction (up, down, east, west, north, or south) of the acceleration vector.
Example
The diagram to the right portrays a Hot Wheels track. the car starts at point A, descends the hill (continually speeding up from A to B); after a short straight section of track, the car rounds the curve and finishes its run at point C. The car continuously slows down from point B to C.
v
v
t
t
a
a
t
t
Sketch the graph of position vs. time and velocity vs. time.
Velocity vs. Time
50
45
45
40
40
35
Velocity (m/s)
Position (m)
Position vs. Time
50
30
25
20
35
30
25
20
15
15
10
10
5
5
1
3
2
Time (s)
4
5
1
2
3
Time (s)
4
5
Describe the motion of the bus during each segment of the journey.
1.
2.
3.
4.
5.
6.
7.
8.
9.
0 ­ A
A ­ B
B ­ C
C ­ D
speeding up
slowing down
constant velocity
positive acceleration
negative acceleration
no acceleration
moving to the left
moving to the right
not moving
D ­ E
E ­ F
Basic Motion Equations
Notes Equation(s) Δv = change in velocity
(vf ­ vi )
Δt = time
Example
A car speeds up from rest to +16 m/s in 4.0 seconds. What is its acceleration?
Example
­
+
A car slows down from 32 m/s to the right, comes to a stop, and goes 8 m/s to the left, all in 5 seconds. What is its acceleration?
Wrap-Up
An object is accelerating if it is moving __________. (circle all that apply)
Explain the reason for you choice(s).
a) with changing speed
c) with constant velocity
e) downward
b) extremely fast
d) in a circle
f) none of the above
An object is NOT accelerating, then one knows for sure that it is _____.
Explain the reason for you choice(s).
a) at rest
c) slowing down
b) moving with a constant speed
d) maintaining a constant velocity
Kinematic Equations
Equation(s) Notes v = velocity
v0 = initial velocity
a = acceleration
t = time
x = position
x0 = initial position
x ­ x0 = change in position
v0 = initial velocity
a = acceleration
t = time
v = velocity
v0 = initial velocity
a = acceleration
x ­ x0 = change in position
Attachments
2013.09.03&04 ­ Linear Motion Exploration.doc