Transcript Chapter 1 Structure and Bonding

Chapter 3 Lecture 1 Falling Objects
I.
Acceleration due to Gravity
A.
Special case of Uniform Acceleration
1) Constant force acting on an object imparts uniform acceleration
2) Gravity is a constant force
3) Any dropped object accelerates uniformly
B.
Measuring the Acceleration due to Gravity
v = d/t =1.2cm/0.05s = 24cm/s
v = d/t =16cm/0.05s = 320cm/s
1)
Compute average velocity for each time period
v
d 1.2cm

 24cm/s
t 0.05s
y
x
C.
2)
Plot velocity versus time
3)
Remember
4)
Compute acceleration due to gravity: g = 9.8 m/s2
v y
a

 slope of velocity graph
t x
a
Δv Δy 464  72cm/s 392cm/s 980cm




 9.8m/s 2
2
Δt Δx 0.45  0.05s
0.40s
s
Air Resistance
1) Which hits first, brick or a feather?
2) g is constant for all things; weight of the object doesn’t matter
3) Shape of a feather and its light weight do cause it to interact with the air
strongly = air resistance
a) Large surface area
b) Small weight
4) In a vacuum (no air) all things fall at the same rate
(on Earth)
II.
Tracking Falling Objects
A.
Ball dropped from a building
1) Find velocity and distance at 0.5-2 s
2) Plot distance vs. time
v = vo + at
d = vot + ½ at2
B.
Throwing a ball straight down
1) Now we have an initial velocity v0
2) Acceleration due to gravity is still g = 9.8 m/s2
3)
1 2
d  v0 t  a t
2
4)
v  v0  a t
5)
C.
Ball is thrown down at 20 m/s. What are the velocities and distances at 1
and 2 seconds?
-a
-a
+v
Throwing a ball straight up
1) Direction of velocity and
acceleration are very important
-v
2) What is the acceleration
at the top of the trajectory?
What is the velocity?
3)
Example: sample exercise 3.2