Transcript Physics Presentation - AEA-TIC

1
Click anywhere in the black area below to see a movie.
B
A
C
A ball is going around in a circle attached to a
string. If the string breaks at the instant shown,
which path will the ball follow?
2
B
A
C
A planet is going in a circular orbit around a star. If the star
was to suddenly disappear at the instant shown, which path
will the planet follow?
2
B
A
C
A ball is going around in a horizontal circle on a
frictionless table attached to a string. If the
string breaks at the instant shown, which path
will the ball follow?
2
Uniform Circular motion
Uniform circular motion in a horizontal plane
N
v
T
r
m
mg
mw
T = m g as it is at rest.
mwg = m v2 / r
w
mw= mass
(downward)
N = mg
Net force = T
F = ma
T = m (v2 / r)
Uniform Circular motion
Uniform circular motion in a horizontal plane
T = m (v2 / r)
N
v
T
r
m
mg
mw
What is the physical meaning of the formula?
(1) What happens if the initial speed v of the bob is forced to move faster?
A greater T is needed, the greater mw, to keep the radius of motion r.
It will slide out to attain a greater r.
(2) What happens if there is energy lost due to friction on the plane?
v decreases  By T = mv2 / r, r decreases
(3) What happen if an additional mass is added to the weight mw?
M moves down  r decreases and T increases  v increases
Uniform Circular motion
Uniform circular motion in a horizontal plane
T = m (v2 / r)
N
v
T
r
m
mg
M
What is the physical meaning of the formula?
(4) What happens if the string suddenly break?
•
•
•
Tension suddenly disappears and there is no more centripetal force.
No more circular motion can be maintained.
The bob will move with its velocity tangent to the circle
at the instant the string breaks



When we see an object
carrying out circular motion,
we know that there must be
force acting on the object,
directed towards the center
of the circle.
When you look at the circular
motion of a ball attached to a
string, the force is provided
by the tension in the string.
When the force responsible
for the circular motion
disappears, e.g. by cutting
the string, the motion will
become linear.


In most cases, the string
force not only has to provide
the force required for
circular motion, but also the
force required to balance the
gravitational force.
Important consequences:
 You can never swing an object
with the string aligned with the
horizontal plane.
 When the speed increases, the
acceleration increases up to the
point that the force required for
circular motion exceeds the
maximum force that can be
provided by the string.




When you drive your car around a
corner you carry out circular
motion.
In order to be able to carry out this
type of motion, there must be a
force present that provides the
required acceleration towards the
center of the circle.
This required force is provided by
the friction force between the tires
and the road.
But remember ….. The friction force
has a maximum value, and there is
a maximum speed with which you
can make the turn.
Required force = Mv2/r.
If v increases, the friction force
must increase and/or the radius
must increase.
You are driving a car with constant speed around a horizontal circular track.
Normal
On a piece of paper, draw a Free Body Diagram (FBD) for the car.
Friction
How many forces are acting on the car?
A) 1
B) 2
C) 3
D) 4
Gravity
E) 5
The net force on the car is
A. Zero
B. Pointing radially inward (centripetal!)
C. Pointing radially outward (centrifugal?)
25

Davain sits on the outer rim of a merry-go-round, and
Diego sits midway between the center and the rim. The
merry-go-round makes one complete revolution every
two seconds.
◦ Diego’s angular velocity is:
Diego
Davian
(a) the same as Davian’s
(b) twice Davian’s
(c) half Davian’s

Uniform Circular Motion
◦
◦
◦
◦
Speed is constant
Direction is changing
Acceleration toward center a = v2 / r
Newton’s Second Law F = ma
Click on this question
to investigate! How does this apply to planets?
Check out this
tutorial! http://www.physicsclassroom.com/
Are you HYPER
enough to learn
and link?
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
Check out this site for simulations of circular motion
And so many other simulations!
http://www.animations.physics.unsw.edu.au/mechanics/
chapter3_circularmotion.html
Citations
Graphics:
Slides 9, 10, 11
Physics Text 1
Chapter 3 Vectors and Two-Dimensional Motion
Slides 12, 13
Physics Text 2
Video:
Planetary Orbits are Elliptical. Discovery Channel School. 2005.
Discovery Education. 17 July 2009
http://streaming.discoveryeducation.com/
Websites:
http://hyperphysics.phy-astr.gsu.edu/hbase/orbv.html#co
http://www.physicsclassroom.com/
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
http://www.animations.physics.unsw.edu.au/mechanics/chapter3_circularmotion.html