View
3
Download
0
Category
Preview:
Citation preview
March 23, 2015
March 23, 2015
Free fall - motion of an object which is only subject to gravitational force (no friction / air resistance considered)
acceleration due to Earth's gravitational pull:
g = 9.80 m/s2
sign convention:
axis of reference is y-axis pointing upwards --> falling object is subject to acceleration of -9.80 m/s2 and has a negative velocity:
y
a = -9.8 m/s2v < 0
v > 0
same equations as for other accelerated motions hold true:
March 23, 2015
y = (-9,8 m/s2) t212
--> inverted half-parabula as position / time graph
--> in the second second of movement object is 9.8 m/s faster than in first second and so on.
--> mass of object does not appear in formula - what does that mean?
Strobe phtograph of a falling ballhttps://www.youtube.com/watch?v=xQ4znShlK5A
https://www.youtube.com/watch?v=AV-qyDnZx0A
March 23, 2015
Do heavy and light objects fall with the same speed?
http://video.mit.edu/watch/feather-and-coin-in-a-vacuum-6407/
compare styrofoam and rubber ball, feather and ball
March 23, 2015
motion of a ball thrown into the air:
- ascend and descend take the same amount of time
- final velocity same magnitude as initial velocity (but opposite sign)
March 23, 2015
Example:
From a hight of 15m an object is thrown into the air with a velocity of 10 m/s. What is the objects flight time and what will its final velocity be?
(vf2 = vi2 + 2a y)
15 m
y = yfinal - yinitial = 0m - 15m = -15m
March 23, 2015
March 23, 2015
March 23, 2015
A car travels at a velocity of 5 m/s for five seconds and then accelerates to a velocity of 10 m/s within 5 sec. What is the average velocity over the course of this movement?
March 23, 2015
Isaac's house lies at a distance of 1500 m from school. Kyle lives on the same street as Isaac but 750 m further from school. On Friday morning they both head out the door at the same time. Isaacs strolls to school at a velocity of 1 m/s. Kyle however has an urgent question to ask his math teacher before his quiz and therefore hurries to school at a velocity of 2 m/s. At what point in time after leaving the house does Kyle overtake Isaac?
March 23, 2015
Air resistance increases with velocity
March 23, 2015
"Real" fall and terminal velocity
initially:
not much air resistance --> skydiver accelerates
later:
the greater the velocity, the greater the air resistance --> drag created by air resistance becomes equal to acceleration due to gravity --> terminal velocity reached.
B
Free Fall: air resistance is not considered (either negligable, or we pretend there is no air). Acceleration formulas hold true, an object will continue accelerating untill it hits the ground.
Real Fall: air resistance increases with velocity and will slow acceleration. A terminal velocity will be reached.
The more massive the object, the greater the gravitational force pulling it down. Therefore a massive object has to reach a higher speed compared to a lighter object, before air resistance counteracts the gravitational pull.
In total: the more massive the object, the greater the terminal velocity.
March 23, 2015
An object, initially at rest, falls from a height of 12 metres. What will its height be when it reaches half of its maximum speed?
March 23, 2015
The graph below shows the speeds of two cars.
At time 0 car B passes car A, which is starting from rest.
1.) what will the distance be between the two cars after 60 sec.?
2.) At what time will car A pass car B?
Recommended