Physics education research, electronic materials, and the musings of Christopher Moore, Ph.D.
Physics education research, electronic materials, and the musings of Christopher Moore, Ph.D.
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So I don't get 8 and 5. The answer to 8 is 1.28e1 N but im getting confused between tangential and angular accelertaion...help!
8. [1 point, 10 tries]
A 162.0 g ball on a 70.2 cm long string is swung in a vertical circle about a point 228.0 cm above the floor. The string suddenly breaks when it is parallel to the ground and the ball is moving upward. The ball reaches a height 510.0 cm above the floor. What was the tension in the string an instant before it broke?
Answer:
Last Answer: 14.75 N
Incorrect, tries 5/10.
4. [1 point, 10 tries]
A 5.20 m diameter merry-go-round is initially turning with a 6.98 s period. It slows down and stops in 17.5 s. Before slowing, what is the speed of a child on the rim?
Correct, computer gets: 2.34e+00 m/s
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5. [1 point, 10 tries]
How many revolutions does the merry-go-round make as it stops? (Do not enter a unit.) Answer:
Last Answer: 2.51
Not yet correct, tries 2/10
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first draw a picture ... be careful where you measure heights from.
second get a formula for the height from release point in terms of the initial upward velocity (speed)
(i ll come back to that)
third the velocity needs to be related to the angular acceleration
fourth and last, from the acceleration you get the force from newtons law
...coming back ....
the kinetic energy of the initial upward velocity needs to be set equal to the gravitational potential energy in terms of the extra height. THATS THE PLAN...simple.
you should know all those formulae.... if not look them up ... that will be useful to see what you have in your notes and texts too....or ask again.
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just in case ....
for cicular motion v=wr, a=wv where w=2 pi f
circular motion x = r cos wt y = r sin wt
KE= (1/2)mv**2
PE= m.g .h
f=ma
NOTE you can easily get stuff like w=v/r, a= w**2 .r and a= v**2/r ...etc from above.
ANSWER f=ma ... a=v**2/r ... v**2 = 2gh
so f=2mgh/r --- this formula has correct units--always good to check.---i actually forgot the m in the PE formula and my f originally came out independant of m which i imediately realised must be wrong -- then i noticed my v depended on m which it shouldnt,,, we know if we drop something then the velocity doesnt depend on the mass -checking units can help.
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