Download con review circ gravity soln

Conceptual Physics
Mrs. Mills
Ch 12-14 Gravity Practice Problems
Don’t forget to review Ch 9 (circular motion) as well
Fg =
Gm1 m2
2
d
G = 6.67 x 10-11 N m
Mass of Earth = 5.98 x 1024 kg
2
kg 2
Radius of Earth = 6.38 x 106 m
1) Given the mass of two objects and distance between them, you should be able to use the equation
above to find the force of gravity between them. I will NOT help you use your calculator during the
test.
Find the force of gravity between a 45 kg person and the Earth when:
a) the person is standing on the surface of the Earth.
Fg = mg = 45 * 9.8 = 441N
b) the person is 1 Earth radius above the surface of the Earth.
Center to center distance is now 2RE. This will decrease the force by 4 since there is
a square in the denominator. So 441/4 = 110N
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c) The person is at the center of the Earth.
Earth will pull on person equally in all directions so the Net force is 0N.
2) If you know the force of gravity acting on an object, you should be able to find the acceleration due to
gravity (F = ma or W=mg). What is the acceleration due to gravity of the person in part a) and b)
above?
a) g=9.8 m/s2
b) g=F/m=110N/45kg=2.44 m/s2
3) A gravitational force exists between the Earth and the Sun. Which applies a greater force on the other.
Which will accelerate more?
Equal force but the Earth accelerates more due to its smaller mass.
4) A toy is swung around in a circle at the end of a string. If the string is cut, what path will he follow?
Straight line path tangent to the circle at the point where it is cut off.
5) The force of gravity acts on all of the apples on a 4 m tall apple tree. Some apples are twice as far
away from the ground as others. These twice as high apples, for the same mass, have
c) practically the same weight
6) If you stepped in a hole bored completely through the Earth and made no attempt to grab the edges at
either end, what kind of motion would you experience?
Not discussed in class this year
7) Explain how your acceleration and velocity change as you go through the hole bored completely
though the Earth.
Not discussed in class this year
8) An object has a mass 0.5 kg. It is transported to the surface of Planet Z where the object's weight (Fg)
is measured to be 20 N. The radius of the planet is 4 x 106 m. Determine the acceleration due to
gravity on Planet Z.
F 20N
F
a = net = g =
= 40m /s2
m
m
.5
9) Using complete sentences, explain why an astronaut on the orbiting space shuttle experiences apparent
€
weightlessness.
Constantly falling along with everything else, no support force
10) If you know the force acting on a object, you should be able to find the acceleration (F = ma)
a=F/m
11) If the moon is constantly being pulled by the Earth, why doesn't it fall into the Earth?
The moon has just the right speed so that it will orbit. As it falls, the Earth is curving away
at the same rate so the moon never gets any closer.
12) Explain how force of gravity between the Earth and Moon is the reason for tides.
Moon pull more on one side of the Earth than the other (about 7% difference) causing the
water to be stretched.
13) The Sun's force of gravity on the Earth is about 200 times greater than the Moon's, why is the Moon's
influence on the Earth's tides greater?
Sun is pulling more on the Earth but since it is so far away the difference between the pull
on each side of the Earth is very small. (0.02% difference as opposed to 7% from Moon)
14) Who invented gravity?
No one, even my baby has discovered gravity on her own.
15) A gravitational force exists between the Earth and the Sun. Which applies a greater force on the other.
Where will acceleration due to gravity be larger?
Well, considering this is the second time this question has appeared you should be able to
answer it by now!
16) True or False. The force of Earth's gravity on a space shuttle in orbit is zero or nearly zero.
False! Explain why using the concept of Apparent weightlessness and also sample
calculations.
Some Circular Motion Problems
17) Horses that move with the fastest linear speed on a merry-go-round are located where?
outside
18) Centrifugal forces feel real to an observer in a reference frame that is:
c) rotating
19) A new Ferris wheel ride has seats halfway between the axis and the outside rim. Compare the
frequency of the passengers to an ordinary Ferris wheel where the people ride at the outer edge.
Same frequency
20) Can an object accelerate if its velocity is constant? If yes how?
No! acceleration is defined as a change in velocity over time.
It can however have an acceleration while moving at a constant speed.
21) Dilbert is driving and Filbert is a passenger. When Dilbert makes a sharp turn to the left, Filbert feels
like he is being pushed to the right. What is the reaction force to this Centrifugal force?
None, it is a fake force
22) A small dog (weight is 50 N) with two broken legs and a strained neck is attached to a 2 meter piece
of string and spun 5 times a second around Elmo's head. What is the dog's linear speed?
d 5(2πr)
v= =
= 62.8m /s2
t
1s
23) When you roll a Dixie cup on a table, why does the open end have a greater linear speed?
€
It travels more distance in every rotation of the cup than the small end.
24) How often does the Earth revolve and around what?
Once a year around the Sun
25) Which way does the acceleration of the spinning object point at position B?
F and a are always toward the center
V is always tangent to the circle
26) Which point A or B has a greater frequency?
same