# Explaining these concepts (circular motion)

#### jrd007

Again, these concepts we discussed in class, but I still do not get them. Can anyone explain why they are true?

1) Will the acceleration of a car be the same when the car travels around a sharp curve at a constant 60 km/hr as when it travels around a gentle curve at the same speed? Explain.

2) Which pulls harder gravitationally, the earth on the moon, or the moon on the Earth? Which accelerates more?

3) When will your apparent weight be greatest, as measured by a a scale in a moving elevator; when the elevators: (a) accel. downward (b) accel. upward (c) is in free fall (d) moves upward at a constant speed? In what case would you weight be the least? When would it bethe same as when you are on the ground?

My instinct for question three is that it is least on the way down, b/c your force, pushing on the scale is less. And it is greatest on the way up, since your force of push is stronger. And I would say in free fall it is the same as ground state?

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#### arildno

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What are your ideas on 1) and 2)?

#### lightgrav

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007, have you ever seen skateboarders in the air (free-fall) lose contact with their board?

Does your instinct tell you why your force of push (on the scales?) would have to be stronger if the scales is accelerating upward?
Does this have anything to do with question 2?

#### jrd007

I am still confused on what you mean by that, lightgrav. Maybe the weight would remain the same at ground level if the speed is constant.

Question 1: I would think that the car will be faster when going around a sharp curve because it is more angled?

Question 2: I would say the moon pulls harder, but the earth accelerates more because gravity is more on earth?

#### arildno

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jrd007 said:
I am still confused on what you mean by that, lightgrav. Maybe the weight would remain the same at ground level if the speed is constant.

Question 1: I would think that the car will be faster when going around a sharp curve because it is more angled?
1. The car has equal velocity, it is its ACCELERATION that might be different..
2. Do you connect the word "radius of curvature" with something relevant here?
Question 2: I would say the moon pulls harder, but the earth accelerates more because gravity is more on earth?
What does Newton's 3.law say?

#### jrd007

Well then the accel will be greater on a sharper curve?

I do not connect what you are saying about radius of curvature? Is there an easier way to explain this to me. As I previous stated, I do not understand the statements. Newtons 3rd law states "For every action, there is an equal and opposite reaction" but what does that have to do with the question?

#### arildno

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As for 1), what is the general expression for the centripetal acceleration of an object?

As for 2), if the moon drags on Earth with a force, what will be the force the Earth drags on the moon with?

#### jrd007

ar=v^2/r

I think I figured out the second question. Both the grav. force and acceleration will be greater on earth.

#### lightgrav

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If the skateboard is not touching the feet, there is no upward Force to counter-act gravity's Force (weight). How would the scales in free-fall be comressed if it isn't even touching the feet?

The Force by A on B is the negative of the Force by B on A.

You accelerate upward in an elevator because the ELEVATOR pushes on YOU. You have to push on the elevator, too, but that just makes the elevator go up more slowly than it would've otherwise.

Haven't you drawn Free-Body Force Diagrams?

Vocabulary : reserve "fast" for speed and velocity, "quick" for acceleration.

If direction changes, the velocity has changed. If the curve is short, the radius (half the diameter of the path) is also short ... it won't take much time to change the velocity.

Gravitational Force:
Calculate it! Force by Earth applied to Moon is
M_Moon * G*(M_Earth)/(dist_from_E_to_M)^2 .
Force by Moon applied to Earth
= M_Earth * G*(M_Moon)/(dist_from_M_to_E)^2 .

Is it easier to change the speed of a large object or a small one? Can you throw a boulder quicker than you can throw a baseball?

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#### jrd007

Gravitational Force:

M_Moon * G*(M_Earth)/(dist_from_E_to_M)^2 .
=1.99 10^20 N

Force by Moon applied to Earth
= M_Earth * G*(M_Moon)/(dist_from_M_to_E)^2 .
same answer....

#### lightgrav

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But opposite direction.
an example of F_by_A_on_B = - F_by_B_on_A .
If you treat A and B together as one system,
the "internal" Forces add to zero so the pair
is only influenced by External Objects (the Sun).

But treating them as two objects, with equal pulls,
which will move quicker, the big Earth or the little moon?

#### jrd007

The moon will accelerate more, but the earth has a greater force, right?

#### lightgrav

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I thought you just calculated these Forces!
Yes, the Moon accelerates much quicker.

By the way: in Physics, "action" means Force applied for a while during motion some distance along a path. So "Newton's 3rd Law" is talking about Forces.

Now, what about the Forces acting on this car to make it accelerate?

#### jrd007

1) Forces acting on the care are: Normal force, weight of car, and friction.

2) The moon accelerates much quicker and also pulls harder gravitationally. Right?

3) Your apparent weight be greatest: as measured by a a scale in a moving elevator; when the elevators: (a) accel. downward and (b) accel. upward

In what case would you weight be the least? (c) is in free fall, it would be zero, no tension.

When would it be the same as when you are on the ground? (d) moves upward at a constant speed?, because the speed is constant therefore the weight will not change.

#### arildno

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Why do you persist in saying the moon pulls harder on the Earth than the other way around??
This is wrong!!!
Again:
WHAT IS NEWTON'S 3.law????
Apply that law!

#### jrd007

Please do not get fustrated with me, I am still learning. #### arildno

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jrd007 said:
Please do not get fustrated with me, I am still learning. That's okay; but what have you found out about the relationship between the force from the moon upon the earth and the force from the earth upon the moon?

#### jrd007

I'll tell you what I know about Newtons third law and maybe that will help.

If object A exerts a force on object B, object B will exert an equal and opposite force onto object A, aka the Action-Reaction Law. (forces come in pairs)

So if this saying that the as the earth pulls on the moon, the moon exerts a equal and opposite force as it pulls on the earth?

#### arildno

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jrd007 said:
I'll tell you what I know about Newtons third law and maybe that will help.

If object A exerts a force on object B, object B will exert an equal and opposite force onto object A, aka the Action-Reaction Law. (forces come in pairs)

So if this saying that the as the earth pulls on the moon, the moon exerts a equal and opposite force as it pulls on the earth?
Yes, that is correct.

#### jrd007

And the reason things accelerate more on the moon is because it's acc is less than 9.8, which is the earth's?

#### arildno

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jrd007 said:
And the reason things accelerate more on the moon is because it's acc is less than 9.8, which is the earth's?
The reason why the moon accelerates more than the earth does even though they are subjected to equally strong forces, is that the Earth has greater mass than the Earth.

An object ON the moon, will experience a very reduced free-fall acceleration there than it would have on the Earth, again, because the Earth has a greater mass.

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