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Newton's 3rd Law - action/reaction pairs

by jamesnb
Tags: action or reaction, newton, pairs
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jamesnb
#1
Oct25-11, 11:06 PM
P: 37
1. The problem statement, all variables and given/known data
I'm a high school physics teacher and the following was a question on a work sheet. I don't really like the question and I'm not certain the the answer is correct.
"A ball hangs from a string being pulled down by gravity. If the action force is Earth’s gravity
pull on the ball, the reaction force (Newton’s third law force) is
a. the string force on the ball.
b. the ball force on the string.
c. the force holding the string from above.
d. the ball’s gravity pull on the Earth."

2. Relevant equations

The key says the answer is D. We haven't covered Universal Gravitation yet and I don't really see it as relevant because the ball isn't levitating.

3. The attempt at a solution

I'm more inclined to say the answer is the tension on the string or the force on what ever the string is tied.
Am I missing something or is this just a bad question? Thanks.
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Andrew Mason
#2
Oct25-11, 11:44 PM
Sci Advisor
HW Helper
P: 6,679
Quote Quote by jamesnb View Post
1. The problem statement, all variables and given/known data
I'm a high school physics teacher and the following was a question on a work sheet. I don't really like the question and I'm not certain the the answer is correct.
"A ball hangs from a string being pulled down by gravity. If the action force is Earth’s gravity
pull on the ball, the reaction force (Newton’s third law force) is
a. the string force on the ball.
b. the ball force on the string.
c. the force holding the string from above.
d. the ball’s gravity pull on the Earth."

2. Relevant equations

The key says the answer is D. We haven't covered Universal Gravitation yet and I don't really see it as relevant because the ball isn't levitating.

3. The attempt at a solution

I'm more inclined to say the answer is the tension on the string or the force on what ever the string is tied.
The correct answer is d.

Action/reaction are unfortunate terms. It suggests one follows the other, which of course is not true. They act simultaneously. They are inextricably tied to each other, always.

There are only two forces that are capable of causing a mass to accelerate: the force of the earth on the ball and the force of the ball on the earth. The other forces (string tension and the force of the earth on the string) are tensions that result from the gravitational forces by constraining the relative motion of the ball and the earth.

If the string breaks, the tension and normal forces go to 0 but the forces of gravity remain the same. So the force of gravity on the ball cannot be the action/reaction pair to the string tension. The force of gravity of the ball on the earth is always equal and opposite the gravitational force of the earth on the ball.

AM
PeterO
#3
Oct26-11, 01:40 AM
HW Helper
P: 2,318
Quote Quote by jamesnb View Post
1. The problem statement, all variables and given/known data
I'm a high school physics teacher and the following was a question on a work sheet. I don't really like the question and I'm not certain the the answer is correct.
"A ball hangs from a string being pulled down by gravity. If the action force is Earth’s gravity
pull on the ball, the reaction force (Newton’s third law force) is
a. the string force on the ball.
b. the ball force on the string.
c. the force holding the string from above.
d. the ball’s gravity pull on the Earth."

2. Relevant equations

The key says the answer is D. We haven't covered Universal Gravitation yet and I don't really see it as relevant because the ball isn't levitating.

3. The attempt at a solution

I'm more inclined to say the answer is the tension on the string or the force on what ever the string is tied.
Am I missing something or is this just a bad question? Thanks.
The key to newtons 3rd law couples is to just interchange the objects, so

the action force is Earth’s gravity pull on the ball [copied from your post]

becomes

the re-action force is ball’s gravity pull on the Earth

grzz
#4
Oct26-11, 02:58 AM
P: 950
Newton's 3rd Law - action/reaction pairs

Quote Quote by Andrew Mason View Post
Action/reaction are unfortunate terms.
Yes those two terms caused me a lot of trouble in my teaching of Physics. The first thing I used to do was to make my students UNLEARN the statement of Newton's 3rd law as they learned it before.

I believe that the great Newton first wrote the law as,'To every action there is an equal and opposite reaction'. BUT Newton knew what that meant! The trouble isthat if students learn it that way most probably they do not understand it. At least good books of Physics do not write the 3rd law like that anymore but put it in the form,'If body A acts with a force on body B then body B will act with an equal and opposite force on body A'.

And there is another thing. Later on in their Physics education the students will meet with the term 'action' with a specific meaning.


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