Newton's 3rd Law - action/reaction pairs

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In summary: They will read that the word action comes from the root word, 'ac', which means to take or to make an attack. But what they don't realize is that the same word is in the word 'react' as well. React comes from the Latin word, 're-act'. So when they hear the word 'action' they think of taking something or making an attack. And when they hear the word 'reaction' they think of the opposite of taking something or making an attack.In summary, action/reaction pairs are two terms that are always inextricably tied to each other and always cause each other to happen.
  • #1
jamesnb
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Newton's 3rd Law -- action/reaction pairs

Homework Statement


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."

Homework 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.

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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  • #2


jamesnb said:

Homework Statement


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."

Homework 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.

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
 
  • #3


jamesnb said:

Homework Statement


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."

Homework 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.

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
 
  • #4


Andrew Mason said:
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.
 
  • #5


I would agree that this is not a well-written question. The answer should be the tension on the string, as that is the force that is directly opposing the Earth's gravity on the ball. The ball's gravity pull on the Earth is not relevant in this scenario, as it is much smaller compared to the Earth's gravity on the ball. This question could be improved by clarifying that the reaction force is the force that directly opposes the action force.
 

1. What is Newton's 3rd Law of Motion?

Newton's 3rd Law of Motion states that for every action, there is an equal and opposite reaction. This means that when an object exerts a force on another object, the second object will also exert an equal and opposite force on the first object.

2. How does this law apply to everyday life?

This law can be observed in many everyday situations. For example, when you push against a wall, the wall pushes back on you with an equal force. Another example is when you jump off a diving board, the diving board pushes back on you, propelling you into the air.

3. Can you give an example of an action/reaction pair?

An action/reaction pair can be seen when a person is rowing a boat. As they push against the water with their oars (action), the water pushes back on the boat, propelling it forward (reaction).

4. Does this law only apply to objects in motion?

No, this law applies to all objects, whether they are in motion or at rest. When an object is at rest, the forces acting on it are balanced, but as soon as it starts moving, it will experience an equal and opposite force.

5. How does Newton's 3rd Law relate to the conservation of momentum?

Newton's 3rd Law is closely related to the conservation of momentum, which states that the total momentum of a closed system remains constant. This means that the momentum of the first object pushing on the second object is transferred to the second object, resulting in equal and opposite momentums for both objects.

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