Understanding how Newton's Third Law is applied?

In summary, Newton's third law states that for every action there is an equal and opposite reaction. Zoro provides a summary of two examples of how this law can be applied in the real world. In the first example, a kick with the same force as a kick of a much more massive object does not produce the same reaction due to the difference in mass. In the second example, a force is applied to a rocket that is attached to a crate, and the total force and acceleration are calculated.
  • #1
Zorodius
184
0
Hello!

I've just begun attempting to teach myself physics, and I'm finding it difficult to understand Newton's third law. Would someone be so kind as to shed a little light on the situation?

The idea of the equal and opposite reaction for every action makes perfect sense to me in some cases (a swimmer's motions push the water behind them, and in response, the water pushes them forward; I lean against a wall, the wall is pushed away from me and responds by exerting a force in the opposite direction that holds me up), however, I keep running into situations where it doesn't seem to make sense. For instance:

1) I kick a pebble and a boulder with the exact same force. My foot experiences almost no noticeable response when I kick the pebble, but when I kick the boulder, I hurt my foot. How can these both be equal reactions?

2) In outer space, a rocket is attached (by a cord, or by a beam, or by whatever would make this example logical without complicating things) to some object, let's say a crate. The rocket is trying to pull the crate. If the rocket fires, the action-reaction pair between the rocket's exhaust and the rocket subjects the rocket to some force x.
Since the rocket is attached to the crate, the rocket also applies force x to the crate. Since the crate is attached to the rocket, it applies a force to slow the rocket down - which, by Newton's third law, is -x. The net force on the rocket is x - x = 0, and so, no matter how hard the rocket fires, it's never going to be able to escape equilibrium.
Where's the error here?

I would really appreciate help understanding where I'm going wrong with this law :wink: Thanks!
 
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  • #3
Without getting into a lot of detail, what you seem tohave missed in your examples is the mass of the object in question (Newton's law, force=mass times acceleration).
 
  • #4
Thanks for the great link!

However, I am still unclear on where I am going wrong in the two examples I mentioned. I realize that the mass of the objects comes into play as part of the explanation, but I don't see where. Could someone offer some clarification here?
 
  • #5
Welcome to PF, zoro.

1) Here, you're not actually kicking them with the same force -- the average force you put on the pebble is much smaller than that on the boulder. In general, force is a bad concept to use when dealing with collisions, because the actual collision forces are very dependent on how much the two colliding thingies deform. But here's what happens:

Force = mass*acceleration. During the kick, the pebble accelerates quickly, but it is so light that the force needed is small. So the acceleration of your leg is negligible and you hardly feel like you hit anything. When you kick the much more massive boulder, the situation is reversed; the 3rd law means it accelerates much less than your leg. So your leg is stopped before the boulder moves very far at all.

2) The rocket doesn't subject the crate to force X; it subjects it to some different force Y. Remember that if they are rigidly attached, that means they have to move at the same velocity always (otherwise one would be getting closer/further) hence their acceleration is the same. The total forces and accels are:

F_rocket = X-Y
F_crate = Y

a_rocket = (X-Y)/m_rocket
a_crate = Y/m_crate

So if the two accels are equal,

Y = X * m_crate/(m_rocket+m_crate)
F_rocket = X * m_rocket/(m_rocket+m_crate)

So the net force on the rocket -> 0 if the crate weighs a lot more, and -> X if the crate is really light.
 

1. How does Newton's Third Law explain the concept of action and reaction?

Newton's Third Law states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another object, the second object will exert a force back on the first object that is equal in magnitude and opposite in direction.

2. Can you give an example of Newton's Third Law in action?

One example of Newton's Third Law is when you push against a wall, the wall pushes back on you with an equal force. Another example is when a swimmer pushes against the water with their arms, the water pushes back on the swimmer, propelling them forward.

3. How is Newton's Third Law applied in sports?

In sports such as soccer, when a player kicks the ball, the ball exerts an equal and opposite force back on the player's foot. This allows the ball to move and the player to maintain balance. In skateboarding, when a skater pushes against the ground with their foot, the ground pushes back with an equal force, propelling the skateboard forward.

4. How does Newton's Third Law relate to everyday life?

Newton's Third Law is present in many everyday situations, such as walking, driving a car, or throwing a ball. When you walk, your feet push against the ground and the ground pushes back, allowing you to move forward. When you drive a car, the wheels push against the road and the road pushes back, propelling the car forward. When you throw a ball, your hand exerts a force on the ball and the ball exerts an equal and opposite force back, causing it to move.

5. Is Newton's Third Law always true?

Newton's Third Law is a fundamental law of physics and is always true in any interaction between two objects. However, the effects of the forces may not always be noticeable or may be canceled out by other forces. In situations where there is no net force acting on an object, the forces will cancel out and there will be no acceleration.

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