Conceptual problem on Newton's Third Law

In summary, the conversation discusses Newton's Third Law, which states that for every action force, there is an equal and opposite reaction force. The example of a basketball player on Earth is used to explain this concept, with the Earth's gravitational force and the player's normal force canceling each other out to keep the player stationary. When the player jumps, there is a net force present and the player exerts a greater force on the Earth (floor) than the Earth exerts on the player, causing the player to accelerate upwards. It is important to understand the different forces at play and how they are related to each other in order to fully grasp the concept of Newton's Third Law.
  • #1
Conservation
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Newton's Third Law states that for every action force, there is a reaction force.

So take a classical problem with a basketball player. The Earth has a gravitational force on the player, and thus player exerts an equal and opposite force on the earth. The player is stationary.

My first question is, is this equal opposite force on the Earth exerted by the player the normal force (yes, it would be numerically equivalent, but by definition)?

Suppose the player decides to jump. Since there is a net force in play due to a certain net acceleration the player, things are no longer at an equilibrium. Does this mean that upon jumping, the player exerts more force on the Earth (floor) than the Earth exerts on the player? And if so, how is this variation from Newton's Third Law possible?

Thank you beforehand.
 
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  • #2
Conservation said:
Newton's Third Law states that for every action force, there is a reaction force.

So take a classical problem with a basketball player. The Earth has a gravitational force on the player, and thus player exerts an equal and opposite force on the earth. The player is stationary.
There are multiple forces acting on the basketball player in this scenario. That the player is stationary has nothing to do with the third law. The player is stationary because those multiple forces cancel. The Earth as a whole is gravitationally pulling the player downward, but at the same time the surface of the Earth is pushing the player upward via the normal force.

So what are the third law reactions? That the Earth attracts the player gravitationally means that the player exerts an equal but opposite force on the Earth, directed upward. That the Earth pushes the player upward means that the player in turn pushes the Earth downward via the normal force.

One final note: Unless the basketball player is at the North or South Pole, the downward gravitational force and the upward normal force do not quite cancel one another. The Earth is rotating about it's axis, and thus so is the player who is more or less stationary with respect to the surface of the Earth. The net force on the player must be just enough to place the player in uniform circular motion about the Earth's rotation axis.
 
  • #3
I'm slightly confused here. So we have two pairs of four forces, labeled 1,2,3,4:

1. Gravitational force (field force) of Earth on player, 2. gravitational force of player on earth

3. The Earth holding the player up (Floor's contact force) on player, 4. the player in turn pushing the Earth down due to normal force

Isn't 1 the same as 4, or directly caused by 4? And my question point about being stationary was meant to simply exemplify the fact that all the forces appeared to cancel out, leading to my question about the player jumping.
 
  • #4
I'm slightly confused here. So we have two pairs of four forces, labeled 1,2,3,4:

1. Gravitational force (field force) of Earth on player, 2. gravitational force of player on earth

3. The Earth holding the player up (Floor's contact force) on player, 4. the player in turn pushing the Earth down due to normal force
Right.

3 and 4 can be seen as caused by 1 and 2. They are not the same.

Suppose the player decides to jump. Since there is a net force in play due to a certain net acceleration the player, things are no longer at an equilibrium. Does this mean that upon jumping, the player exerts more force on the Earth (floor) than the Earth exerts on the player?
No, the forces between player and Earth (via the ground, 3 and 4 in your list) both increase and get larger than the gravitational forces (1 and 2). The forces on the objects are no longer balanced and they will accelerate. The Earth starts to move backwards/downwards a tiny bit.
 
  • #5
First part: There are two action-reaction pairs at play when the person is stationary. The first pair consists of the force that the player's feet exert on the ground and the equal and opposite reaction force that the ground exerts on the feet. The second pair is the gravitational force exerted by the Earth on the player and the equal and opposite gravitational reaction force that the player exerts on the Earth (every object in the universe attracts every other object, according to Newton's universal gravitation). These pairs of forces have essentially nothing to do with each other.

It just so happens that, in this example, the upward/normal force exerted by the floor on the person exactly balances the downward pull of the Earth. So, the guy stays stationary.

Second part: When the player goes to jump, he uses his legs to exert a greater force on the ground than is present when he is just standing. The reaction force, equal and opposite, that the ground exerts on the person is greater than the downward force of gravity, accelerating the player upwards.

It's very important to realize which forces are acting where, and which objects are providing them. You mixed up the action/reaction forces in your introduction of the problem.
 
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  • #6
Conservation said:
Isn't 1 the same as 4, or directly caused by 4? And my question point about being stationary was meant to simply exemplify the fact that all the forces appeared to cancel out, leading to my question about the player jumping.

NO! They are not the same forces. They are in fact causally related, but that has no bearing on the resultant acceleration, or lack thereof, of the object in question (the person). Forces 1 and 3 are equal and opposite, but not because of action/reaction, just because that's the way it is in this particular situation.
 
  • #7
Hi Conservation,

Forces are always caused by some sort of interaction between two objects. When two objects interact through gravity then there will be a gravitational force from each object on the other object. When two objects interact through contact then there will be a contact force from each object on the other object. These forces are equal and opposite and form 3rd law pairs.

So the important characteristics of a 3rd law pair are
1) they each are of the same "kind"
2) they each act on different objects

If the player is at rest then the gravitational force is equal and opposite to the normal force, but one is a gravitational force and the other is a contact force and they are acting on the same object. So they are not a third law pair because they don't meet those two characteristics. Gravitational forces and contact forces are not the same "kind" of forces, and the player is a single object.
 
  • #8
I see; thanks for all the replies. I now see how the two pairs of forces are separate, so, just to sum up for confirmation:

There are four forces acting on the basketball player.

At rest, the pair of gravitational forces (1 Earth on player, 2 player on earth) exist along with pair of contact forces (3 floor on player, 4 player on floor). Since the player is not exerting any extra force on the floor aka earth, the force of the player on floor equals the gravitational force on earth. Hence, Force 1=Force 2 due to Newton's third law, Force 1=Force 4 due to no further exertion of force by the player on the floor, and Force 3=Force 4 due to Newton's third law.

However, while jumping, the player exerts more force on the floor than previously, which only accounted for his weight. Force 1 no longer equals force 4; force 4 is the sum of the force applied by the player and the weight of the player. However, 1=2 and 3=4 still hold due to Newton's third law. Thus, Force 3 (the floor's upward contact force) becomes greater than force 1, allowing the player to "defy gravity" momentarily.

At least, I think that's how that works...right?
 
  • #9
There are four forces acting on the basketball player.
Only 2. The other two are acting on earth.
At least, I think that's how that works...right?
Right.
 
  • #10
Conservation said:
At least, I think that's how that works...right?
Seems like you got it!
 

Related to Conceptual problem on Newton's Third Law

1. What is Newton's Third Law?

Newton's Third Law, also known as the Law of Action and Reaction, 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 exert an equal and opposite force back on the first object.

2. How does Newton's Third Law apply to everyday life?

Newton's Third Law can be seen in many everyday situations, such as when you push against a wall and the wall pushes back on you, or when you jump off a diving board and the board pushes you upwards. It also applies to the way objects move, as every movement is the result of a force being applied.

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

One example of Newton's Third Law is a rocket launching into space. The rocket's engines exert a force downwards, which causes an equal and opposite force upwards, propelling the rocket upwards. This is also seen in a balloon being released and flying around the room.

4. What is the significance of Newton's Third Law in physics?

Newton's Third Law is significant in physics because it helps us understand the relationship between forces and motion. It explains how objects interact with each other and why they move the way they do. It is also an essential principle in the study of momentum and energy.

5. Are there any exceptions to Newton's Third Law?

There are a few exceptions to Newton's Third Law, such as when there is a non-contact force, like gravity. Gravity is a force that acts between two objects without direct contact, so there is not always an equal and opposite reaction. Additionally, in some cases of elastic collisions, the forces may not be exactly equal and opposite due to energy being transferred between the objects.

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