Forces misconception hammer and nail

  • Context: High School 
  • Thread starter Thread starter eulerddx4
  • Start date Start date
  • Tags Tags
    Forces
Click For Summary

Discussion Overview

The discussion centers around the mechanics of how a hammer drives a nail, particularly in relation to Newton's third law of motion. Participants explore the implications of force interactions between the hammer and nail, as well as analogous scenarios like pushing a coffee mug or a car. The conversation includes conceptual misunderstandings and attempts to clarify the nature of forces and acceleration in these contexts.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants express confusion about how a nail can accelerate when a hammer strikes it, given that Newton's third law states that the nail exerts an equal and opposite force on the hammer.
  • Others suggest that the hammer's mass and the external forces acting on the system (like the hand and gravity) contribute to the nail's acceleration.
  • One participant questions the concept of net force, stating that they do not see a net force present in the hammer-nail interaction.
  • Another participant explains that the forces between the hammer and nail are internal forces and do not affect the overall acceleration of the system, which includes external forces.
  • Some participants draw parallels to pushing a coffee mug or a car, discussing how the forces involved do not cancel out but rather result in different accelerations based on mass.
  • There is a mention of free-body diagrams as a tool to visualize the forces acting on each object in the system.
  • A later reply emphasizes that the forces do not "just go away" and that they are real and affect the motion of both the hammer and the nail.

Areas of Agreement / Disagreement

Participants generally express confusion and seek clarification on the concepts discussed, indicating that multiple competing views remain. There is no consensus on the understanding of how forces interact in the hammer-nail scenario, and the discussion remains unresolved.

Contextual Notes

Some participants reference specific scenarios (like pushing a car or a coffee mug) to illustrate their points, but these examples may not fully capture the complexities of the hammer-nail interaction. The discussion includes various assumptions about mass, acceleration, and external forces that are not uniformly agreed upon.

eulerddx4
Messages
23
Reaction score
0
Can someone please explain how a hammer can drive a nail. A person swings a hammer in the air and exerts a force on a nail but Newton's third says the nail exerts the same force back on the hammer. So if Newton is right then how does the nail accelerate.

I just don't get it.
 
Physics news on Phys.org
eulerddx4 said:
Can someone please explain how a hammer can drive a nail. A person swings a hammer in the air and exerts a force on a nail but Newton's third says the nail exerts the same force back on the hammer. So if Newton is right then how does the nail accelerate.

I just don't get it.

What exactly don't you get about it?
 
Last edited by a moderator:
eulerddx4 said:
Can someone please explain how a hammer can drive a nail. A person swings a hammer in the air and exerts a force on a nail but Newton's third says the nail exerts the same force back on the hammer. So if Newton is right then how does the nail accelerate.

I just don't get it.

The hammer does get accelerated backwards (as in up). What do you think stops the motion of the hammer as you slam down on the nail?
 
Last edited by a moderator:
I guess my question might be is the hammer able to exert a force on the nail because it has mass and acceleration?
Also if the nail exerts the same force back then how does the nail accelerate
 
If i have a coffee mug on a table why am i able to push it with my hand if whenever i push it the coffee mug pushes back.
 
Thats what I am trying to get at and i feel extremely stupid right now but I am just not getting it
 
eulerddx4 said:
Also if the nail exerts the same force back then how does the nail accelerate
Why should the force on the hammer prevent the nail from accelerating?
 
Last edited by a moderator:
because the force on the hammer is equal to the force on the nail
 
so shouldn't the forces on the nail and hammer just go away if Newtons law is true
 
  • #10
eulerddx4 said:
If i have a coffee mug on a table why am i able to push it with my hand if whenever i push it the coffee mug pushes back.

You exert the same force that the coffee mug exerts on you. However, since you have a far far greater mass, by F = ma, the coffee mug will provide a very small (and to you, unnoticeable) acceleration back on you compared to the acceleration you provide to the mug.
 
Last edited by a moderator:
  • #11
oooooo
 
  • #12
eulerddx4 said:
because the force on the hammer is equal to the force on the nail
But it is acting on the hammer. Why should it affect the nail?
 
Last edited by a moderator:
  • #13
eulerddx4 said:
so shouldn't the forces on the nail and hammer just go away if Newtons law is true
Why? They act on two different objects.
 
Last edited by a moderator:
  • #14
well I am thinking that there's a force acting on the hammer just as there's a force acting on the nail
 
  • #15
eulerddx4 said:
well I am thinking that there's a force acting on the hammer just as there's a force acting on the nail
Yes, and therefore they both are accelerated in opposite directions.
 
Last edited by a moderator:
  • #16
i just don't get why there's acceleration if the force of the hammer is the same and opposite the force on the nail. To me it seems like the force should just go away if that's the case
 
  • #17
says to self: "why is this so hard for me to understand"
 
  • #18
Pengwuino said:
You exert the same force that the coffee mug exerts on you. However, since you have a far far greater mass, by F = ma, the coffee mug will provide a very small (and to you, unnoticeable) acceleration back on you compared to the acceleration you provide to the mug.

but why should i be providing acceleration if the force is canceled. Doesn't there have to be some outside force. When i push on a car the car pushes on me. The only way for me to move the car / accelerate it is if i push on the ground really hard.
 
  • #19
eulerddx4 said:
i just don't get why there's acceleration if the force of the hammer is the same and opposite the force on the nail. To me it seems like the force should just go away if that's the case
They don't cancel, because they act on different objects.
 
Last edited by a moderator:
  • #20
eulerddx4 said:
When i push on a car the car pushes on me. The only way for me to move the car / accelerate it is if i push on the ground really hard.
This is not correct. If the ground were frictionless so that neither you nor the car could push on the ground then when you push on the car you would accelerate in one direction and the car would accelerate in the opposite direction.
 
Last edited by a moderator:
  • #21
so then how does someone push something like a car or a box
 
  • #22
last year when i took physics my physics teacher said if you have a game of tug of war, the only way to win is if one side pushes harder on the ground because when you pull on the other side the other side pulls with the same exact magnitude in the opposite direction. Is my physics teacher completely misinformed?
 
  • #23
i don't understand why there is any acceleration at all. In my mind there has to be a net force for there to be acceleration but i see no net force here
 
  • #24
eulerddx4 said:
i see no net force here
Draw the free-body diagrams for the problem and label all of the forces acting on each body. Then you should clearly see net forces.
 
Last edited by a moderator:
  • #25
I think eulerddx4's confusion is pretty understandable, and most of you really aren't helping by acting like the answer to his problem is so obvious.

So I'm going to try to actually help him out, please correct me if I make any mistakes...The term "net force" has to do with what you make your system. Here your system is the hammer and the nail, and there are external forces on the system: your hand, gravity, and contact forces in the wood you're nailing. These EXTERNAL forces are transmitted through the hammer and nail, and cause acceleration. The force between the hammer and the nail is an internal force, and causes no acceleration of the system as a whole. So if there was truly no net force - no external force - that is the hammer was flying at the nail in space - it becomes a question of conservation of momentum. The hammer knocks into the nail, giving it a certain amount of momentum. In this case, the center of mass of the system remains at a constant velocity (no force), but remember, your system is NOT just the nail, its the hammer AND the nail. If you're only looking at the nail, you have an external force - the impulse given by the hammer.

But in the case of "driving" the nail, what's really happening is your hand is applying an external force to the system - the force between the nail and hammer does "cancel out" in the sense that it does not affect the motion of the center of mass of your system.Does this help dude?
 
Last edited by a moderator:
  • #26
Yes thank you for taking the time to answer the question! No one would give me a legit answer haha
 
  • #27
The carpenter who hits his thumb with the hammer will tell you that no net force hurts like a sonuva...
 
  • #28
eulerddx4 said:
so shouldn't the forces on the nail and hammer just go away if Newtons law is true

Perhaps it's mostly a matter of precise formulation. According to Newton's laws forces do not "just go away"!

If you accelerate a cop of coffee (or a nail), the force that you need to accelerate it is determined by the force with which it (and friction with the table or wall) withstands that acceleration.
So, taking Newton's second and third law together, that tells you that the force with which you must push to accelerate the cup is equal to the force with which the cup pushes back against your hand due to its acceleration - it's the same force, between your hand the cup.

And as others already mentioned, in principle when you accelerate the cup away by pushing against it, also you and the Earth accelerate in the opposite direction due to the cup that is pushing back against you.

Does that help?

Harald
 
Last edited by a moderator:
  • #29
Ok So now I understand! The forces don't cancel out. They are real, they exist and they act on both the nail and the hammer/ person assembly. when you hit the nail you get pushed back but you don't get moved back very much because you are standing on the ground and you have significant mass so gravity is exerting a relatively large force keeping you in place.

What really made the idea click is a gun. When you shoot a gun the bolt exerts a force on the bullet and the bullet exerts a force back. The gun shoots the force doesn't just go away because there is a force exerted back. This is crazy. Rather you actually feel the force back its the recoil. I really had to come up with an example like this to make it click so i would suggest using something like this example in the future because I was really stumped at first. Anyway I'm glad I understand now! Thanks to the people that pointed me in the right direction and I'm also kind of happy that no one came out and explained the whole thing. I feel like I solved the problem myself in a way by coming up with this example.
 
  • #30
eulerddx4 said:
i don't understand why there is any acceleration at all. In my mind there has to be a net force for there to be acceleration but i see no net force here
I see why you are confused. If we have a law that says every action is met by an equal and opposite reaction it makes it sound like every force is automatically in equilibrium and nothing can ever move anything else. Hehe.

What it actually means, though, is that once object A has moved object B, object A no longer has all the force it had before it moved object B. It now has less force. How much less? Exactly that amount it took to move object B. Object A loses an amount of force exactly equal to the amount it took to accelerate object B. That's what "equal and opposite" means. Exactly what is added to B must be subtracted from A.

It's baby simple: If I give you $5, you experience a change of +$5 and I experience a change of -$5. The changes are equal and opposite! The action and reaction are automatically equal and opposite!

It does not mean: If I give you +$5 you must give me +$5. That's not what Newton III is saying at all. Newton III is saying if I give you exactly $5 I automatically suffer a loss of exactly $5. Equal, but OPPOSITE. You: +$5, me: -$5.

Newton's Third Law guarantees that if you take x amount of force from me, I lose exactly x amount of force. If I lose x amount of momentum to you, you gain exactly x amount of momentum. No more, no less. It is the foundation of all the conservation laws. You experience a net force of +$5, so to speak, and I simultaneously experience a net force of -$5.
 
Last edited by a moderator:

Similar threads

High School Hold a hammer at the top vs bottom of the handle
  • · Replies 7 ·
Replies
7
Views
2K
High School Resistive force that a nail will have to overcome to penetrate a piece of wood
  • · Replies 4 ·
Replies
4
Views
4K
High School Hammer & a nail -- Calculating penetration depth into wood for first strike
  • · Replies 9 ·
Replies
9
Views
5K
Undergrad Why is it harder to hammer in a nail the deeper it gets?
  • · Replies 4 ·
Replies
4
Views
2K
High School Wanting to do some impact force experiments
  • · Replies 15 ·
Replies
15
Views
2K
High School Why Is an Object's Weight Equal to the Force of Gravity It Experiences?
  • · Replies 51 ·
2
Replies
51
Views
5K
High School Action-reaction of a sailing boat and wind
  • · Replies 8 ·
Replies
8
Views
2K
Graduate Impact force of Rigid objects and no displacement
  • · Replies 61 ·
3
Replies
61
Views
12K
High School Frying Pans vs. Hammers: Which Drives Nails Farther with Same Mass and Velocity?
  • · Replies 20 ·
Replies
20
Views
4K
High School Physics teacher demonstrated Newton's 3rd Law
  • · Replies 14 ·
Replies
14
Views
14K