Force Problem -- projectile launcher fires from a platform on a frictionless platform

  • #1
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Homework Statement


A projectile launcher fires a projectile horizontally from a platform which rests on a flat, icy, frictionless surface. Just after the projectile is fired and while it is moving through the launcher, the projectile has an accelleration of 25m/s^2. At the same time, the launcher has an accelleration of 0.25m/s^2. The mass of the projectile is 0.20kg. Calculate the mass of the launcher.

Given: acceleration of projectile: 25m/s^2[forwards]. Accelleration of launcher: 0.25m/s^2[backwards]. mass of projectile- 0.20kg

Homework Equations



Fnet=ma

The Attempt at a Solution


I have no clue how to solve this. I checked the solution, and according to that, you have to solve for Fnet of the projectile using Fnet=ma, and the net force of the launcher is equal in magnitude but opposite in direction to that of the projectile. Then you use Fnet=ma for the launcher and solve for mass. I don't understand why the net force of the launcher is equal in magnitude but opposite in direction to that of the launcher. Can someone explain?
 

Answers and Replies

  • #2
haruspex
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I don't understand why the net force of the launcher is equal in magnitude but opposite in direction to that of the launcher.
Newton... action and reaction... any memories stirring?
 
  • #3
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As haruspex has said, you need to know about Newton's third law
 
  • #4
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As haruspex has said, you need to know about Newton's third law
It just says that any action force will produce a reaction force, what does that have to do with net force? The net force inolves gravity in this case, and since the masses of these 2 obects are different, how can you assume that their net forces are equal in magnitude?
 
  • #5
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It just says that any action force will produce a reaction force, what does that have to do with net force? The net force inolves gravity in this case, and since the masses of these 2 obects are different, how can you assume that their net forces are equal in magnitude?
right, my mistake,it must be conservation of momentum
 
  • #6
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right, my mistake,it must be conservation of momentum
We haven't learned that yet, so I don't know how we would be expected to solve this/
 
  • #7
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i will try figuring a solution
 
  • #8
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No, i was right, we must use newton's third law, the mass is different but acceleration is different too
 
  • #9
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No, i was right, we must use newton's third law, the mass is different but acceleration is different too
Can you please explain further?
 
  • #10
haruspex
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any action force will produce a reaction force
Yes. That is, the force body A exerts on body B is equal and opposite to the force body B exerts on body A.
what does that have to do with net force?
The net force on a body is from summing all forces acting on the body. Any reaction on it from another body contributes.
The net force inolves gravity in this case
There is a gravitational force, but there is also a normal force, which is here guaranteed to be exactly what is need to prevent it falling through the ice, and no more. Thus, can assume they will cancel.
What does that leave for the net force on the platform?
 
  • #11
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The force that makes the projectile move also affects on the launcher
 
  • #12
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Yes. That is, the force body A exerts on body B is equal and opposite to the force body B exerts on body A.

The net force on a body is from summing all forces acting on the body. Any reaction on it from another body contributes.

There is a gravitational force, but there is also a normal force, which is here guaranteed to be exactly what is need to prevent it falling through the ice, and no more. Thus, can assume they will cancel.
What does that leave for the net force on the platform?


Are the projectile and the launcher in contact? I don't think so. What normal forces ?
 
  • #13
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The force that makes the projectile move also affects on the launcher

Is there a normal force on each object?
 
  • #14
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Are the projectile and the launcher in contact? I don't think so. What normal forces ?
It is the force that perpendiculars to the ground
 
  • #15
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It is the force that perpendiculars to the ground
But they're not both in contact with the ground
 
  • #16
haruspex
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Are the projectile and the launcher in contact?
By what magic does the projectile move?
What normal forces ?
By what magic does the platform avoid falling through the ice?
 
  • #17
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By what magic does the projectile move?

By what magic does the platform avoid falling through the ice?
I think god can do that but not human and especially that we are under the influence of physics laws
 
  • #18
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By what magic does the projectile move?

By what magic does the platform avoid falling through the ice?

Could the inner diameter of the launcher not be wide enough so the projectile is not in contact with the launcher? In that case, it would not be in contact with any surface- no normal force
 
  • #19
haruspex
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Could the inner diameter of the launcher not be wide enough so the projectile is not in contact with the launcher? In that case, it would not be in contact with any surface- no normal force
Action and reaction does not necessarily consist of contact forces. What force made the projectile move? What was the launcher's role in that?
If the projectile is not in contact with the launcher at all while moving through the muzzle, what is stopping it from falling?
 
  • #20
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Action and reaction does not necessarily consist of contact forces. What force made the projectile move? What was the launcher's role in that?
If the projectile is not in contact with the launcher at all while moving through the muzzle, what is stopping it from falling?
The launcher exerted a force on the projectile, making it move. Does it have to be in contact with the launcher in order to not fall or could the force exerted on it horizontally be large enough to allow the projectile to only move horizontally(at least, initally)?
 
  • #21
haruspex
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The launcher exerted a force on the projectile, making it move.
Whatever force it exerted on the projectile, and however it exerted it, the projectile exerted an equal and opposite force on the launcher at each instant. The law of conservation of momentum follows from his.
Does it have to be in contact with the launcher in order to not fall or could the force exerted on it horizontally be large enough to allow the projectile to only move horizontally(at least, initally)?
No amount of horizontal force will affect the vertical motion.
 
  • #22
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Whatever force it exerted on the projectile, and however it exerted it, the projectile exerted an equal and opposite force on the launcher at each instant. The law of conservation of momentum follows from his.

No amount of horizontal force will affect the vertical motion.
Ok, so they must've been in contact
 
  • #23
haruspex
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Ok, so they must've been in contact
OK, so where are we at in resolving this thread?
 
  • #24
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OK, so where are we at in resolving this thread?


This was from a few days ago, but if I remember correctly, I was able to solve the problem.
 
  • #25
haruspex
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This was from a few days ago, but if I remember correctly, I was able to solve the problem.
Ok, I'll mark it solved.
 

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