How to Solve a Physics Problem Involving Newton's Laws and Weight Measurements

In summary, the conversation is about a physics problem involving two weights on a rope hanging off a table, with a scale on the rope. The question is what the scale reads, and the discussion revolves around the forces acting on the system, the importance of a clear diagram, and the possibility of acceleration. Ultimately, it is determined that the scale will read the combined weight of the two weights at the moment, assuming there are no other forces like friction.
  • #1
Number Lover
8
0
Hey, new member here, and I'm stumped on a physics problem.
Two weights are tied onto each end of a piece of rope and set on a table. The rope has a scale on it. The two weights are hanging over the edge of the table - one has Fg of 100 N, the other has Fg of 50 N. What does the scale read?
My physics teacher presented this problem the other day, and I've been obsessed with it ever since... but I don't quite know how to achieve an answer. He gave us the hints that the answer is not 50, 75, or 100, but it's still in that range (50 < x < 100). If anybody could just put me on the right track to solving this problem, I'd be much obliged.
 
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  • #2
If the system is symmetric, except for the amount of weight on each side,
my hint would be that there are two forces which keep the system from accelerating sideways. The two forces are probably not equal ... .
 
  • #3
I'm not sure what you mean by the rope having a scale "on it."
 
  • #4
cscott said:
I'm not sure what you mean by the rope having a scale "on it."

Picture: http://img228.imageshack.us/img228/2940/physicsproblem0yj.jpg
 
Last edited by a moderator:
  • #5
NumberLover,

Your diagram shows 100 N applied upward to the bottom of the left-hand piece of metal ... WHAT?

You're not likely to answer interesting Force questions with careless diagrams.

The whole thing WOULD accelerate leftward, not rightward ...
except for 2 important Forces which keep the whole thing still ...

Unless this is an unrealistic problem where it DOES accelerate leftward .
 
  • #6
lightgrav said:
NumberLover,

Your diagram shows 100 N applied upward to the bottom of the left-hand piece of metal ... WHAT?

You're not likely to answer interesting Force questions with careless diagrams.

The whole thing WOULD accelerate leftward, not rightward ...
except for 2 important Forces which keep the whole thing still ...

Unless this is an unrealistic problem where it DOES accelerate leftward .

I'm sure those arrows are only pointing towards the blocks giving their weight. The diagram was to show the general layout and I doubt was intented to be a FBD.
 
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  • #7
The point is that a neat diagram of applied Forces is the FIRST thing to do,
(while you're still reading the question) with those Forces labeled by source.

(Setting the Sum of Forces equal to ma is next.)
 
  • #8
cscott said:
I'm sure those arrows are only pointing towards the blocks giving their weight. The diagram was to show the general layout and I doubt was intented to be a FBD.

Yeah that was it, but I'll be more careful when doing drawings in the future. The weights are hanging off the table, so I believe the only Force affecting each individual weight would be gravity, yes?
 
  • #9
Doesn't the string contact each cylinder?

Does the string contact the table?

Aren't there any upward Forces?

Does the whole thing accelerate leftward?
 
  • #10
lightgrav said:
Doesn't the string contact each cylinder?

Does the string contact the table?

Aren't there any upward Forces?

Does the whole thing accelerate leftward?

The ends of the string each have a weight attached to them, the string is on the table. I don't believe there are any upward Forces, and based on the information given, the whole thing will probably accelerate to the left.
 
  • #11
I'm not fully comprehending how acceleration can aid in solving this problem, though my teacher hinted that it might.
 
  • #12
If there is not other forces like friction, the rope will accelerate and keep moving to the heavier side. And ultimately will hit the ground. That's it.
 
  • #13
myxinjie said:
If there is not other forces like friction, the rope will accelerate and keep moving to the heavier side. And ultimately will hit the ground. That's it.

But the question is asking what the scale reads at this moment in time.

(I bet that wasn't too clear either - I apologize. I've been working probabilities of poker in my head all night, so I'm a little exhausted.)
 
  • #14
OK, wait, I think I've got it now. Thanks.
 

1. What are Newton's Three Laws of Motion?

Newton's Three Laws of Motion are a set of principles that describe the behavior of objects in motion. The first law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. The second law states that the force applied to an object is directly proportional to its mass and acceleration. The third law states that for every action, there is an equal and opposite reaction.

2. How are Newton's Laws used in everyday life?

Newton's Laws are used in various applications in everyday life. For example, the first law is applied in seatbelts, which keep passengers in a car moving at a constant speed in case of a sudden stop. The second law is used in sports, where the force from a player's body is transferred to an object, such as a ball, to make it move. The third law is applied in rocket propulsion, where the force from the gas expelled out of the rocket pushes it upward.

3. What is the difference between mass and weight in relation to Newton's Laws?

Mass and weight are two different concepts in Newton's Laws. Mass is the amount of matter an object contains, while weight is the force of gravity acting on an object. According to Newton's second law, an object with a greater mass will require more force to accelerate, while weight remains constant regardless of location.

4. Can Newton's Laws be applied to non-inertial reference frames?

Yes, Newton's Laws can be applied to non-inertial reference frames, but they may require additional forces to account for the acceleration of the reference frame. This is known as the fictitious force or inertial force. An example of this is the centrifugal force experienced by objects in a rotating reference frame.

5. How do Newton's Laws relate to other laws of physics?

Newton's Laws of Motion are fundamental principles that form the basis of classical mechanics. They are closely related to other laws of physics, such as the law of conservation of energy and the law of conservation of momentum. These laws work together to explain the behavior of objects in motion and are essential in many areas of physics, such as dynamics, kinematics, and fluid mechanics.

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