Momentum-impulse vs. rectilinear motion equations

In summary, The conversation is about solving a pulley problem using different approaches. The person tried using the rectilinear motion equations with a constant acceleration, but the book's solution used the momentum-impulse equations and got a different answer. The other person points out that the first approach is incorrect because the objects are not freely falling and suggests using the sum of forces to find the acceleration. The first person realizes their mistake and agrees to use the correct approach.
  • #1
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Here we are again...We have apully system as seen on page 1 of the attachment. I tried using the rectilinear motion equations, assuming a=constant, but the book's solution used the momentum-impulse equations and got a different answer.

I don't understand why my approach is incorrect... Thanks.
 

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  • #2
Nothing wrong with using kinematics with constant acceleration. But how did you solve for the acceleration?
 
  • #3
I just assumed the acceleration was gravity...I'm guessing there's something wrong with that assumption...but I don't know what...
 
  • #4
Its not gravity because its not free falling. I can't believe I made that mistake. I would have to use Sum of forces=ma and find the acceleration if i wanted to use kinematics, right?
 
  • #5
The "acceleration due to gravity" applies to a freely falling object--where the only force acting is gravity. But these objects are attached to ropes and pulleys--they are not freely falling! Solve for the acceleration like in any other pulley problem.
 
  • #6
cipotilla said:
Its not gravity because its not free falling. I can't believe I made that mistake. I would have to use Sum of forces=ma and find the acceleration if i wanted to use kinematics, right?
Exactly. (I knew you'd snap out of it. :smile: )
 

What is momentum?

Momentum is a measure of an object's motion, determined by multiplying its mass and velocity. It is a vector quantity, meaning it has both magnitude and direction.

What is impulse?

Impulse is the change in momentum of an object over a period of time. It is calculated by multiplying the force applied to an object by the time it is applied.

What is the difference between momentum and impulse?

Momentum is a measure of an object's motion at a specific moment, while impulse is the change in momentum over a period of time. Momentum is a vector quantity, while impulse is a scalar quantity.

How do momentum-impulse equations differ from rectilinear motion equations?

Momentum-impulse equations take into account the change in momentum of an object over a period of time, while rectilinear motion equations only consider an object's position, velocity, and acceleration at a specific point in time. Additionally, momentum-impulse equations involve the force applied to an object, while rectilinear motion equations do not.

What are some real-world applications of momentum-impulse equations?

Momentum-impulse equations are used in various fields such as sports, engineering, and transportation. For example, they can be used to analyze the impact force of a collision between two vehicles, or to calculate the required force and time for a rocket to launch into space.

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