Solving Impulse & Momentum for Time Needed for Block A to Reach 2 m/s

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In summary, the homework statement states that if blocks are released from rest, determine the time required for block A to attain a speed of 2 m/s. If you use impulse and momentum, then you can find t using the initial conditions.
  • #1
joemama69
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Homework Statement



Note Diagram.

If they are released from rest, determine the time required for block A to attain a speed of 2 m/s. Neglect the mass of the ropes


Homework Equations





The Attempt at a Solution



I = mk2 = (30)(.252) = 1.875 kg m2

X Direction

mv + [tex]\int[/tex] Fdt = mv2

0 + Oxt = 0

Y Direction

mv + [tex]\int[/tex] Fdt = mv2

0 + Oyt - 30(9.81)t = 0

Oy = 294.3

Moments

Iw1 + [tex]\int[/tex]MOdt = Iw2

0 + 25(.3) - 10(.18) = 1.875w2

w = [tex]\sqrt{3.04}[/tex]

How do i relate this to velocity and time. I can use v = wr, but how can i solve for t when v = 2 m/s

2 = .3w, therefore i need to find t when w = 2/.3
 

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  • #2
I'm having trouble understanding the image. Can you get a better one or explain the image further?
Anyway, it seems you are making things much more complicated than they have to be.
Draw a FBD, and apply Newton's second law in linear and angular forms to each mass. The rest should just be kinematics.
 
  • #3
The spool has two radiuses. one is .18m and one is .3m. they start from rest. when the blocks are released, block A weighs more so it will fall, thus lifting block B. When will block a reach a speed of 2 m/s.
 
  • #4
But you i don't think i needed most of what i posted first. Heres what I am at

I = mk2 = 30(.25)2 = 1.875

25(.3) - 10(.18) = 1.875w2

w = [tex]\sqrt{3.04}[/tex]

w = v/r

[tex]\sqrt{3.04}[/tex](.3) = v

how do i get a t in there
 
  • #5
I think that I would take a different approach. You know that

[tex]\sum M =I\alpha[/tex]

and there is a relationship between alpha and the acceleration of the blocks.

So you can write the sum of the moments in terms of a

Since [itex]a=\frac{dv}{dt}[/itex] you should be able to find t using the initial conditions.

Edit: Was there a reason you thought that it was an Impulse & Momentum problem? I am only wondering in case I overlooked something.
 
  • #6
okok i think i got it

I = 1.875

Iw2 + + [tex]\sum\int[/tex]Mdt = Iw2

0 + [tex]\int[/tex](25(.3) - 10(.18)dt = 1.875w2

5.7t = 1.875w2 therefore w = [tex]\sqrt{3.04t}[/tex]

w = v/r

[tex]\sqrt{3.04t}[/tex](.3) = v = 2

3.04t = 4

t = 1.32 seconds

Is this right
 
  • #7
I thought it was impulse and momentum because the chapter is called

Planar Kinetics of a Rigid Body: Impulse and Momentum
Section 19.2 Principle of Impulse and Momentum

Im assuming there is more than one way to solve it, I am just suppose to use impulse and momentum
 
  • #8
Edit: I can multiply :smile: I think you're good!

You can always do it the other way as a check.

You have all the numbers necessary.
 
Last edited:
  • #9
but u do agree with my answer based on my method
 

Related to Solving Impulse & Momentum for Time Needed for Block A to Reach 2 m/s

1. How do you calculate impulse?

Impulse is calculated by multiplying the force applied to an object by the change in time it takes for the object to come to a stop.

2. What is momentum?

Momentum is the product of an object's mass and velocity. It is a measure of an object's motion and is in the same direction as its velocity.

3. How can momentum be used to solve for the time needed for a block to reach a certain velocity?

Using the equation for momentum (p=mv), we can rearrange it to solve for time (t=p/mv). By plugging in the known values for momentum and mass, we can solve for time needed for the block to reach a certain velocity.

4. Is the time needed for a block to reach a certain velocity affected by the mass of the block?

Yes, the time needed for a block to reach a certain velocity is affected by the mass of the block. The greater the mass, the longer it will take for the block to reach the desired velocity.

5. What are some real-world applications of solving impulse and momentum for time needed for a block to reach a certain velocity?

Some real-world applications include calculating the time needed for a car to come to a stop after applying the brakes, determining the time needed for a rocket to reach a certain speed, and calculating the time needed for a ball to reach a certain height after being thrown into the air.

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