What constant force must Bob exert on the rock to throw it with this speed?

[aligass2004]

Homework Statement

Bob, who has a mass of 85 kg, can throw a 450g rock with a speed of 30m/s. The distance through which his hand moves as he accelerates the rock forward from rest until he releases it is 1.8m. a.) What constant force must Bob exert on the rock to throw it with this speed? b.) If Bob is standing on frictionless ice, what is his recoil speed after releasing the rock?

Homework Equations

F=ma

The Attempt at a Solution

I have no idea how to start this problem. There are way too many elements of the problem for me to know how to start.

 

[learningphysics]

Think of it like a kinematics problem... find the acceleration, then you can find the force.

 

[aligass2004]

Ok. I tried using Vxf^2 = Vxi^2 + 2ax(delta x) to find the acceleration, but that didn't work.

 

[learningphysics]

Ok. I tried using Vxf^2 = Vxi^2 + 2ax(delta x) to find the acceleration, but that didn't work.

What number did you get for acceleration?

 

[aligass2004]

-250 m/s^2

 

[learningphysics]

-250 m/s^2

You shouldn't have the minus sign...

 

[aligass2004]

Ok, I got that one, 112.5N. Now how do I start part b?

 

[learningphysics]

Ok, I got that one, 112.5N. Now how do I start part b?

Use conservation of momentum.

 

[aligass2004]

We haven't gone over momentum yet.

 

[learningphysics]

We haven't gone over momentum yet.

Oh... you can also do it this way. What is the force acting on Bob (use Newton's third law). Then you can find Bob's acceleration... Also find the time over which Bob has contact with the rock (you can get this now easily, because you have acceleration of the rock)

Using bob's acceleration and time, you can get bob's velocity at the end of the time period...

 

[aligass2004]

Using Newton's third law would make the force acting on Bob -112.5N. So using that I found the acceleration to be -1.324m/s^2. Then I used Vxf = Vxi + ax (delta time). The answer I got was 30.079, which was wrong.

 

[learningphysics]

Using Newton's third law would make the force acting on Bob -112.5N. So using that I found the acceleration to be -1.324m/s^2. Then I used Vxf = Vxi + ax (delta time). The answer I got was 30.079, which was wrong.

How are you getting 30.079? Vxi = 0. ax = -1.324. delta t = 30/250=0.12s.

 

[aligass2004]

I was using the wrong time. I didn't think to use acceleration = change in velocity/time. I got -0.159, which is still wrong.

 

[learningphysics]

I was using the wrong time. I didn't think to use acceleration = change in velocity/time. I got -0.159, which is still wrong.

Did you try 0.159?

 

[aligass2004]

The positive answer was right, which is good because that was my last guess at the problem.

 

[learningphysics]

The positive answer was right, which is good because that was my last guess at the problem.

glad you got it.