Ok you guys helped with the first one lets try another

[Finns14]

A projectile of mass 0.676 kg is shot straight up with an initial speed of 18.0 m/s. (a) How high would it go if there were no air resistance? (b) If the projectile rises to a maximum height of only 7.04 m, determine the magnitude of the average force due to air resistance.

Here is the problem

 

[Finns14]

No takers I have another problem that you could help me out with too

 

[BigJ]

Come on. At least put a little effort into the first part.

 

[Finns14]

Well the first part I know how to do its the second part I have a problem with, the first part you know the inital velocity and you set the final velocity to 0 and the acceleration is g

 

[freakflag]

This is basically a gravity problem, correct?

 

[Finns14]

Originally posted by: freakflag
This is basically a gravity problem, correct?

The first part is I'm not sure how to do the second part

 

[SaturnX]

For the second part, you've got the distance, mass, initial velocity and you know final velocity is 0, you need to find the acceleration (it will no longer be g, because of the added force of resistance), once you have the accel, apply Netwon's law:

FTotal=m(g + a), and you've got m, and found g + a from above. So just calculate the ma portion and that will be force due to the resistance.

I get an answer of ~9 N

That should be answer, then again I haven't really done kinematic problems in about 2 years, so I may be a little off.



--Mark

 

[heelside23]

first part is kinetic energy transfered into potential energy, solve for height

 

[marketsons1985]

Use Energy conservation.

Find the work done by Air resistance, and use W=FD to find average force

 

[Finns14]

Originally posted by: heelside23
first part is kinetic energy transfered into potential energy, solve for height

I used Vf^2=Vi^2 +2ax

isn't that easier?

 

[marketsons1985]

Originally posted by: Finns14

Originally posted by: heelside23
first part is kinetic energy transfered into potential energy, solve for height

I used Vf^2=Vi^2 +2ax

isn't that easier?

Both will give you the same thing.

 

[BigJ]

Originally posted by: Finns14

Originally posted by: heelside23
first part is kinetic energy transfered into potential energy, solve for height

I used Vf^2=Vi^2 +2ax

isn't that easier?

From your previous posts, your professor is trying to teach you about KE, PE, and energy conservation.

You can do whatever the hell you want on the HW, but on the test he may force you to solve the problem using only the equations related to those topics.

 

[Finns14]

Nah tests are muliple choice but I am still confused about the second part

 

[heelside23]

.5v^2=gh
162=9.8h
h=16.5meters
I haven't studied this stuff in years, but it seems simple enough

 

[SaturnX]

Ok, for the second part, think of it this way:

When the project is shot upwards, there are two doward forces acting on the body:

1. Gravity
2. Air Resistance

Draw yourself a free body diagram if you need to see it clearer, but either way, the values of these forces are going to be the following:

1. Gravity: Fg = mg
2. Air Resistance: Fr = mA

Where g is the acceleration due to gravity, and A is the acceleration of the object due to air resistance.

Thus the sum of the forces will yield:

Ftotal = Fg + Fr = mg + ma = m(g + A)

Using the following kinematic equation:

Vf^2=Vi^2 +2ax

We know:

Vf = 0
Vi = 18
x = 7.04
a = unknown (the TOTAL accleration of the projectile)

rearrange and solve for a

this value of a is EQUAL to (g + A) above

so you can get A = a - g

Therefore, from the sum of forces

Fr (air) = mA

Which yields ~9N

--Mark

 

[Yossarian]

ah but what's the acceleration at max height, that's the real question.

 

[BigJ]

Originally posted by: Yossarian
ah but what's the acceleration at max height, that's the real question.

Just ask the poor mouse who was strapped to the projectile

 

[heelside23]

9.8m/s^2 right?

 

[BigJelly]

For the second part you just find the energy dissipated (find the initial energy substract the potential energy at the top and that equals the dissipated energy). Then since E=F*d. So average force is the energy dissipated divided by the max height.

Then again I took physics 4 years ago.