How Much Force to Lower Two Boxes Down a Ramp at Constant Speed?

[truthlies217]

Homework Statement

can someone help me solve this problem .. You are lowering two boxes, one on top of the other, down the ramp shown in Figure 5.53 by pulling on a rope parallel to the surface of the ramp. Both boxes move together at a constant speed of 15.0 cm/s. The coefficient of kinetic friction between the ramp and the lower box is 0.444, and the coefficient of static friction between the two boxes is 0.800. (a) What force do you need to exert to accomplish this? (b) What are the magnitude and direction of the friction force on the upper box?

Figure here > http://www.flickr.com/photos/53947633@N06/4988606991/

Homework Equations

Sum of F = m a

The Attempt at a Solution

i just sum the weight of the two boxes to find the force for part one and i still need the acceleration so how can i find it ?

 

[tiny-tim]

welcome to pf!

hi truthlies217! welcome to pf!

i just sum the weight of the two boxes to find the force for part one …

yup!

and i still need the acceleration so how can i find it ?

hmm, let's see …

"Both boxes move together at a constant speed of 15.0 cm/s." …

so the acceleration is … ? ​

 

[truthlies217]

opss .. ><

Ok then i found F= 346.05 N for part a and fk= 627.2 for part B the direction is 207.2 degree .. I'm not sure is it right ?

 

[tiny-tim]

hi truthlies217!

if you want us to check your results, you need to show us your full calculations

 

[truthlies217]

part A :

mg= 32+48= 80 kg*9.8 = 784N
the angel = shift tan ( 2.50/4.75) = 27.7 degree
/x =784+Cos27.7 + F - Fk = 0.0
/y =784+Sin27.7+n=0.0
so n=784 N
F=784+Cos27.7-(784*0.444) = 346.05 N

part B :

fk=un = (0.800*784)=627.2 N
direction = 27.7 + 180 = 207.7 degree ..

 

[tiny-tim]

/x =784+Cos27.7 + F - Fk = 0.0
/y =784+Sin27.7+n=0.0
so n=784 N

(please don't use + to mean "times" )

no, your cos and sin are the wrong way round (you should always check by asking whether it gives the right result if the angle is zero)

and n = 784 N doesn't follow from the previous line anyway

 

[truthlies217]

i just found my mistakes .. thanks for the help :)