How Do You Calculate Forces Acting on a Hamster and a Wedge on a Scale?

[bcjochim07]

Force Body Diagram Hamster-Please Help!

Homework Statement

A 200 g hamster sits on an 800 g wedge shaped block. The block, in turn rests on a spring scale.

a) Initially static friction is sufficient to keep the hamster from moving. In this case the hamster and the block are effectively a single 1000g mass and the scale should read 9.8 N. Show that this is the case by treating the hamster and the block as separate objects and analyzing the forces.

Homework Equations

The Attempt at a Solution

I am reviewing, and I know I did this problem correctly before, but now it's not working for me. Could someone please tell me what I am doing wrong

Hamster: I drew a force body diagram, tilting the x & y axes

Sum of forces on y = Force of Block on hamster - mgcos40=0

Force of block on hamster = (.200kg)(9.80)(cos40)

Block: For the block I didn't tilt the coordinates

Sum of forces on y= normal force - Force of hamster on block in y direction - Mg
= n - (.200kg)(9.80)(cos40)(cos40) - (.800kg)(9.80)
n= -8.99 N which is the answer for the second part of the question, when the static friction goes away.

 

[tiny-tim]

Hi bcjochim07!

You're not asked for the separate friction force at this stage, so just calculate R, the total reaction force (normal + friction).

That makes only two forces on the hamster … R and mg.

So R = … ?

Then use minus R for the block.

 

[bcjochim07]

Ok I got it, I forgot that friction has an equal opposite force