Question

FIGURE CP7.56 shows a 200 g hamster sitting on an 800 g wedge- shaped block. The block, in turn, rests on a spring scale. An extra-fine lubricating oil having mu_s = mu_k = 0 is sprayed on the op surface of the block, causing the hamster to slide down. Friction between the block and the scale is large enough that the block does not slip on the scale. What does the scale read, in grams, as the hamster slides down?

Answer 1

This question got me like wuut... common sense or at least my first instinct is to think that the scale will read 1000 g lol but I know it's more complex than that

Answer 2

ayye @agent0smith :D

Answer 3

Intuition always helps in getting started in problems like these

Answer 4

Maybe start with a FBD of the hamster

Answer 5

ok :)

Answer 6

Dude you gotta draw FBDs in proper orientation.

Answer 7

I thought I did, it's just that I made my incline be the x axis hehe

Answer 8

No i mean, if the object is on a slope, rotate your axes to reflect that.



mg should never point anywhere but straight down.

Answer 9

Let's also look at your question quick.

Why wouldn't the scale simply show 1000g ?

Answer 10

Ganeshie has a good point. Imagine if the slope were steeper, like an extreme 90 degrees - at that point, is ANY of the hamsters weight pressing on the block?

Answer 11

it'll fall off lol

Answer 12

like no not at all

Answer 13

Well yeah lol but what i mean is, the steeper the angle, the less the hamsters weight acts on the slope.

Answer 14

ohh I see

Answer 15

why tho

Answer 16

Key thing to notice here is that the hamster is accelerating

Answer 17

Because remember that the force that the block "feels" from the hamster - that is the normal force of the block on the hamster.

Answer 18

The component of mg that is acting parallel to the ramp - that force does not act directly on the block

Answer 19

It may help to recall the elevator problem if you're familiar with it
http://astarmathsandphysics.com/ib-physics/mechanics/weightlessness-html-m24728bcf.gif

Answer 20

Btw
all my replies so far are an attempt to address your second question
`This question got me like wuut... common sense or at least my first instinct is to think that the scale will read 1000 g lol but I know it's more complex than that`

Once you see why common sense doesn't work here, you can use F=ma in your FBD to solve the main problem.

Answer 21

Ok so the angle of the incline is pretty significant.

Answer 22

This is a pretty tricky question, though. I think that we can ignore the horizontal force that the block exerts on the scale. I think the force we need from the hamster, will be a component of the normal force.

Answer 23

oh ok

Answer 24

so like we only need to focus on the vertical forces

Answer 25

I think the force we need is that one. The horizontal component of the normal should be equivalent to the horizontal force the block exerts on the scale. Which I think we can ignore.

Finding that component isn't very difficult... I'm just not 100% sure this is correct. @ganeshie8

Answer 26

ohhhh ok I think we use the y component mgcos40