You are Here: Home >< Physics

# Challenging question - forces

Announcements Posted on
TSR's new app is coming! Sign up here to try it first >> 17-10-2016
1. (Original post by mik1a)
Ah. If you have different container shapes then different volumes then different masses then different weights therefore different forces.

The pressure of the water according to rho*g*h is a hydrostatic pressure of the water exerted on the inside of the container. That's an internal force that doesn't affect the total weight.

The pressure of the container on the scale is F/A, where F is the weight. Since they have the same area at the base, the pressure is higher for the cubic container since the weight is higher.
perfect, that makes sense now. I actually used the wrong relationship there.

going back to the interview question now to finish everything up. So it seems they wanted us to assume the downward force is same, but even when they are same, the left one will still go down since force from water will act on sides of the container and have a little extra vertical force for the right container. hence forces are unbalanced. correct?
2. I really don't understand this. There is a force balance on every bit of container wall, balancing the water pressure inside with the air pressure outside and, where these are different, the container wall will slightly deform outwards and that deformation will generate a restoring force that balances the pressure difference.

I don't see how that affects the scales though.
3. (Original post by mik1a)
I really don't understand this. There is a force balance on every bit of container wall, balancing the water pressure inside with the air pressure outside and, where these are different, the container wall will slightly deform outwards and that deformation will generate a restoring force that balances the pressure difference.

I don't see how that affects the scales though.
same here. It's weird

but this force is the one responsible maybe (wanting to produce anticlockwise movement)

anyway, if I do reach to something about it, I will let you know.
4. Hmmmm. But that force is still an internal force, i.e. it doesn't actually act on the lever.

It's like if you set a mousetrap, there are a bunch of internal forces as the spring is loaded, but that wouldn't affect the weight when compared to a mousetrap that's not loaded.
5. (Original post by mik1a)
Hmmmm. But that force is still an internal force, i.e. it doesn't actually act on the lever.

It's like if you set a mousetrap, there are a bunch of internal forces as the spring is loaded, but that wouldn't affect the weight when compared to a mousetrap that's not loaded.
ye i got you.

won't this make the net weight less (not right physics here, but i mean down force) ?
6. Shouldn't make a difference. The downforce would change when the trap springs, but it will be the same after as it was before.
7. (Original post by mik1a)
Shouldn't make a difference. The downforce would change when the trap springs, but it will be the same after as it was before.
if we apply my statement to the containers example, would that make a change? since the force is acting on the wall in an upward direction (as if it's being held up slightly by a string). the balance will read less maybe?
8. No that doesn't work because the force is not acting on the lever. The fact that some water pressure acts upwards on the inside of the container is irrelevant. There's a Newton's 3rd force pair from the container acting back into the fluid, which completely cancels the water pressure when summing. This is, in general, why internal forces don't affect the lever.
9. (Original post by mik1a)
No that doesn't work because the force is not acting on the lever. The fact that some water pressure acts upwards on the inside of the container is irrelevant. There's a Newton's 3rd force pair from the container acting back into the fluid, which completely cancels the water pressure when summing. This is, in general, why internal forces don't affect the lever.
aha yes that's right. this question brought a headache to you haha

then the only thing i can say is that interviewers are looking at how you can suggest explanations by looking at the problem even if it is wrong (how you can get out of the problem you are stuck at), according to how you are approaching it. you know what i mean

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: September 5, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### How does exam reform affect you?

From GCSE to A level, it's all changing

### Who would you like to thank?

Poll
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.