Physics question (Deformation of solids)

method looks ok havnt checked the numbers though, which ones weren't you sure about?

Reply 2

The young modulus is a measure of stiffness. What you have written is correct, but u shoud write both.

If ur answer for the stress is correct, then the force is incorrect. For some reason u have done
E = (f/a)/l

The 1.8 on the bottom should be 3.06 x10^-4

Reply 3

JamesF

maybe i should have checked properly! (*feels very stupid and certain to fail her exam now!*)

Reply 4

FaerieLand

maybe i should have checked properly!

Hehe.

Btw i get 7.344N as the force for part ii
And 1.1 mm as the extension in part iii

Reply 5

JamesF

Yep, you're right - I don't know why I put 1.8 at the bottom. I'm always making silly mistakes like this :frown:

. For part (c)(i) of the question, is it correct to say that if the Young's Modulus of the material is halved then the extension is doubled? Less stiffness = More extension?

fm2.jpg38.3KB

Reply 6

JamesF

Hehe.

Btw i get 7.344N as the force for part ii
And 1.1 mm as the extension in part iii

agreed!

Reply 7

FaerieLand

agreed!

Another question solved :biggrin:

Reply 8

nofronting

Yep, you're right - I don't know why I put 1.8 at the bottom. I'm always making silly mistakes like this :frown:

. For part (c)(i) of the question, is it correct to say that if the Young's Modulus of the material is halved then the extension is doubled? Less stiffness = More extension?

Yep.

Reply 9

FaerieLand

agreed!

See, u'll do fine in the exam.

Reply 10

lgs98jonee

JamesF

See, u'll do fine in the exam.

how come u know so much....u know everything :eek:

Reply 11

lgs98jonee

how come u know so much....u know everything :eek:

Because it's on our physics sylabus :smile:

Reply 12

Okay, here's some more questions. I reckon the first two parts are corect but i'm not so sure about part 3.

** part ii is meant to be 2.5^5 J not 2.5^-5 J.

fm3.jpg35.2KB

Reply 13

JamesF

Because it's on our physics sylabus :smile:

somebody's been good!

Reply 14

nofronting

Okay, here's some more questions. I reckon the first two parts are corect but i'm not so sure about part 3.

** part ii is meant to be 2.5^5 J not 2.5^-5 J.

Only the last part is wrong. The breaks are only applied after the thinking time, so the distance is 40m.

Also, i dont know what u have done there, a better approach is to consider the energy.
All the KE must be turned into work done by the brakes, so
Work done = F * d
2.5x10^5 = 40F
F = 6250N

Reply 15

FaerieLand

somebody's been good!

Im not sure about that, I haven't done any physics revision yet :redface:

I've gotta decide which question to do in the exam too, materials or astro-physics....

Reply 16

Hm, okay thanks - i understand now. I tried to work it out using one of the equations of motion to find the deceleration of the car and then use F = ma to determine the braking force. :confused:

u = 25ms-1
v = 0ms-1
s = 40m
a = a1

Reply 17

nofronting

Hm, okay thanks - i understand now. I tried to work it out using one of the equations of motion to find the deceleration of the car and then use F = ma to determine the braking force. :confused:

u = 25ms-1
v = 0ms-1
s = 40m
a = a1

Ok, then u would use
v^2 = u^2 - 2as (which is what you did)
0 = 25^2 - 80a
625 = 80a
a = 625/80

F = ma = 625/80 * 800 = 6250N

Reply 18

JamesF

Ok, then u would use
v^2 = u^2 - 2as (which is what you did)
0 = 25^2 - 80a
625 = 80a
a = 625/80

F = ma = 625/80 * 800 = 6250N

This is why i love physics - there's usually more than one way to get an answer. Isn't the formula v^2 = u^2 + 2as?

Reply 19