1,000,000 Question Thread :D

[kenhung123]

Hookes law F=-kx, why do graphs with positive gradient obey Hookes law?

[Studyinghard]

Another stupid question from me

An object is dropped from a tower 125 m high.
a What is the speed of the object that falls freely from rest for a distance of 128 m?

[kenhung123]

Ug=mgh=m*9.8*128m
Ug=Ek=1/2mv^2 as conservation of energy
v^2=(m*9.8*128)/(1/2*m)
     =(9.8*128)/0.5
v=((9.8*128)/0.5)^0.5

[kenhung123]

Hookes law F=-kx, why do graphs with positive gradient obey Hookes law?

No idea with the question below

[Akirus]

For A:







For B:







For C:








Haven't done this for a while so I'm not sure if it's correct, get someone to confirm.

[kenhung123]

Yea, you are right, but why negative and do you know how to do question 4?

[Akirus]

I believe the negative sign represents opposite direction to the force being applied to the spring.

For question 4... wouldn't it just be 1:2?


/EDIT: Checked the textbook for you,

Newton’s third law tells us that an extended or compressed spring in equilibrium is able to exert a restorative force equal in size but opposite in direction to the force that is being applied to it.

[kenhung123]

But when we write the answer, we don't put negative? When we interpret graphs its also a positive gradient. So they basically ignored the negative sign. Why is this the case?

[Aden]

The spring constant is just the gradient of a force-compression graph (). If you plug it into Hooke's Law to find k, you will always get the opposite signed answer, since Hooke's Law finds the force 'exerted by a spring, but opposite in direction to the spring's extension of compression'.

Studyinghard: Since your object is allowed to free-fall for 128m, use (where the initial velocity is 0, displacement is 128 and acceleration is 9.8 due to gravity):


*Although, I'm not sure what the information about the tower being 125m high is for...

[kenhung123]

What does the term horizontal road or horizontal round about mean?

[Studyinghard]

What does the term horizontal road or horizontal round about mean?

could it mean like a flat road. so no elevation?

A question:)

Question 6
A block of wood of mass 15.0 kg, initially at rest, is pulled 4.0 m
along a rough fl oor by a force of 120 N. The frictional force opposing
the motion of the block of wood is 30 N.
What is the change in velocity of the block of wood?

[kenhung123]

What does the term horizontal road or horizontal round about mean?

could it mean like a flat road. so no elevation?

A question:)

Question 6
A block of wood of mass 15.0 kg, initially at rest, is pulled 4.0 m
along a rough fl oor by a force of 120 N. The frictional force opposing
the motion of the block of wood is 30 N.
What is the change in velocity of the block of wood?

Possibly, but I have no idea why they didn't call it flat?
u=0, v=?, x=4, a=?
F=ma
120-30=15xa
a=6m/s

u=0, v=?, x=4, a=6
v^2=u^2+2ax
      =2*6*4
v=48^0.5

[kenhung123]

Aden do you know how to do question 4? Thanks very much

[Aden]

I swear I've answered this somewhere before =\

Kinetic energy of ball:



Subbing into elastic potential energy:


or

[/0]

I haven't done Structures and Materials in a while so I'm a bit rusty, but here's what I got for 4

For 4,




When the stored energy is the same,









Ratio of compression of spring a to spring b is