VCE Physics Question Thread!

[Adequace]

Well work = force * distance, but the first two of those formulae aren't in terms of a force, so they are wrong.

The first equation, whoever made it was probably going for W = ΔKE which leads to v^2 = u^2 + 2ax.

The second equation, close but not quite... Work = Force * distance = GMm/r^2 * distance for gravity. Since for most situations the Δr is very small it's possible to approximate GMm/r^2 to mg, so Work = mg * x = mgx = mgh.

The third one is correct but usually friction is W(friction) < 0 and friction opposes the motion and hence removes energy from a system.

Ah cheers.

Hopefully I just copied the notes down incorrectly, lmao..

[Orson]

Can someone confirm if these energy/work formulas are legitimate, they were in our notes from my teacher but people have told me they're wrong.

Ek: work done by force - W=1/2mv^2 * x
Ug: work done by gravity - W=mgh * x
Thermal Energy: work done by friction - W=Fr * x

Cheers

I've never seen these before... I think your better off not using these derivations...Just a thought...

[paper-back]

How do you prove that maximum speed occurs at the center of oscillation?
In VCAA 2014 Q2(d), they've stated that maximum speed occurs at the centre of oscillation in the examiners report, did we have to know this as a part of our 'prerequisite' knowledge or was there a way to work this out?

[Orson]

How do you prove that maximum speed occurs at the center of oscillation?
In VCAA 2014 Q2(d), they've stated that maximum speed occurs at the centre of oscillation in the examiners report, did we have to know this as a part of our 'prerequisite' knowledge or was there a way to work this out?

SPOILERS! 

I haven't seen the question yet, but I assume it is an energy question...Sounds like a Spring/Energy situation....

[odeaa]

How do you prove that maximum speed occurs at the center of oscillation?
In VCAA 2014 Q2(d), they've stated that maximum speed occurs at the centre of oscillation in the examiners report, did we have to know this as a part of our 'prerequisite' knowledge or was there a way to work this out?

Make a table with the gpe, ke and spe at the top, middle and bottom of the oscillation

Because the ke is 0 at both top and bottom, it makes sense that it would be the highest in the centre of oscillation

[paper-back]

Make a table with the gpe, ke and spe at the top, middle and bottom of the oscillation

Because the ke is 0 at both top and bottom, it makes sense that it would be the highest in the centre of oscillation

Oh I see
Thanks!

[odeaa]

How come when a plane is flying in a vertical circle, it has an apparent weight at the top? Isn't there no normal reaction there? Simple question but it's got me confused

[Orson]

How come when a plane is flying in a vertical circle, it has an apparent weight at the top? Isn't there no normal reaction there? Simple question but it's got me confused

I think it's because there is a reaction force to the centripetal force exerted on the plane. I don't really understand it...but I've come to accept it.

[lzxnl]

This depends actually on the radius of the circle and speed of the plane. At the top of the circle, there is a 'normal reaction' force provided by the air which pushes upwards on the plane. This may partially balance the downwards gravitational force on the plane.

[Orson]

This depends actually on the radius of the circle and speed of the plane. At the top of the circle, there is a 'normal reaction' force provided by the air which pushes upwards on the plane. This may partially balance the downwards gravitational force on the plane.

Ohhhh! So is this called 'lift'? Is this the force created by the aerodynamics (vents and shapes) and whatnot?

Thanks!

[lzxnl]

Yes. This is the Newton's third law reaction to the plane pushing down on the air. Its size is determined by the shape of the plane.

[Cosec]

Ohhhh! So is this called 'lift'? Is this the force created by the aerodynamics (vents and shapes) and whatnot?

Thanks!

I could go in to quite some detail, but ill spare it. Basically. Lift is based on a variety of factors, including speed, angle of attack, area of the wing, and some coefficients based on air density, etc. The lift force itself is like the normal for a car on the air. Its the force that keeps them vertically stationary. If a plane is to ascend, it has to have a greater amount of lift than its weight. Vice versa for descending. So while it isnt a normal force, the reaction force is whats keeping the plane flying and thus because their is no 'normal' force but their is a reaction force present, the plane is not going to be apparent weightlessness.

[odeaa]

Just to clarify, if a question asks for net force of the system, is it just f=ma? as opposed to the driving force which is ma+fric?

[zsteve]

Looks like you're using ready made formulas - there's no set formula for 'driving force', you derive that.
'Net force' is the sum of all vector forces, i.e. . on a system. It is not the 'driving force'.
Acceleration is only calculated from net force, i.e. . So if you know the acceleration of a system and its mass, you essentially have its net force.

[odeaa]

Looks like you're using ready made formulas - there's no set formula for 'driving force', you derive that.
'Net force' is the sum of all vector forces, i.e. . on a system. It is not the 'driving force'.
Acceleration is only calculated from net force, i.e. . So if you know the acceleration of a system and its mass, you essentially have its net force.

so friction isnt taken into account for net force if you have the acceleration?