jinny1's Physics Questions Thread

[FromBaghdadWithLove]

ohh God!, please tssm 2009 was a killer i got to the bus question and stopped lol... too hard

anyways does anyone know how to solve this?

[cranberry]

ah k cool! thanks xZero and vinc

[onur369]

@ FromBaghdadWithLove

The correct answer is D.
stress= force/area
= 600MPA
strain = 7.5x10^-3

Change in Length= strain x length = 0.0015 = 1.5cm

[FromBaghdadWithLove]

how did u know that strain was 7.5x10^-3?

[onur369]

Wait I think thats wrong, sorry.

[FromBaghdadWithLove]

lol, that was in the answers sheet,

[jinny1]

Am i right in saying that a ball at rest has the same FORCE as a ball traveling constantly at 2000000 km/h ??

or that a ball released from a very high cliff has the same force near the start of its fall as it does near the end of it (where it is much faster).

It's kinda hard to comprehend this, since if a ball traveling at 2000000km/h hits your face, then the "force" will decapitate you... when compared to a still ball..

[jinny1]

Also TSFX says that air resistance will increase the time of flight in projectile motion. How can this be true?

Q13. TSFX 2007.

[xZero]

Am i right in saying that a ball at rest has the same FORCE as a ball traveling constantly at 2000000 km/h ??

or that a ball released from a very high cliff has the same force near the start of its fall as it does near the end of it (where it is much faster).

It's kinda hard to comprehend this, since if a ball traveling at 2000000km/h hits your face, then the "force" will decapitate you... when compared to a still ball..

You're confusing net force with momentum, a ball hitting you in the face is calculated by p=mv but their net force will still be 0 right before it hits you. You can say that the net force of the ball at rest is the same as the ball traveling at a constant velocity.

As for the air resistance, it can increase the time. Lets say that you dropped a ball from 100m above the ground, if there's air resistance then its downward velocity will be less than without air resistance. Hence increasing the time of flight.

[tony3272]

Also TSFX says that air resistance will increase the time of flight in projectile motion. How can this be true?

Q13. TSFX 2007.

At their lectures they said that at the top of the flight, the projectile will experience a sense of "hangtime" which will increase the total flight time to that which is greater than the time w/o any air resistance.

[jinny1]

Also TSFX says that air resistance will increase the time of flight in projectile motion. How can this be true?

Q13. TSFX 2007.

At their lectures they said that at the top of the flight, the projectile will experience a sense of "hangtime" which will increase the total flight time to that which is greater than the time w/o any air resistance.

cheers for that

Am i right in saying that a ball at rest has the same FORCE as a ball traveling constantly at 2000000 km/h ??

or that a ball released from a very high cliff has the same force near the start of its fall as it does near the end of it (where it is much faster).

It's kinda hard to comprehend this, since if a ball traveling at 2000000km/h hits your face, then the "force" will decapitate you... when compared to a still ball..

You're confusing net force with momentum, a ball hitting you in the face is calculated by p=mv but their net force will still be 0 right before it hits you. You can say that the net force of the ball at rest is the same as the ball traveling at a constant velocity.

As for the air resistance, it can increase the time. Lets say that you dropped a ball from 100m above the ground, if there's air resistance then its downward velocity will be less than without air resistance. Hence increasing the time of flight.

Thanks mate.

regarding the momentum/netforce.

Force is also given by change in momentum/time . so if we use that equation for Force then, in this scenario, our force would be massive.

but if we use F=ma , then our force would be zero??? what am i misunderstanding here??

[xZero]

there's your problem, it should be Fnet=ma, not just F=ma

to clear up any misunderstanding, net force = 0 means that all forces acting on you cancels each other out, but it doesn't mean that there's no forces acting upon you

[jinny1]

I have a couple of questions

1. Are columns in tension at all??? TSSM certainly thinks that a column is in more tension than a House foundation where people walk over it ( i would think this would have more tension).


2. This is from TSSM 2010 exam,



Question 14
Determine the spring coefficient of the mat required to bring the ball to rest at a height of
0.2 m. You must show your working.

I've attached the worked solutions below. It differs from what i did.

I did:  

GPE(f) + SPE = GPE(i) + KE(i)    and then solved for 'k' in the SPE     (i,f stand for inital and final respectively)

they seem to have forgotten that there was an initial GPE and just though of the final GPE.

or did i do something wrong???

[Vincezor]

2. This is from TSSM 2010 exam,

(Image removed from quote.)

Question 14
Determine the spring coefficient of the mat required to bring the ball to rest at a height of
0.2 m. You must show your working.

I've attached the worked solutions below. It differs from what i did.

I did: 

GPE(f) + SPE = GPE(i) + KE(i)    and then solved for 'k' in the SPE     (i,f stand for inital and final respectively)

they seem to have forgotten that there was an initial GPE and just though of the final GPE.

or did i do something wrong???

The way i see that is the total energy at the top =



Now since the ball doesnt hit the bottom on it's way down, the height it has from the ground is it's new GPE which is



So the rest of the energy was converted to spring potential energy





Is this correct? :S

[ttn]

Yeap, that's how I would do it also.