questions about physics (1 Viewer)

[psmao]

Hey, I am doing physics past paper and I met a problem which is confusing me.

this is from HSC 2018 question 19, basically its talking about on a bus, there is a ball hanging on it and what would happen to the ball if the string is cut and the bus suddenly stops.

I understand the perspective from a passenger (perspective from inside) , since there is a force towards the head of the bus (due to inertia), and there is a force downwards (due to gravitational acceleration), these two vectors add together to give you a oblique linear vector(motion).

But why for a guy outside the bus, he can observe a parabolic path?

thanks for helps x))))

 

[Ian Ch]

Hey, I am doing physics past paper and I met a problem which is confusing me.

this is from HSC 2018 question 19, basically its talking about on a bus, there is a ball hanging on it and what would happen to the ball if the string is cut and the bus suddenly stops.

I understand the perspective from a passenger (perspective from inside) , since there is a force towards the head of the bus (due to inertia), and there is a force downwards (due to gravitational acceleration), these two vectors add together to give you a oblique linear vector(motion).

But why for a guy outside the bus, he can observe a parabolic path?

thanks for helps x))))

Hmm maybe because the velocity of the ball relative to yours (in the bus) is the same and therefore a vertical motion is all that can be seen from the ball falling. But from the perspective of a observer thats stationary, when the ball is suddenly released (cut), the ball experiences a horizontal velocity same as the bus is experiencing, and hence the velocity the ball experiences relative to the stationary observer should look like the ball has a parabolic trajectory.

 

[ABBAS38]

An observer inside the bus will see the the ball move towards the front of the bus. From the perspective of a passenger, the ball has both a forward (due to the 3m/s2 slowdown of the bus) and downward acceleration; this will result in a linear motion! The initial velocity of the ball from the perspective of the passenger is zero.

 

[psmao]

An observer inside the bus will see the the ball move towards the front of the bus. From the perspective of a passenger, the ball has both a forward (due to the 3m/s2 slowdown of the bus) and downward acceleration; this will result in a linear motion! The initial velocity of the ball from the perspective of the passenger is zero.

nice copy from matrix lol but u didn’t answer my question :’(

 

[Drdusk]

Hey, I am doing physics past paper and I met a problem which is confusing me.

this is from HSC 2018 question 19, basically its talking about on a bus, there is a ball hanging on it and what would happen to the ball if the string is cut and the bus suddenly stops.

I understand the perspective from a passenger (perspective from inside) , since there is a force towards the head of the bus (due to inertia), and there is a force downwards (due to gravitational acceleration), these two vectors add together to give you a oblique linear vector(motion).

But why for a guy outside the bus, he can observe a parabolic path?

thanks for helps x))))

Think about it carefully. When the ball is attached to the string the ball is essentially a 'part' of the bus in the sense that whatever motion the bus will undergo, the ball will undergo too (from an outsiders perspective). Suppose the bus breaks while the ball is attached to the string, this acceleration will cause a force to be exerted on the string which will exert a Tension/reaction force on the ball.

Now when the spring is cut the ball and the bus become two 'separate things'. The ball and the bus are not attached and hence are now considered two separate objects. So if we look from an outsiders perspective the only force acting on the ball is gravity which is known as projectile motion. It's literally the same as if there was no bus 'around' the ball, as the ball is just falling in free air UNTIL it touches the floor of the bus.

 

[akkjen]

Think about it carefully. When the ball is attached to the string the ball is essentially a 'part' of the bus in the sense that whatever motion the bus will undergo, the ball will undergo too (from an outsiders perspective). Suppose the bus breaks while the ball is attached to the string, this acceleration will cause a force to be exerted on the string which will exert a Tension/reaction force on the ball.

Now when the spring is cut the ball and the bus become two 'separate things'. The ball and the bus are not attached and hence are now considered two separate objects. So if we look from an outsiders perspective the only force acting on the ball is gravity which is known as projectile motion. It's literally the same as if there was no bus 'around' the ball, as the ball is just falling in free air UNTIL it touches the floor of the bus.

but it undertakes a parabolic path as viewed by a person outside the bus, not vertically down

 

[Drdusk]

but it undertakes a parabolic path as viewed by a person outside the bus, not vertically down

................ projectile motion is parabolic is it not?

The ball has an initial horizontal velocity as the bus is moving before the string breaks... i.e. it will undergo projectile motion which wouldn't be vertically down because there's a horizontal velocity

 

[akkjen]

................ projectile motion is parabolic is it not?

The ball has an initial horizontal velocity as the bus is moving before the string breaks... i.e. it will undergo projectile motion which wouldn't be vertically down because there's a horizontal velocity

so what if a projectile is let go without horizontal force? it goes vertically down. so NO projectile motion isn't always parabolic

 

[ABBAS38]

so what if a projectile is let go without horizontal force? it goes vertically down. so NO projectile motion isn't always parabolic

Yes projectile motion is always parabolic. Ur missing the point, in the bus when u cut it, it only has vertical velocity so therefore goes straight down and in turn linear motion, and when viewing it on the outside, gravity acts on it and therefore follows a parabolic path as it has vertical and horizontal velocities at different points therefore is a projectile!

 

[Drdusk]

so what if a projectile is let go without horizontal force? it goes vertically down. so NO projectile motion isn't always parabolic

Of course, this is what I explained in my second paragraph. Oh and remember it's not a horizontal 'force' but horizontal velocity. You would lose marks if it was a long response question and you said horizontal force. Be careful =)

Yes projectile motion is always parabolic. Ur missing the point, in the bus when u cut it, it only has vertical velocity so therefore goes straight down and in turn linear motion, and when viewing it on the outside, gravity acts on it and therefore follows a parabolic path as it has vertical and horizontal velocities at different points therefore is a projectile!

This is an incorrect explanation. The ball doesn't fall just vertically downwards if your in the bus. You will observe it to go horizontally as well but not in a parabolic path. Also gravity is always acting on the ball, so it is incorrect to say it follows a parabolic path just because gravity acts on it. You gotta specify and say because you are stationary outside, gravity is the only acceleration present between the two reference frames which leads to parabolic projectile motion.

Also a ball falling vertically downwards is still projectile motion. Projectile motion is defined by when something falls with only the force of gravity acting on it(we exclude air resistance).

 

[Arrowshaft]

i’ve never seen these relativity of simultaneity and these sorta questions crop up in any trials or sample questions? Is it likely they could assess us on this?

 

[Arrowshaft]

Also I would’ve reasoned that the reason the object travels parabolically is because when it is cut the horizontal component of the bus’ velocity coupled with gravity results in a projectile motion, since there is net 0 horizontal acceleration since it detached from the bus. Sorry for being clunky and unorthodox but cbbs. Is my reasoning somewhat right?

 

[Drdusk]

i’ve never seen these relativity of simultaneity and these sorta questions crop up in any trials or sample questions? Is it likely they could assess us on this?

Yeah why not. If they wanna ask a question that is somewhat conceptually difficult to explain for a lot of the Physics cohort they would go with relativity and give some weird scenario.

Also I would’ve reasoned that the reason the object travels parabolically is because when it is cut the horizontal component of the bus’ velocity coupled with gravity results in a projectile motion, since there is net 0 horizontal acceleration since it detached from the bus. Sorry for being clunky and unorthodox but cbbs. Is my reasoning somewhat right?

Of course that's right. That's exactly what I wrote but more concise. Don't @ me, I was trying to explain it to him

 

[Arrowshaft]

Yeah why not. If they wanna ask a question that is somewhat conceptually difficult to explain for a lot of the Physics cohort they would go with relativity and give some weird scenario.


Of course that's right. That's exactly what I wrote but more concise. Don't @ me, I was trying to explain it to him

Haha sorry, I didn’t read your post, I just saw the conflict and jumped in

 

[Arrowshaft]

But I still have one question: why is it that in the passenger’s frame the object travels obliquely? Why is it linear, if it experiences a forward acceleration shouldn’t it be non linear? Or do the gravitational arc and the forward arc appear to cancel out or something?

 

[Arrowshaft]

Yeah why not. If they wanna ask a question that is somewhat conceptually difficult to explain for a lot of the Physics cohort they would go with relativity and give some weird scenario.

Yeah, I’m not too worried to be honest, I’m just asking because it used to be a dot point and a prac involving it in the previous syllabus analysing it. The principles are pretty simple once you study it, but people who haven’t learnt it might struggle on the spot in the exam

 

[Drdusk]

But I still have one question: why is it that in the passenger’s frame the object travels obliquely? Why is it linear, if it experiences a forward acceleration shouldn’t it be non linear? Or do the gravitational arc and the forward arc appear to cancel out or something?

Think about it this way. When your inside the bus the string is cut the very moment the bus begins to decelerate. What this means is you have a horizontal acceleration compared to the ball which has zero horizontal acceleration. Now as we know the ball has gravity acting on it which is purely vertical acceleration.

Now if you remember, the thing with acceleration is that you can switch it around. You can say that instead of you accelerating away from the ball at whatever m/s, the ball is accelerating away from you are the same rate(It's not actually but it's just an explanation I say so that you can understand why we can just add the two vectors). So now we can use this can say the ball has two acceleration vectors, one away from you purely horizontally and the other purely downwards.

Now we all know how to add two vectors together. If you have two vectors that are perpendicular they form a right angled triangle and you get that the vector sum of these is a diagonal vector which is the direction that the net acceleration points. This means the ball has a net acceleration in one direction diagonally which is the direction the ball will move in. Hence it moves in a linear motion diagonally.

 

[Arrowshaft]

Think about it this way. When your inside the bus the string is cut the very moment the bus begins to decelerate. What this means is you have a horizontal acceleration compared to the ball which has zero horizontal acceleration. Now as we know the ball has gravity acting on it which is purely vertical acceleration.

Now if you remember, the thing with acceleration is that you can switch it around. You can say that instead of you accelerating away from the ball at whatever m/s, the ball is accelerating away from you are the same rate(It's not actually but you can it's just an explanation I say so that you can understand why we can just add the two vectors). So now we can use this can say the ball has two acceleration vectors, one away from you purely horizontally and the other purely downwards.

Now we all know how to add two vectors together. If you have two vectors that are perpendicular they form a right angled triangle and you get that the vector sum of these is a diagonal vector which is the direction that the net acceleration points. This means the ball has a net acceleration in one direction diagonally which is the direction the ball will move in. Hence it moves in a linear motion diagonally.

Oh okay, and the reason an object moves parabollically is if it has a vertical acceleration but zero acceleration horizontally - however it has a horizontal velocity, meaning that it kinda drags that downward vector along with it in its path which accentuates more the longer the time passes. Is that visualisation correct? Ps. I know all the stuff here about the linear motion, I just need a solid visualisation to consolidate it.

 

[Drdusk]

Oh okay, and the reason an object moves parabollically is if it has a vertical acceleration but zero acceleration horizontally, however it has a velocity which means that it kinda drags that downward vector along with it in its path which accentuate more the longer the time passes. Is that visualisation correct? Ps. I know all the stuff here about the linear motion, I just need a solid visualisation to consolidate it.

Correct. If you think about it in terms of vectors you gain a much better understanding. The acceleration in the vertical direction keeps on increasing the length of the vertical velocity vector but the horizontal velocity vector stays the same. This constantly changes the direction of the net velocity vector which gives rise to parabolic motion. Of course you can prove specifically it's parabolic and not Quartic for example through calculus.

 

[Arrowshaft]

Correct. If you think about it in terms of vectors you gain a much better understanding. The acceleration in the vertical direction keeps on increasing the length of the vertical velocity vector but the horizontal velocity vector stays the same. This constantly changes the direction of the net velocity vector which gives rise to parabolic motion. Of course you can prove specifically it's parabolic and not Quartic for example through calculus.

Thank you, makes sense