The Final Straw

[Motor Daddy]

I measured and marked a straight level road a distance of 1 km. I marked the road every meter for the entire 1,000 meters. I placed a flag on a pole at the 500 meter mark half way between the two ends of the marked distance. I placed two switches, one at each end of the marked distance 1,000 meters apart to start the two timers for both halves of the distance.

 

Two cars each traveled at the constant velocity of 10 m/s towards the flag pole at the 500 meter mark. They both activated the timers and continued on the collision course towards each other until they each hit the flag pole at the exact same time, which stopped each timer when each car hit the pole.

 

Both timers read 50 seconds when they stopped.

 

What was the velocity of the flag pole relative to each of the cars?

Edited June 25, 2008 by Motor Daddy

[Mr Skeptic]

(+/-) 10 m/s

[ecoli]

it's a simple matter of having a different reference frame. I suppose it can be a tricky concept... but it's not that tricky.

[Motor Daddy]

it's a simple matter of having a different reference frame. I suppose it can be a tricky concept... but it's not that tricky.

 

So both are correct to assume the pole and Earth are moving towards them, and they are "stationary?"

[ecoli]

So both are correct to assume the pole and Earth are moving towards them, and they are "stationary?"

 

yep, in that case the ground is moving with a negative velocity "under" the car. The physics will work out to be the same... it's just that some reference frames are easier to work with then others.

 

For example, the earth is also orbiting the sun, so to consider the sun as stationary to measure the velocity of the car isn't very useful for a simple kinematics exercise.

[Motor Daddy]

yep, in that case the ground is moving with a negative velocity "under" the car. The physics will work out to be the same... it's just that some reference frames are easier to work with then others.

 

For example, the earth is also orbiting the sun, so to consider the sun as stationary to measure the velocity of the car isn't very useful for a simple kinematics exercise.

 

What is a negative velocity?

 

Also, when I turn my ceiling fan on, is it actually the Earth that starts spinning faster?

Edited June 25, 2008 by Motor Daddy
multiple post merged

[ecoli]

What is a negative velocity?

Forget what you thought you know about simple physics for a second. What you're thinking about velocity is actually a speed. It's a scalar quantity without direction.

 

True velocity has a magnitude (m/s) and direction.

 

so 10 m/s is a speed. 10 m/s Northwest is a velocity.

 

When I say 10 m/s towards the flagpole, that's a velocity. -10m/s is the same speed, but in the opposite direction.

 

So, the car can be considered as going towards the flagpole at 10m/s OR the ground and flagpole can be moving towards the car at -10m/s. The negative just denotes the direction of movement.

 

Think of it this way... you're traveling in a car at 60mph. A car comes along side you traveling at 70 mph. To you, this car is only traveling 10mph faster than you. So, in reference to you (being stationary in the car) the other car is only traveling 10mph. To an outside observer, someone watching from the street, they see you going at 60 and 70 mph, respectively (and respective to the ground). The guy in the car going at 70 sees you getting left behind. From this frame of reference, you're traveling forward at -10 mph.

 

 

Also, when I turn my ceiling fan on, is it actually the Earth that starts spinning faster?

 

image you were sitting on top of your fan blade. IF you could imagine them being stationary, it is indeed the earth spinning underneath you.

 

 

Keep in mind, however, that since the earth has gravity, you'll still probably get dizzy and fall off.

[swansont]

image you were sitting on top of your fan blade. IF you could imagine them being stationary, it is indeed the earth spinning underneath you.

 

 

Keep in mind, however, that since the earth has gravity, you'll still probably get dizzy and fall off.

 

 

You can tell when you're accelerating. It's not an inertial frame. So you have to be careful when you say that the earth is spinning faster. In a non-inertial frame the physics isn't the same — there will be an additional term or terms to make Newton's laws work (pseudoforces) while in inertial frames, the physics is all the same, which is why we can say that there is no preferred inertial frame.

 

———

 

Now, is there some reason we're going over this same ground, AGAIN?

[Edtharan]

What is a negative velocity?

As I have explained (due to you asking this exact same question) in other threads, a negative speed/velocity is a linguistic simplification. Instead of restating all the "relative to" information and calculating the vectors, you can simplify the calculations in certain circumstances (when all the vectors are in line with each other) as a negative value.

 

Only in the circumstances where the vectors line up can such simplifications be applied.

 

So what exactly is a negative velocity? Nothing more than a simplification due the special circumstance that the vectors line up.

 

Do you actually read my posts?

Also' date=' when I turn my ceiling fan on, is it actually the Earth that starts spinning faster? [/quote']

image you were sitting on top of your fan blade. IF you could imagine them being stationary, it is indeed the earth spinning underneath you.

Actually you can determine that it is the fan that is turning because rotation is an acceleration reference frame. As an acceleration reference frame is different to a constant motion reference frame, you can always determine hat it is the accelerating reference frame that is moving.

 

The fan blades under go acceleration. In fact, two types of acceleration. The first type is as the fan blades rotate faster. The force that is doing this is called torque and comes from the motor. The second type is due to the fact that it is rotating. All rotational motion, even constant rotation has two vectors associate with it. The first is the tangential velocity, this vector lies at a tangent to the rotation.

 

The second is actually how that tangential vector is changing over time. Remember a vector that is changing over time is actually an acceleration (a vector has a speed and when you divide the vector by a scalar, in this case Time, you get the speed component of the vector divided by time and speed divided by time is acceleration). This acceleration is always at right angles to the tangential vector.

 

This means that the two vectors (one a constant vector, the other an acceleration vector) combine together to specify the rotational motion. As the tangential vector is changed due to the acceleration vector, the acceleration vector must remain at right angles to the tangential vector and so it now points in a different direction.

[Motor Daddy]

Thanks for the replies so far. I appreciate it.

 

What is the torque of each axle on each of the cars traveling towards the flag pole?

 

Could you explain the torque on the rear axles and the direction of the "twist" of the axles of each of the cars as they are traveling towards the pole with zero acceleration? I'm not quite sure I understand torque and HP.

 

Also, since I can consider myself to be traveling at a velocity of -10m/s, does that mean I get -10 MPG (miles per gallon), since I am traveling at the velocity of -10 m/s, and I usually get 10 MPG at 10 m/s?

 

When I say 10 m/s towards the flagpole, that's a velocity. -10m/s is the same speed, but in the opposite direction.

 

What is the basis for the direction of travel of each of the cars? Is there a reference point of that direction of travel? Can I use the other car as the reference point of direction of travel, since the pole and the other car are both traveling the same direction towards me?

Edited June 25, 2008 by Motor Daddy
multiple post merged

[doG]

I measured and marked a straight level road a distance of 1 km. I marked the road every meter for the entire 1,000 meters. I placed a flag on a pole at the 500 meter mark half way between the two ends of the marked distance. I placed two switches, one at each end of the marked distance 1,000 meters apart to start the two timers for both halves of the distance.

 

Two cars each traveled at the constant velocity of 10 m/s towards the flag pole at the 500 meter mark. They both activated the timers and continued on the collision course towards each other until they each hit the flag pole at the exact same time, which stopped each timer when each car hit the pole.

 

Both timers read 50 seconds when they stopped.

 

What was the velocity of the flag pole relative to each of the cars?

 

Does this road run north-south, east-west, or on some other angle relative to the Earth's rotation? Me thinks you have not provided enough data to get the answer you are looking for.

[Motor Daddy]

Does this road run north-south, east-west, or on some other angle relative to the Earth's rotation? Me thinks you have not provided enough data to get the answer you are looking for.

 

I wanted to give a n/s/e/w direction of travel, but I have no clue which direction North and South are, and If I keep traveling in the same direction the direction seems to change if I give a North South reference.

[doG]

I wanted to give a n/s/e/w direction of travel, but I have no clue which direction North and South are, and If I keep traveling in the same direction the direction seems to change if I give a North South reference.

 

Yes but, if you want to ask a question as you did where you expect an answer that considers the rotation of the Earth then it matters whether it is n-s, e-w or some other direction.

[Motor Daddy]

Yes but, if you want to ask a question as you did where you expect an answer that considers the rotation of the Earth then it matters whether it is n-s, e-w or some other direction.

 

 

I'm not concerned with the rotation of the Earth, only how far I traveled on the Earth compared to the Earth, and how long it took me to travel the distance on the Earth. I can see a difference in force with the rotation of the Earth, but not d/t when I measured the distance and time traveled on the Earth compared to the Earth.

 

As I have explained (due to you asking this exact same question) in other threads, a negative speed/velocity is a linguistic simplification. Instead of restating all the "relative to" information and calculating the vectors, you can simplify the calculations in certain circumstances (when all the vectors are in line with each other) as a negative value.

 

Only in the circumstances where the vectors line up can such simplifications be applied.

 

So what exactly is a negative velocity? Nothing more than a simplification due the special circumstance that the vectors line up.

 

Could you just give me the straight scoop and not the simplified linguistic version? That may be the source of my confusion, that people mix the simplified linguistic version with the not so simplified linguistic version.

Edited June 25, 2008 by Motor Daddy

[Mr Skeptic]

Also, since I can consider myself to be traveling at a velocity of -10m/s, does that mean I get -10 MPG (miles per gallon), since I am traveling at the velocity of -10 m/s, and I usually get 10 MPG at 10 m/s?

 

Yeah. If you drive in reverse, you actually get fuel. If you drive in reverse all the time, you can sell all the fuel you get.

[Motor Daddy]

Yeah. If you drive in reverse, you actually get fuel. If you drive in reverse all the time, you can sell all the fuel you get.

 

Really? So if I drive 10 miles in 10 minutes when the transmission is in "D" (drive), and then I put it in "R" (reverse) and go back to where I came from when I first started traveling in "D", I actually traveled zero miles, and the "time" is the same as when I first started traveling the 10 miles in "D?"

[iNow]

Now, is there some reason we're going over this same ground, AGAIN?

Excellent question. I propose the following:

Motor Daddy is like a bored cat, and he is trying to treat the membership here like a ball of yarn to entertain himself.

[Motor Daddy]

Excellent question. I propose the following:

Motor Daddy is like a bored cat, and he is trying to treat the membership here like a ball of yarn to entertain himself.

 

No, I am having extreme difficulty with SR, as it goes against everything I know to be true, as measured with a tape measure and stop watch.

 

I am having extreme difficulty locating the -10 meter mark on my tape measure.

 

Also, why does my torque wrench not show a torque when I spin it in the air, but when I place it on a bolt that resists the rotation, the torque wrench reads a torque?

 

Now, is there some reason we're going over this same ground, AGAIN?

 

Yes, because I still don't understand it.

 

Why are people so patient with other people that think the earth is a tree, and they continue to entertain themselves at the OP's expense, but nobody cares to entertain themselves at my expense?

[doG]

What was the velocity of the flag pole relative to each of the cars?

 

I'm not concerned with the rotation of the Earth, only how far I traveled on the Earth compared to the Earth, and how long it took me to travel the distance on the Earth. I can see a difference in force with the rotation of the Earth, but not d/t when I measured the distance and time traveled on the Earth compared to the Earth.

 

Which is it? The velocity of the flag pole relative to each of the cars is relative to it's own velocity with Earth and the relative velocity of each of the cars.

[Mr Skeptic]

Really? So if I drive 10 miles in 10 minutes when the transmission is in "D" (drive), and then I put it in "R" (reverse) and go back to where I came from when I first started traveling in "D", I actually traveled zero miles, and the "time" is the same as when I first started traveling the 10 miles in "D?"

 

No, I was just seeing if you had any common sense.

 

---

 

The "-" is the direction opposite to the "+" direction. If "+" is north, then "-" is south. Since you gave no directions in your problem and there are two directions mentioned in it, one direction is the "+" direction and the other is the "-" direction.

[Motor Daddy]

Which is it? The velocity of the flag pole relative to each of the cars is relative to it's own velocity with Earth and the relative velocity of each of the cars.

 

The event is over, there is no motion. The distance is not increasing or decreasing, and the timer has stopped.

 

How do you measure distance and time while you are traveling and the timer continues to "tick?"

 

No, I was just seeing if you had any common sense.

 

I have good logic and common sense, but it doesn't agree with SR. That is my problem.

 

No, I was just seeing if you had any common sense.

 

---

 

The "-" is the direction opposite to the "+" direction. If "+" is north, then "-" is south. Since you gave no directions in your problem and there are two directions mentioned in it, one direction is the "+" direction and the other is the "-" direction.

 

So would each of their velocities have switched if they didn't hit the pole, but continued to each others "start switch," which then stopped each timer, instead of the timer stopping when each car reached the pole?

[Phi for All]

it goes against everything I know to be true

This "knowing" is your biggest impediment to learning. You keep asking the same questions because you've already made up your mind.

[Motor Daddy]

This "knowing" is your biggest impediment to learning. You keep asking the same questions because you've already made up your mind.

 

I've made up my mind that distance is measured with a tape measure, and time is measured with the timer. I have no clue of how to measure distance and time without those two items, do you?

 

I am, however, open to suggestions. I am not beyond changing directions of travel, I just need to do a little accelerating to do that, though.

[Mr Skeptic]

I have good logic and common sense, but it doesn't agree with SR. That is my problem.

 

That statement seems directly at odds with all our observations about you

 

So would each of their velocities have switched if they didn't hit the pole, but continued to each others "start switch," which then stopped each timer, instead of the timer stopping when each car reached the pole?

 

Would a car that was going north suddenly start going south, is that what you are asking?

 

---

 

Anyhow, I'm out of troll food.

[Motor Daddy]

That statement seems directly at odds with all our observations about you

 

 

 

Would a car that was going north suddenly start going south, is that what you are asking?

 

Yes. For example, I am 10 feet from the "North pole," or whatever the true axis of the rotation of the Earth is at the moment. I travel to the pole and keep going. Which direction am I, or did I travel?