# Kinematics question - a bet I made with a friend

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## Main Question or Discussion Point

He was giving me a lift home after class, where we studied kinematics (our 3rd class), and we started arguing about whether if there was a fly flying in the car in the same place, would he change his position in the car if the car accelerates. I said definitely, that the fly would be thrown at the back of the car. He initially said no, maybe, not sure, then no again, but when I told him "wanna bet?" he seemed to have retracted and said that I'm probably right.

Then we had a second argument. Would the fly stick the back side of the car if we were just driving at constant velocity without acceleration?

I again said yes, he again said no. That's when we made our bet.

Then I gave him another example. I asked if I put an apple somewhere in the car, would it fly backwards if the car moved at constant velocity? He said no, I said I agree. But, then I asked, what if there's no friction?

He said it would still stay in place. I said it would move backwards.

Who's right?

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He was giving me a lift home after class, where we studied kinematics (our 3rd class), and we started arguing about whether if there was a fly flying in the car in the same place, would he change his position in the car if the car accelerates. I said definitely, that the fly would be thrown at the back of the car. He initially said no, maybe, not sure, then no again, but when I told him "wanna bet?" he seemed to have retracted and said that I'm probably right.
Yep!
Fly gets thrown to the back of the car (or rather: is not accelerated forward as the car is).

Then we had a second argument. Would the fly stick the back side of the car if we were just driving at constant velocity without acceleration?

I again said yes, he again said no. That's when we made our bet.
I don't quite understand.
If the fly is on the outside of the car, it might get thrown off by the air moving around the car, but not by any acceleration effects.

Then I gave him another example. I asked if I put an apple somewhere in the car, would it fly backwards if the car moved at constant velocity? He said no, I said I agree. But, then I asked, what if there's no friction?

He said it would still stay in place. I said it would move backwards.

Who's right?
At constant velocity there's no horizontal force on the apple.
So with or without friction, it will stay in place.

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I don't quite understand.
If the fly is on the outside of the car, it might get thrown off by the air moving around the car, but not by any acceleration effects.
The fly is inside the car

At constant velocity there's no horizontal force on the apple.
So with or without friction, it will stay in place.
Really?

How interesting. So if I stand on a bus that drives in a constant velocity, I use the same effort/muscles to stay standing as I do if I was just standing in my room?

In other words, I wouldn't feel that velocity, only the changes in acceleration?

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The fly is inside the car

Really?

How interesting. So if I stand on a bus that drives in a constant velocity, I use the same effort/muscles to stay standing as I do if I was just standing in my room?

In other words, I wouldn't feel that velocity, only the changes in acceleration?
Yep!

You've just summarized the first law of Newton.

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What if the bus drives constant velocity of 150 km/h, and I wanna walk in it. Would I feel the same way as walking in my room?

And what about my example with the fly being inside the car with the car at constant velocity. It wouldn't be thrown at the back of the car in this case?

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What if the bus drives constant velocity of 150 km/h, and I wanna walk in it. Would I feel the same way as walking in my room?

And what about my example with the fly being inside the car with the car at constant velocity. It wouldn't be thrown at the back of the car in this case?
Yep!

That is, until the bus turns a corner! ;)

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Okay, looks like we were both right and we were both wrong :) Thanks for settings us straight, king of the castle^^

sophiecentaur
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When you are flying in an aircraft and have a cup of coffee, is the surface of the coffee in the cup level or tilted on straight and level flight? If you, the table and the coffee are flying at the same speed and there is no acceleration then there is no detectable horizontal force between you. A fly in the car is in the same situation.

When the car accelerates, air molecules at the rear (inside) are hit slightly more often than molecules at the front (inside) so there will be a slight pressure difference, causing all of the air to speed up as the car speeds up. This will push the fly forwards because the air is so viscous. A heavy pendulum, suspended in the car would be 'left behind' as the car accelerates and would lean backwards until the forward component of tension in the string pulled it forwards. This would be a 'temporary new vertical' within the frame of the car cab. The fly would have to fly using this vertical as a reference if it wanted to hover 'stationary' just in front of your nose.

russ_watters
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How interesting. So if I stand on a bus that drives in a constant velocity, I use the same effort/muscles to stay standing as I do if I was just standing in my room?

In other words, I wouldn't feel that velocity, only the changes in acceleration?
Yes.....you've never stood up on a bus/train/plane before?

How could they serve drinks on a plane if the speed of the plane had an effect on those inside the plane?

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Good points, guys. :) I humbly submit to your logic.

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Good points, guys. :) I humbly submit to your logic.
Actually, I think you nailed it when you were creative enough to think of the example of a bus!

Good points, guys. :) I humbly submit to your logic.
I know I'm beating a dead horse here, but let me drive the point home :)
How could you stand on the earth when it's moving ~60,000mph around the sun? And then probably a few hundred thousand mph around some other thing in space?

This is actually a big reason why everyone initially refused to believe that the earth moves around the sun...

sophiecentaur
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Yes.....you've never stood up on a bus/train/plane before?
Chauffeur driven Rolls Royce mainly, perhaps?

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My friend from the bet just called me and I let him know the results :)

He then said that he made an experiment and threw a ball FORWARD in his car while he was driving at a constant velocity, and he said the ball flew BACKWARDS! He said he did the same experiment driving backwards, throwing the ball backwards, and the fall flew forwards!

I told him that it's probaby because he had a tough time maintaining constant velocity and minor events of acceleration would do to the results he got.

Was I right?

How could you stand on the earth when it's moving ~60,000mph around the sun? And then probably a few hundred thousand mph around some other thing in space?

I never thought about it! That would mean that....

*ties herself with a rope to a pole!*

*hammers her shoes with nails to the floor!*

Phew!

[great example actually ;) )

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My friend from the bet just called me and I let him know the results :)

He then said that he made an experiment and threw a ball FORWARD in his car while he was driving at a constant velocity, and he said the ball flew BACKWARDS! He said he did the same experiment driving backwards, throwing the ball backwards, and the fall flew forwards!

I told him that it's probaby because he had a tough time maintaining constant velocity and minor events of acceleration would do to the results he got.

Was I right?
Yep!

I expect that when he threw the ball, he inadvertently jerked the gas pedal at the same time.
To do it right he should either use cruise control, or have someone else throw the ball.

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Aha!

I'll put him in his place, don't worry ;)