Rock dropped inside of a moving vehicle

[Perseverence]

Homework Statement

A truck is moving at constant velocity. A rock is dropped from a storage compartment located at the midpoint of the ceiling of the truck. Where does the rock land on the floor.

2. Homework Equations
Ax=0

The Attempt at a Solution

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The given answer states that the rock lands exactly below the midpoint of the ceiling. In other words it drops straight down in a straight line just below the exact spot from which it was dropped. The rationale is that there is no horizontal velocity an object in free fall. So what I don't understand is that the truck is still moving, the rock is not moving with it, so why does the rocket land in the same spot it would if the truck were standing still. Wouldn't the horizontal velocity of the rock habitue match the horizontal velocity of the truck, for the rock to the land in the same place it would if the truck were standing still?

 

[berkeman]

Wouldn't the horizontal velocity of the rock habitue match the horizontal velocity of the truck,

Yes, exactly. When the rock is being held in place on the roof, it has the same horizontal velocity as the truck. When you drop it, the only force acting on it is the force due to gravity (straight down).

 

[Perseverence]

But in the time that the rock is falling the truck is moving forward. So the rock should land behind the midpoint of the ceiling? Why not?

 

[berkeman]

But in the time that the rock is falling the truck is moving forward. So the rock should land behind the midpoint of the ceiling? Why not?

Are you familiar with Free Body Diagrams (FBDs) yet? How about moving (not accelerating) reference frames?

 

[kuruman]

If you jump up in the air and stay off the ground for about one second, you will land back to the spot where you took off from. Yet, in that one second the Earth has rotated under your feet by about 1000 feet, more or less depending on your latitude. How come you land at the same spot?

 

[Perseverence]

If you jump up in the air and stay off the ground for about one second, you will land back to the spot where you took off from. Yet, in that one second the Earth has rotated under your feet by about 1000 feet, more or less depending on your latitude. How come you land at the same spot?

You know what? I have no idea why that is. If on a skateboard and I jump up and lose contact with it, it will move horizontally in front of me and I will not land on the skateboard. I will win behind the skateboard. I will not land on the same spot that I will not land on the same spot that I started from on the skateboard. How is that different than being inside a moving car?

 

[Perseverence]

The answers I found your questions seem to say that when you jump up you actually don't jump straight up and down but you actually jump on which means you would have horizontal. But the answer to my original problem states that there is no horizontal velocity for the rock.

 

[haruspex]

The answers I found your questions seem to say that when you jump up you actually don't jump straight up and down but you actually jump on which means you would have horizontal. But the answer to my original problem states that there is no horizontal velocity for the rock.

Your original post had:

The rationale is that there is no horizontal velocity an object in free fall.

Is that the exact wording? A translation perhaps?
E.g. perhaps it said/meant there is no change in horizontal velocity of an object in free fall?

 

[Perseverence]

Your original post had:

Is that the exact wording? A translation perhaps?
E.g. perhaps it said/meant there is no change in horizontal velocity of an object in free fall?

Ok. I understand why you land in the same place. Because no force is applied so horizontal velocity stays the same.

But then that just been solve the skateboard question...

If on a skateboard and I jump up and lose contact with it, it will move horizontally in front of me and I will not land on the skateboard. I will win behind the skateboard. I will not land on the same spot that I will not land on the same spot that I started from on the skateboard. How is that different than being inside a moving car?

 

[haruspex]

If on a skateboard and I jump up and lose contact with it, it will move horizontally in front of me and I will not land on the skateboard.

If you were to jump up from the skateboard purely by exerting a vertical force then, if anything, the opposite would happen. The skateboard, experiencing rolling resistance from the ground, would slow down more than you, so you would overtake it.
But it is hard to judge exactly the direction in which you apply the force. It would be very easy to exert some small forward force, leading to the result you describe, without being consciously aware of it.

 

[Perseverence]

If you were to jump up from the skateboard purely by exerting a vertical force then, if anything, the opposite would happen. The skateboard, experiencing rolling resistance from the ground, would slow down more than you, so you would overtake it.
But it is hard to judge exactly the direction in which you apply the force. It would be very easy to exert some small forward force, leading to the result you describe, without being consciously aware of it.

Okay. So the difference between the car and the skateboard is that the force you exert on the skateboard would actually affect the velocity of the skateboard. Whereas the force you exert by jumping up inside of a car or on the planet is minimal in comparison and would not affect the velocity of the car or the planet

 

[Perseverence]

I finally understand! Thank you everyone for your help!