Ball drop in vacuum

[Ramzi]

Hi guys,

This is the scenario: a car moving at 20 mph in a vacuum (gravity exists). I drop a ball (just drop, not throw) out the window. What happens to the ball? Straight down? Forward? Backward?

 

Thanks

[Strange]

Hi guys,

This is the scenario: a car moving at 20 mph in a vacuum (gravity exists). I drop a ball (just drop, not throw) out the window. What happens to the ball? Straight down? Forward? Backward?

 

Depends on the frame of reference. From your point of view in the car it will drop straight down. Someone standing by the side of the road will see it fall while moving forwards at 20 MPH.

[Ramzi]

Why would the ball move forward? I assumed in a vacuum, the only force acting upon the object was gravity. So, when i let go of the ball, shouldn't it be affected only by gravity and therefore drop straight down?

[Strange]

Inertia. It is travelling along with your car and will continue to do so when you let go of it. This is the same thing that throws you forward when you put the brakes on: inertia means you want to keep moving forward at 60MPH even though the car is stopping.

[swansont]

Why would the ball move forward? I assumed in a vacuum, the only force acting upon the object was gravity. So, when i let go of the ball, shouldn't it be affected only by gravity and therefore drop straight down?

But it's moving at 20 mph forward. That won't change, since there's no force in the horizontal direction.

[pzkpfw]

Why would the ball move forward? I assumed in a vacuum, the only force acting upon the object was gravity. So, when i let go of the ball, shouldn't it be affected only by gravity and therefore drop straight down?

You can try this in a real car (please be careful). Of course you won't be able to do this in a vacuum, but what you'll see is that the thing you drop (say, a rock) will land ahead of the location where you let go of it. It might be hard to check - maybe have a friend (outside the car) video the experiment.

 

The air will slow down the rocks forward movement (i.e. it will go backwards, relative to the car), but just as noted in posts above, that thing you drop will start with that forward (relative to the ground) movement. It's certainly not going to land behind the location where it's been let go.

 

 

Come to think of it, you could do this inside a van. As the air inside moves with the van, there would be no horizontal effect on the dropped thing.

 

i.e. carefully mark (A) the centre of the roof of the van, and (B) the centre of the floor - directly below. Drive along at 100 km/h and let go of a rock from the roof (A). Where do you expect the rock to hit the floor? At (B)? Behind (B)? Or ahead of (B)?

[Klaynos]

Doing it with a skateboard or similar should make it safer but still visible on a video.