Does Pushing a Ship from a Ramp Affect Its Movement?

  • #1
Deej
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TL;DR Summary
Question about pushing a ship
If you push a floating object from land, the object should move away.

If you push a floating object from the floating object, the force should be equal and opposite, and floating object won't move.

What if you push a floating object from a ramp, where the ramp is connected to the floating object but also resting on the shore?

Think of a roll-on roll off-ferry, where vehicles drive on to the ferry via a ramp. But imagine there is a closed door at the top of the ramp and a car is on the ramp pushing against the closed door. (Ignore momentum and inertia - ie assume the car gently rests against the door before increasing force gradually and that the force is enough to over come the ship's inertia)
 
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  • #2
Do you have a diagram of your set-up?
 
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  • #3
No but I'll get one. May take a while

Note - I'd have assumed force on door equals force on ramp and the vessel doesn;t move but we had an analogous incident recently where the boat did move. Perhaps coincidentally due to something else of course...
 
  • #4
Deej said:
What if you push a floating object from a ramp, where the ramp is connected to the floating object but also resting on the shore?
What matters is if there is a horizontal force between the ramp and the shore. Does the force of the car on the ramp tend to change the ramp inclination, in a way that could lead to such forces?
 
  • #5
Not really. The position of the car on the ramp effects where the weight is and so how high the boat sits in the water, but the act of pushing against the door won't effect this much. And it shifting of weight/angle of ramp happens under normal circumstances - ie every time a car comes on/off - without affecting the ramp.
 
  • #6
Is the door attached to the ramp or the land?
 
  • #7
Deej said:
TL;DR Summary: Question about pushing a ship

the force should be equal and opposite,
Do not confuse Forces in Equalibrium with Newton's third law. There is a reaction force from the quayside and also a reaction force against your foot (or whatever) from the boat (unless there's oil on the deck). The force from your foot is NOT equal to the force from the water so you and the boat will move away from the quay.
To get an idea of what really goes on here, use a simpler model before bringing in ramps and things. Newton rules here, as everywhere else.

When a RORO ship comes to shore, they tie it to the shore or the wheels of every car leaving it will be pushing it off backwards. Also a car with front wheel drive will tend to pull the ship onto the shore.

People who are used to getting on and off boats get into the habit of using their feet to keep the boat I contact with the quay, getting on or off.
 
  • #8
Ibix said:
Is the door attached to the ramp or the land?
the door being pushed on it part of the ship and located at the top of the ramp - ie if open you drive up the ramp and through the door on to the deck
 
  • #9
sophiecentaur said:
Do not confuse Forces in Equalibrium with Newton's third law. There is a reaction force from the quayside and also a reaction force against your foot (or whatever) from the boat (unless there's oil on the deck). The force from your foot is NOT equal to the force from the water so you and the boat will move away from the quay.
To get an idea of what really goes on here, use a simpler model before bringing in ramps and things. Newton rules here, as everywhere else.

When a RORO ship comes to shore, they tie it to the shore or the wheels of every car leaving it will be pushing it off backwards. Also a car with front wheel drive will tend to pull the ship onto the shore.

People who are used to getting on and off boats get into the habit of using their feet to keep the boat I contact with the quay, getting on or off.
In this situation, the car is not in direct contact with the shore. It's on the ramp, (the ramp is attached to the ship and touching the shore.)

(Also, assume the ferry is not tied to the shore for this example.
And I don't think it matters for this example, but if it does, assume rear drive)
 
  • #10
Deej said:
In this situation, the car is not in direct contact with the shore. It's on the ramp, (the ramp is attached to the ship and touching the shore.)
So the ship and ramp are a single rigid object.
The car is on the ramp/ship.

1695300658944.png


Its force will have no effect on the ship' position relative to the shore.

Why do you think it would?
 
  • #11
DaveC426913 said:
So the ship and ramp are a single rigid object.
The car is on the ramp/ship.

Its force will have no effect on the ship' position relative to the shore.

Why do you think it would?

I don't. But we had similar situation, where the ship moved, and we're not sure why. My assumption is it was just a coincidence as there's a fair few other things going on - tide, wind, current, ship's engine is used to push on to the slip to prevent movement as vehicles move on to the ramp etc.... but thought I'd check
 
  • #12
Deej said:
...there's a fair few other things going on - tide, wind, current, ship's engine is used to push on to the slip to prevent movement as vehicles move on to the ramp etc....
This.

There is one thing the car can do. Start by removing the door from the scenario. If the car accelerated and moved any significant distance/speed on the boat, including the ramp, it could - in principle - cause the boat to move in the other direction.

It would require some unlikely conditions:
  1. A relatively high ratio of car mass to boat mass.
  2. A relatively aggressive movement of the car.
  3. Relatively little friction between shore and ramp, boat and water.
But it could be done.

(If a person in a canoe decides to get up and move to the other end of the canoe, the canoe will move underneath him in the opposite direction.)
 
  • #13
DaveC426913 said:
But it could be done.
There are plenty of occasions where a mass being moved around on a boat will cause the boat to move. That can be because the centre of mass of the boat plus the mass remains stationary so the CM of the boast must move.

I think the problem here is that the precise details of the OP's scenario are too vague. The "Bonk bonk" could imply that a moving car hits the boat and it could be that the car's speed was caused by its acceleration whilst on the ground. It then shares its momentum with the boat on impact.

To 'explain' situations like this and resolve apparent paradoxes you have to 1. Believe that Newton's Laws are followed. 2. That Newton's Laws are followed. 3.,4.,5.,6.. You need to find where the apparent paradox occurs and then believe it's not a paradox so you have to look for the error in describing the situation.
 
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  • #14
sophiecentaur said:
There are plenty of occasions where a mass being moved around on a boat will cause the boat to move. That can be because the centre of mass of the boat plus the mass remains stationary so the CM of the boast must move.

I think the problem here is that the precise details of the OP's scenario are too vague. The "Bonk bonk" could imply that a moving car hits the boat and it could be that the car's speed was caused by its acceleration whilst on the ground. It then shares its momentum with the boat on impact.

To 'explain' situations like this and resolve apparent paradoxes you have to 1. Believe that Newton's Laws are followed. 2. That Newton's Laws are followed. 3.,4.,5.,6.. You need to find where the apparent paradox occurs and then believe it's not a paradox so you have to look for the error in describing the situation.
I said the car rests gently against the door before starting to push :)
 
  • #15
DaveC426913 said:
If a person in a canoe decides to get up and move to the other end of the canoe, the canoe will move underneath him in the opposite direction.

You might have cracked it. If a large vehicle was to starts to move in the opposite direction - ie as to go back off the boat - it could shift the vessel away from the shore?

Real life scenario was quite complex. A large lorry was pushing against a roll of rubber matting which was attached to the ship. The lorries cab was still on the ramp and the rest of the lorry fully on to the boat (note they reverse on). After pushing against the rubber matting the lorry then moved a short distance quite quickly back towards the shore.

To complicate things further, the ramp was resting on a slightly sloping edge, and pushing down on it probably creates force to push the vessel off.
 
  • #16
Deej said:
Real life scenario was quite complex.
Then so must your prediction / explanation be. As I implied earlier, you are chasing your tail if you want to find a mechanical system that fails to obey Newton's laws of motion. You have to quantify and define every single variable involved and the answer will be there.

If you are after finding what actually happened during an actual incident or accident for the purposes of a legal case then I could suggest you are wasting your time. If there was an actual accident the someone may have contravened the safety regs. That would be the only way to get satisfaction; ignore the Physics; the judge wouldn't want to get involved with that.
 

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