What Is the Speed of the Man Relative to the Ground?

[Captain Levi]

Homework Statement

If a river is going 6 km/hr relative to the ground, a boat is going 14 km/hr relative to the river, and a man is jumping from one end of the boat to the other at 6 km/hr relative to the river. What is the speed of the man relative to the ground?

I'm stuck between the answers: 14 km/hr and 26 km/hr.

They are all moving in the same direction

Homework Equations

None that I'm aware of

The Attempt at a Solution

Well if you subtract:
River - Ground=6 km/hr
Boat - River= 8 km/hr
Man - River= 0 km/hr so, it equals 14 km/hr
I got 26 km/hr by just adding them together[/B]

 

[CWatters]

It says the man is jumping at 6km/h relative to the _river_ is that correct?

 

[Captain Levi]

It says the man is jumping at 6km/h relative to the _river_ is that correct?

Yes.

 

[haruspex]

boat is going 14 km/hr relative to the river

From that, how do you get:

Boat - River= 8 km/hr

?

From this:

man is jumping from one end of the boat to the other at 6 km/hr relative to the river.

how do you get:

Man - River= 0 km/hr

?

 

[Captain Levi]

From this video that shows examples of similar problems:

 

[haruspex]

From this video that shows examples of similar problems:

I don't know what part of that you have misunderstood.
Just try using commonsense. If a boat is moving at 14km/h relative to the river, what is the difference in their speeds as measured relative to the bank?

 

[Captain Levi]

Well the ground doesn't move correct? So the river moves 6 km/hr relative to the ground. Since the river is moving 6 km/hr and the boat is moving 14km/hr relative to the river, then you subtract their speeds and so from the perspective of the river the boat is moving 8km/hr.

Honestly I'm just following what my teacher told us and it was something like what I explained in the previous paragraph.

 

[CWatters]

I asked my question above for a reason but you missed it.

If the man moves at 6km/h relative to the river do you even need to know the velocity of the boat?

 

[haruspex]

a boat is going 14 km/hr relative to the river

the boat is moving 14km/hr

Spot the difference.

 

[Captain Levi]

Spot the difference.

Sorry the correct one is the boat is going 14km/hr relative to the river. Edited it.

 

[haruspex]

the boat is moving 14km/hr relative to the river

so from the perspective of the river the boat is moving 8km/hr.

"Relative to the river" and "from the perspective of the river" are the same thing, so how come the speeds are different?

 

[CWatters]

Well the ground doesn't move correct? So the river moves 6 km/hr relative to the ground. Since the river is moving 6 km/hr and the boat is moving 14km/hr relative to the river, then you subtract their speeds and so from the perspective of the river the boat is moving 8km/hr

Think intuitively rather than following a rigid process. ...

If you were in a plane going 100mph relative to the ground and you walk from the back to the front at 10mph relative to the plane, how fast are you going relative to the ground?

If you are walking forwards would you be going faster or slower than the plane?