Solving 2 Motion Questions: Boat and Plane

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The discussion revolves around solving two physics problems involving a boat towing skiers and a plane in circular motion. For the boat question, participants clarify that if the skiers are moving at constant velocity, the net force is zero, meaning the tension in the ropes equals the force exerted by the boat. In the plane question, the focus shifts to calculating the radius of the loop using centripetal force, with participants noting that the mass cancels out in the equations. The conversation emphasizes understanding the relationship between forces and motion rather than needing all variables explicitly. The participants work collaboratively to clarify concepts and derive solutions.
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Homework Statement


1. Boat is towing two water skiers, each skier has it's own rope that's at 40 degrees the boats axis and the tension is 450N, how much force does the boat have to have to keep them all moving?

2. The plane is traveling at 200 km/h at the bottom of the loop, and the scale (the pilot happens to be sitting on one) reads 4x normal weight/Fg. What is the radius of the loop.

Homework Equations


1. I don't really know...

2. I don't really know...

The Attempt at a Solution


My teacher put these on an assignment that's due tomorrow, so I really need some info fast.

I'm thinking she missed out on some variable... Please correct me if I'm wrong.
 
Last edited:
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tornzaer said:

Homework Statement


1. Boat is towing two water skiers, each skier has it's own rope that's at 40 degrees the boats axis and the tension is 450N, how much force does the boat have to have to keep them all moving?

2. The plane is traveling at 200 km/h at the bottom of the loop, and the scale (the pilot happens to be sitting on one) reads 4x normal weight/Fg. What is the radius of the loop.

Homework Equations


1. I don't really know...

2. I don't really know...

The Attempt at a Solution


My teacher put these on an assignment that's due tomorrow, so I really need some info fast.

I'm thinking she missed out on some variable... Please correct me if I'm wrong.
"I don't really know" isn't an attempted solution. There are no missing variables, the questions are complete and can be answered as set.
 
Last edited:
See, the equations I got in my lessons are of circular motion (v=2piR/T) and a pulley equation a=ft/m.

I don't know how to fit them in. Could you please at least direct me in the right direction?
 
tornzaer said:
See, the equations I got in my lessons are of circular motion (v=2piR/T) and a pulley equation a=ft/m.

I don't know how to fit them in. Could you please at least direct me in the right direction?
Well both those equations are irrelevant for the first question since the skiers aren't undergoing circular motion.

For the second question (which is circular motion), do you have an equation connecting centripetal acceleration/force and velocity?
 
I have many equations for circular motion: v=2piR/T a=2piv/T a=4piR/T^2 a=v^2/R

Also centripetal force: Fcent=mv^2/R Fcent=m4pi^2R/T^2 Fcent=m2piv/T

Basically, for circular motion, I don't possesses the period and for the centripetal force, I don't have mass and period.
 
tornzaer said:
Fcent=mv^2/R
This equation looks appropriate for the second question.
 
Hootenanny said:
This equation looks appropriate for the second question.
I was looking at that too, but I need a mass variable, which the questions is lacking. Am I overlooking something?
 
tornzaer said:
I was looking at that too, but I need a mass variable, which the questions is lacking. Am I overlooking something?
What is the net force acting on the pilot at the bottom of the loop?
 
Right, it's 4x Fg.

What I'm understanding is this:
Fg=mg
Fg=m(9.8m/s^2)

Then Fg would be multiplied by 4, which will give me the Fcent. Then just plug it into the equation to get R.
 
  • #10
tornzaer said:
Right, it's 4x Fg.
You're close, but not quite correct. 4mg would be the reading on the scales, which is the normal reaction force of the scales, but this is not the same as the net force acting on the pilot. You need to find the net force acting on the pilot and use this as your centripetal force.

Do you follow?
 
  • #11
I know that at the bottom of a "loop", Fcent is Ft-Fg. Is this what you are getting at?
 
  • #12
tornzaer said:
I know that at the bottom of a "loop", Fcent is Ft-Fg. Is this what you are getting at?
Looks good to me :approve:
 
  • #13
Very well. Still, I'm in need of a mass, which the question lacks...

As for my first question, could you steer me as well? My substitute teacher gave us this and I think she messed up on a couple of lessons.
 
  • #14
tornzaer said:
Very well. Still, I'm in need of a mass, which the question lacks...
Are you sure you need to know the mass? :wink:
tornzaer said:
As for my first question, could you steer me as well? My substitute teacher gave us this and I think she messed up on a couple of lessons.
If you assume that the skiers are traveling at a constant velocity, what can you say about the net force acting on them?
 
  • #15
Hootenanny said:
Are you sure you need to know the mass? :wink:

If you assume that the skiers are traveling at a constant velocity, what can you say about the net force acting on them?
Mass: not following you there, I'm afraid.

1st question: No acceleration means no net force. Fnet=Ft? So Ft would be 0.
 
  • #16
tornzaer said:
Mass: not following you there, I'm afraid.
Try writing out the equation that I quoted before.
tornzaer said:
1st question: No acceleration means no net force. Fnet=Ft? So Ft would be 0.
Correct and the same goes for the boat.
 
  • #17
Alright, first is done. :P

Second:
Fcent=mv^2/R
Ft-Fg=m*(55.55m/s)^2/R
Please clarify on the next step.
 
  • #18
tornzaer said:
Alright, first is done. :P

Second:
Fcent=mv^2/R
Ft-Fg=m*(55.55m/s)^2/R
Please clarify on the next step.
Well what is Ft-Fg?
 
  • #19
Ft-Fg=Fcent=Fnet
 
  • #20
Please respond... this thing is due tomorrow.
 
  • #21
tornzaer said:
Ft-Fg=Fcent=Fnet
That is correct, but explicitly in terms of mg?
tornzaer said:
Please respond... this thing is due tomorrow.
Please be patient, we all provide help here voluntarily. Plus, when you posted your last reply is was gone 2AM in the UK - we all have to sleep :zzz:
 
  • #22
Right, the g is 9.8m/s^2. What about the mass though? That's where I've been stuck for a while.

And sorry for being impatient. :P Didn't know you were in the UK.
 
  • #23
tornzaer said:
Right, the g is 9.8m/s^2. What about the mass though? That's where I've been stuck for a while.
If you write out the equation, you'll find that you don't need to know them explicitly.
 
  • #24
I see... so you are suggesting giving the answer with an unsolved variable, like m(whatever)?
 
  • #25
tornzaer said:
I see... so you are suggesting giving the answer with an unsolved variable, like m(whatever)?
No, I'm saying that if you write out the equation then you will be able to cancel the masses.

Just try writing out the equation.
 

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