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Question of Force - why it differs in reality?

  • #1
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Homework Statement


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Homework Equations




The Attempt at a Solution


could you give me the reason for part i. Can't figure the reason
for the second part. it says ans is 1080N to left. shouldn't it be 900N. and why is it to the left?
 

Answers and Replies

  • #2
CWatters
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What's the difference between an ideal winch and a real winch?
 
  • #3
CWatters
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for the second part. it says ans is 1080N to left. shouldn't it be 900N. and why is it to the left?
The tension T is 900N but that's not the only force on the drum/axle.

The tension and one other force act to the right. If the drum/axle doesn't accelerate to the right there must be another force acting to the left so that the sum of all forces on the drum/axle is zero.
 
  • #4
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What's the difference between an ideal winch and a real winch?
is work done as friction. what exactly would be the wasted energy here?
The tension T is 900N but that's not the only force on the drum/axle.

The tension and one other force act to the right. If the drum/axle doesn't accelerate to the right there must be another force acting to the left so that the sum of all forces on the drum/axle is zero.
if it's the reaction of 180N, won't it be on the axle itself, but how does the trailer exert this force. the axle is in the air?
 
  • #5
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The tension T is 900N but that's not the only force on the drum/axle.

The tension and one other force act to the right. If the drum/axle doesn't accelerate to the right there must be another force acting to the left so that the sum of all forces on the drum/axle is zero.
another thing. by calculation, i went as follows, Tx0.08 = 180x0.4 giving T = 900N
why it don't understand why it's this way. isn't there actually a resultant moment there. so, why should we equate the moments. if it was force, then I understand from newton's 3rd law, its a reaction. but why is it this way here??? i only got the numerical value without understanding what's really happening and why? could you explain plz.
 
  • #6
NascentOxygen
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if it's the reaction of 180N, won't it be on the axle itself, but how does the trailer exert this force. the axle is in the air?
The vertical supports holding the axle are part of the trailer. If the axle is to remain in place, there must be a horizontal force to the left balancing the sum of horizontal forces to the right.
 
  • #7
NascentOxygen
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another thing. by calculation, i went as follows, Tx0.08 = 180x0.4 giving T = 900N
That's the tension in the rope.
so, why should we equate the moments. if it was force, then I understand from newton's 3rd law, its a reaction. but why is it this way here??? i only got the numerical value without understanding what's really happening and why? could you explain plz.
The winch's handle length vs. drum radius offer a mechanical advantage. A narrower drum means the handle will be easier for you to turn, but you'll have to turn it more times to draw in the same length of rope.
 
  • #8
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That's the tension in the rope.

The winch's handle length vs. drum radius offer a mechanical advantage. A narrower drum means the handle will be easier for you to turn, but you'll have to turn it more times to draw in the same length of rope.
yeah, but why equate the moment. shoudl there be a resultant moment, for the drum to rotate
 
  • #9
CWatters
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is work done as friction. what exactly would be the wasted energy here?
Nothing to do with energy... The axle is subject to three torques that sum to zero if it's rotating at constant (angular) velocity. The input torque, the output torque and the torque due to friction.

yeah, but why equate the moment. shoudl there be a resultant moment, for the drum to rotate
No. If you assume the drum is revolving at constant (angular) velocity then it's not accelerating and the torques must sum to zero.

if it's the reaction of 180N, won't it be on the axle itself, but how does the trailer exert this force. the axle is in the air?
Via the bearing.

If it's not accelerating the linear forces sum to zero.
 
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  • #10
NascentOxygen
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If the boat is being drawn in at a steady speed, you can consider that an equilibrium, so sum of forces is zero.
 
  • #11
CWatters
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Might help if you can see the similarity between...

Force = mass * acceleration
and
Torque = moment of inertia * angular acceleration
 

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