Physics Help Forum Work, power and rotational motion problem

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 Jan 5th 2018, 07:01 AM #1 Member   Join Date: Jan 2015 Posts: 96 Work, power and rotational motion problem Hi Everyone, Happy new year to you all. It's been a wee while since I was on the forum .. been a bit unwell for a few weeks .. but I'm better again. I have a problem that has taxed my understanding of the subject matter in the title, namely work-power and rotational motion. The problem is as follows - PROBLEM The carbide tips of the cutting teeth of a circular saw are 8.6 cm from the axis of rotation. (a) The no load speed of the saw when it's not cutting anything is 4800 rev/min. Why is it's no load power negligible ? There is a part (b) to this problem but it is part (a) that has got me stumped. I have no idea why this should be especially when in part (b) it says that when cutting wood the angular speed halves (2400 rev/min) yet the power output here is 1.9 hp ???? I have racked my brains and gone over again the chapter in the book to which the problem pertains, but I am no further forward in understanding this ... can anyone help me to understand what is going on here ? Thank you. Jackthehat
Jan 5th 2018, 08:47 AM   #2
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 Originally Posted by jackthehat Hi Everyone, Happy new year to you all. It's been a wee while since I was on the forum .. been a bit unwell for a few weeks .. but I'm better again. I have a problem that has taxed my understanding of the subject matter in the title, namely work-power and rotational motion. The problem is as follows - PROBLEM The carbide tips of the cutting teeth of a circular saw are 8.6 cm from the axis of rotation. (a) The no load speed of the saw when it's not cutting anything is 4800 rev/min. Why is it's no load power negligible ? There is a part (b) to this problem but it is part (a) that has got me stumped.I have no idea why this should be especially when in part (b) it says that when cutting wood the angular speed halves (2400 rev/min) yet the power output here is 1.9 hp ???? I have racked my brains and gone over again the chapter in the book to which the problem pertains, but I am no further forward in understanding this ... can anyone help me to understand what is going on here ? Thank you. Jackthehat
Hello Jack,

The conditions will apply once the saw has reached its steady speed.
Stopping and starting are not included.

So when the saw is running at a steady speed the power to accelerate the blade from standing still has already been expended.

So what power is required to keep the blade running?

Only the power to overcome the running resistance which has two parts.

The bearing friction and the blade friction against the air.

What do you think the designers do about these?

There will be a couple of % extra because no machine is 100% efficient, but electric motors on no load are at least 98% efficient.

What do you think happens to the 2% ineficiency plus the frictional losses?

Jan 5th 2018, 10:03 PM   #3
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Join Date: Apr 2017
Posts: 410
 Originally Posted by jackthehat .... it says that when cutting wood the angular speed halves (2400 rev/min) yet the power output here is 1.9 hp ????
Don't think because the blade is spinning fast that the power output/input is high . When not cutting it's not doing any work ... except as studiot pointed out , some air resistance , bearing resistance, so this is when it draws the least electrical power from the supply.

As soon as the blade starts to cut , this will slow the speed ,and the motor will draw more power from the electricity supply..

I think studiot , might be off on efficiency ... the type of motors normally used for saws and angle grinders are brushed they have two carbon 'brushes' feeding power to a split ring commutator , these brushes wear out and need replacing , but this type of motor gives high speed and power for it's size , efficiency never more than 80% ... and when no load(max revs) , or jammed (zero revs) ... the motor is 0% efficient

" Definition. Electric motor efficiency is the ratio between power output (mechanical) and power input (electrical)"

 Jan 6th 2018, 04:46 AM #4 Member   Join Date: Jan 2015 Posts: 96 Hi Studiot, Thank you for taking the time to respond to my problem. I see it now that you have explained the situation so succinctly. I was thinking that because it was moving at speed then it had to be expending considerable power ... my understanding of the situation in this case was flawed and so I couldn't think why the power should be in fact negligible. I have got it now. Thanks again Regards, Jackthehat
 Jan 6th 2018, 04:47 AM #5 Member   Join Date: Jan 2015 Posts: 96 Hi Oz93666, Thank you for taking the time to respond to my problem. I see it now that you have explained the situation so succinctly. I had thought that because it was moving at speed then it had to be expending considerable power ... my understanding of the situation in this case was flawed and so I couldn't think why the power should be in fact negligible. I have got it now. Thanks again Regards, Jackthehat

 Tags motion, power, problem, rotational, work

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