Physics Help Forum Help calculating power required to lift a mass

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 Jun 12th 2019, 07:20 AM #1 Junior Member   Join Date: Jun 2019 Posts: 2 Help calculating power required to lift a mass Hello, I realize that this is a Physics 101 pop quiz problem, but it's been 45 years since I was in Physics 101 I'm trying to figure out what size of a 12VDC motor to purchase to perform the following work. I have a 40 Kg tool that I want to lift 30cm within 60 seconds. If you need the length of a lever arm to calculate this, then assume a gear wheel of 2.5 cm in diameter. Any help would be appreciated. Thanks, John Last edited by CantBeRocketScience; Jun 12th 2019 at 07:26 AM.
 Jun 12th 2019, 08:07 AM #2 Senior Member   Join Date: Oct 2017 Location: Glasgow Posts: 373 Minimum work done for lifting: $\displaystyle \Delta W = m_{tool} g \Delta h$ If this is to be achieved in a time limit, then the minimum power is: $\displaystyle P = \frac{\Delta W}{\Delta t} = m_{tool} g \frac{\Delta h}{\Delta t}$ Therefore, the minimum power of the generator, $\displaystyle P_{generator} = P = m_{tool} g \frac{\Delta h}{\Delta t}$ Substituting values: $\displaystyle P_{generator} = 40 \times 9.81 \times \frac{0.3}{60} = 1.962 W$ However, this doesn't take into account many things, such as: i) efficiency of the power generator and other equipment that may be involved; ii) a driving force that may not be perpendicular to the vertically upwards direction; iii) additional forces that oppose the driving force (like friction);
 Jun 12th 2019, 08:31 AM #3 Junior Member   Join Date: Jun 2019 Posts: 2 Thank you benit13 Hmm, The math seems right. 40kg x 9.81 = 392.4 Newtons 392.4 Newtons X 0.3 meters = 117.72 Newton-meters 117.72 Newton-meters / 60 seconds = 1.962 Newton-meters/second 1 Newton-meter/second = 1 W thus 1.962 Watt However, 1.962 W doesn't seem like very much if 1 HP = 746 Watts. I would have ballparked it in the vicinity of 1/4 hp (+/- 1/8hp) or 10 times the calculated power needed. John Last edited by CantBeRocketScience; Jun 12th 2019 at 09:19 AM.
Jun 12th 2019, 09:20 AM   #4
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 Originally Posted by CantBeRocketScience Hmm, 1.962 W doesn't seem like very much if 1 HP = 746 Watts. I would have ballparked it in the vicinity of 1/4 hp (+/- 1/8hp) John
A lift of 30cm in 60 seconds is pretty slow.

Ignoring any non-conservative losses, a power of 1/8 HP would raise the 40kg mass 30 cm in a little over a second.

Jun 12th 2019, 02:55 PM   #5
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 Originally Posted by CantBeRocketScience However, 1.962 W doesn't seem like very much if 1 HP = 746 Watts. I would have ballparked it in the vicinity of 1/4 hp (+/- 1/8hp) or 10 times the calculated power needed. John
Agreed, it's a minimum power; it's the value required to raise the object without any waste energy over the whole 60 seconds.

If the additional factors taken into account at the end of my post are considered, the model will become more sophisticated, but will yield larger power values that will be more realistic.

 Jun 12th 2019, 05:18 PM #6 Senior Member   Join Date: Apr 2017 Posts: 486 This motor is perfect for what you want ... 7W ..too much power ...24V or 12V ... there are dozens of different designs on eBay , many different RPMs https://www.ebay.co.uk/itm/FJ-107RPM...YAAOSwsQNcviTH The one above has built in gears.... Motors of the size pictured above have an efficiency better than 80%( output feeding gears/electrical power IxV) ... Then losses in the gears perhaps 30% .... maybe 10% loss in mechanism to lift ....So a 4Watt motor wold do it ...lets say 5W to be sure.... Last edited by oz93666; Jun 12th 2019 at 05:56 PM.

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