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Classical Mechanics Level 1

Climbing out of bed is sometimes hard in the morning. If you have a mass of 60 kg 60~\mbox{kg} , what is the minimum amount of work in joules you have to do to get out of bed? Assume your center of mass when lying in your bed is 0.75 m 0.75~\mbox{m} above the floor, and your center of mass when standing is 1.25 m 1.25~\mbox{m} above the floor.

Details and assumptions

  • The acceleration due to gravity is − 9.8 m/s 2 -9.8~\mbox{m/s}^2 .


The answer is 294.

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David Mattingly by David Mattingly

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9 solutions

Muneeb HaXan
Dec 2, 2013

the formula for computing the work done is W=fd. for 'f' we have f=mg (m=60kg and g=9.8) f=588 now we fine distancd 'd' d=1.25-0.75=0.5 hence w=294

11 Helpful 0 Interesting 0 Brilliant 0 Confused

mgh = 60 X 9.8 X 0.5 = 294.

7 Helpful 0 Interesting 0 Brilliant 0 Confused

like it

Rajput Rajput - 7 years, 6 months ago
Alex Dias
Dec 4, 2013

Using the Formula of Gravitational Potential Energy , So We Have :

E p = m g h E_{p} = mgh

m = M a s s m = Mass

g = G r a v i t y g = Gravity

h = H e i g h t h = Height

h = 1 , 25 − 0 , 75 = 0 , 5 h = 1,25 - 0,75 = 0,5

E p = 0 , 5 × 60 × 9 , 8 = 294 E_{p} = 0,5 \times 60 \times 9,8 = 294

4 Helpful 0 Interesting 0 Brilliant 0 Confused

this is so easy

Ali Raza Soomro - 7 years, 6 months ago

I was first not able to understand the question properly but now got it that we need to find the resultant work done = final work done - initial work done . that has been demonstrated by finding the difference of the given hights . well solved !!

Devesh Rai - 7 years, 6 months ago
Vinay Saketh
Dec 2, 2013

the energy required to get up is the work done in getting up.. thus w=fXs...i.e forceXdisplacement... force=60X9.8=588 displacement=let us asume a quadrant axis thus on bed center of mass is (0.75,0) on standing center of mass is (1.25,0)... thus the displacement i.e x2-x1=1.25-0.75=0.5 workdone=588X0.5=294

2 Helpful 0 Interesting 0 Brilliant 0 Confused
Neha Sharma
Dec 7, 2013

we will use here formula:-(mgh) h=difference in center of mass =60 9.8 (1.25-0.75)=294

1 Helpful 0 Interesting 0 Brilliant 0 Confused
Rittik Das
Dec 5, 2013

We all know that F=M A.Mass is already given n that is 60kg.Acceleration due to gravity is -9.8m/sec^2..Displacement = 1.25m-0.75m =0.50m.Work done =f s...

1 Helpful 0 Interesting 0 Brilliant 0 Confused
Tiara Novianti
Dec 5, 2013

Diketahui = m = 60 kg s = 0.5 m (1.25-0.75) g = 9.8 m/s2 Ditanya = W...? Jawab = W = F.s

F= m.a F= 60 x 9.8 F= 588 Newton

W= F.s W= 588.05 W= 294 Joule

1 Helpful 0 Interesting 0 Brilliant 0 Confused

First, we must convert mass in to weight in Newtons (or force) by multiplying 60kg by 9.8 m/s^2 . This gives 588N. Next, given that work= force x distance, we use this value to multiply by 1.25m to give 735 J, and doing the same with 0.75m we get 441 J. Finally, we subtract 441J from 735J to get our answer: 294J.

1 Helpful 0 Interesting 0 Brilliant 0 Confused
Pham Tung
Dec 8, 2013

You use Law of conservation of energy : Delta W = m . g .(h1 - h2 ) = 60 x 9,8 x 0,5 = 294 (J)

0 Helpful 0 Interesting 0 Brilliant 0 Confused

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