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Also find the power delivered by gravity

Displacement of the object $$s=\frac 12 at^2$$ $$s=\frac 12.4$$ $$s=2m$$ Work done $$W=F.s$$ Force applied is $$F=ma$$ $$F=100$$ Therefore $$W=200J$$ Power $$P=200/2=100W$$ But the answer is $2200W$. What’s going wrong?

Aditya
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1 Answers1

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We have that

$$F-mg =ma\implies F=1100 N \implies dW=F\cdot tdt \implies P(t)=\frac{dW}{dt}=F\cdot t$$

Note that

  • the mean power is $\frac{W_{tot}}{t_{fin}}=1100 W$

  • the power at time $t=2$ is $P(2)=F\cdot 2=2200 W$

user
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  • Okay, that was a very bad mistake on my part. Thanks a lot though! – Aditya Nov 03 '19 at 12:26
  • @Aditya You are welcome! Bye – user Nov 03 '19 at 12:28
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    The force acted by gravity is equal to $-1000 N$ therefore the power by gravity at time t=2 is equal to $-2000 W$. Indeed we had $200 W$ without gravity and we need $2000 W $ more to lift. – user Nov 03 '19 at 12:33