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# Physics Work, Energy and Power watch

1. can someone help me with this question:
A small electric motor is 20% efficient. It's input power is 9.6W when it is lifting a mass of 0.5kg at a steady speed v.
What is the value of v?
A 0.39m/s
B 2.0m/s
C 2.8m/s
D 3.8 m/s
2. (Original post by josiehicks99)
can someone help me with this question:
A small electric motor is 20% efficient. It's input power is 9.6W when it is lifting a mass of 0.5kg at a steady speed v.
What is the value of v?
A 0.39m/s
B 2.0m/s
C 2.8m/s
D 3.8 m/s
Is it C? I worked it out and got that
3. (Original post by Sunethra)
Is it C? I worked it out and got that
unfortunately not the answer is A according to the textbook.
4. (Original post by josiehicks99)
unfortunately not the answer is A according to the textbook.
I got 2.77ms-1
5. (Original post by josiehicks99)
unfortunately not the answer is A according to the textbook.
A person in my class told me you use P=Fv to find v where P is 9.6 x 0.2 and F is 0.5g and that gives you A.
6. (Original post by josiehicks99)
can someone help me with this question:
A small electric motor is 20% efficient. It's input power is 9.6W when it is lifting a mass of 0.5kg at a steady speed v.
What is the value of v?
A 0.39m/s
B 2.0m/s
C 2.8m/s
D 3.8 m/s
Apply the known relation . You know the force on the object as .

From this you can calculate the answer knowing that the motor is only 20% efficient (so the effective power is 0.2 times the input power). (though in reality electric motors are far far more efficient than this unless operating at very high speeds).

7. here you go. ^¬^ hope you can understand what I did
8. thanks all x
9. (Original post by RizK)

here you go. ^¬^ hope you can understand what I did
WHAT?! Since when did 1.92÷4.9 = 3.191???

1.92÷4.9 = 0.39...
10. (Original post by Jeeves21)
WHAT?! Since when did 1.92÷4.9 = 3.191???

1.92÷4.9 = 0.39...
whoops
11. 20% of 9.6 is 1.92 so divide 1.92 by weight we get 0.39

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