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10 Common Problems of Work and Power - Junior Physics

physics.fisikastudycenter.com - Learning work and power in 10 common questions and the solutions. The work done by the forces, the power and the difference of gravitational potential energy will be involved. Junior high school grade 8.

Problem 1
A body moves through a displacement of 4 m while a force F of 12 Newton acts on it.



What is the work done by the force on the body?

Answer
Work = force x displacement
W = F x S
W = 12 x 4
W = 48 joule

Problem 2
A block is pulled by a constant force of 40 Newton. If the work done by the force on the block is 680 joule what is the displacement of the block?

Answer
The displacement of the block
W = F x S
680 = 40 x S
S = 680 / 40
S = 17 meter

Problem 3
A block displacement cause of a force in a horizontal surface is 13 meter and the work done by the force on the block is 15.6 joule. Find the magnitude of the force!

Answer
Work = force x displacement
W = F x S
15.6 = F x 13
F = 15,6 / 13
F = 1.2 Newton

Problem 4
Two forces that are F1 = 10 N and F2 = 5 N act on a body in a frictionless floor. The displacement of the body is 5 m, what is the work done by the forces on the body!



Answer
W = (F1 + F2) x S
W = (10 + 5) x 5
W = 15 x 5
W = 75 joule

Problem 5
Given two forces act on a block. F1 = 15 N and F2 = 7 N. If the displacement of the block while the two forces act, find the work done on the block!



Answer
W = (F1 − F2) x S
W = (15 − 7) x 6
W = 8 x 6
W = 48 joule

Problem 6
A work done on a block through two forces is 120 joule.



If the displacement of the block is 5 m, determine the magnitude of F2!

Answer
W = (F1 − F2) x S
120 = (36 − F2) x 5
120 / 5 = 36 − F2
24 = 36 − F2
F2 = 36 − 24
F2 = 12 Newton

Problem 7
A boy lifts up a book from the floor to the table. The mass of the book is 300 gram and the height of the table is 80 cm.



Find the work done by the boy when the acceleration due to gravity is 10 m/s2

Answer
Finding the work done by calculating the difference of gravitational potential energy of the book, where the gravitational potential energy is Ep = m x g x h

W = Δ Ep
W = m x g x h
W = 0,300 x 10 x 0,80
W = 2,4 joule

Problem 8
A boy of 450 N of weight climbs a wood steps with a lenght of 3 m. The boy takes 6 seconds to get the end of the steps. What is the power done by the boy on this activity!

Answer
The power (P), the work (W) and the time (t) relationship:
P = W/t

where W = (weight of the boy) x (the boy displacement) = 450 x 3 = 1350 joule

So then :
P = W/t
P = 1350 / 6
P = 225 watt

Problem 9
In two minutes the energy dissipated by a lamp is 3000 joule. Find the power of the lamp!

Answer
The data, 2 minutes = 120 sec

P = W/t
P = 3000 / 120
P = 25 watt

Problem 10
What is the work done by this boy?



A box of 50 Newton of weight is brought by a boy from A to B and then brought back to A.

Answer
W = 0 joule
(There is no displacement of the box)

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