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Question

Question 1: If April lifts two books from the floor and balances them on her head. Does she does work? Why?

Question 2: If april now walks across the room with the books on her head does she work? Why?
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Question 3: Is work done when you move a book from the top of the desk to the floor? Why?

Answer 1

Work is done when the force and the direction of motion have parallel components, that is, they aren't perpendicular. So in Q1, she does work because she lifts the books against gravity by applying an upwards force and moving them upwards. In Q2, she does no work, because she doesn't exert a force to the side as she moves at constant velocity, but that's the only direction the books move; we typically ignore friction and air resistance in this case. In Q3, she does negative work, because the only force she exerts is upwards, to support the books against gravity, but they move downwards.

Answer 2

as you might know,
work done = Force * Displacement * cos(x); where x is the angle between the direction of force and the displacement.

In english, this means, work is done by a force, when it is able to produce displacement in its own direction[given by the cos(x) component of displacement.

so, in this case, the only force is gravity
ans 1)
Gravity is downwards, displacement is upward (lifting of book).
Hence cos(x) = cos(180)= -1. Work is done when lifting the book.

Balancing on head means no displacement. So no work

ans 2)

walking across room: displacement is horizontal,force (gravity) is vertical
so, cos(X)=cos 90 = 0. Hence no work.

ans 3) moving from top of desk to floor: displacement is vertical. It supplements gravity. So cos x =cos 0 =1. So work is done.

Answer 3

Actually there are two concepts of work.
One is the work done by sb/sth and the other is the work done to sb/sth.

Q. 1
April lifted the books from the floor and up to her head.
The work done by her = the work done to the books
= F.d where F=force and d=distance
= ma.d (Since Newton's 2nd law, F=ma)
d=the distance from the surface of the floor to her head.

The work done to her = ma.d
= ma.(0)
= 0


Q. 2
The work done by her = ma.d
where d=the distance she travelled across the room


Q. 3
Yes, the work is done when u move a book from the top of the desk to the floor. To be exact, the work in that particular case is what we called the potential energy.

The work done to the book = ma.d

d=the distance from the top of the desk to the floor.




How simple physics are

Answer 4

1: yes,because she lift the books against the gravity
2: no, because the books are at the same level.
3: no, because you did not lift against the gravity but follow the direction of gravity.

Answer 5

1 yes because any work takes place when push or pull isd done and picking up the book is a pull .

2) yes because there is movement

3)yes

Answer 6

...QI.....April does the work of picking up the book and balancing it on her head......
...Q2...now April works of balancing the book on her head and walk across the room....
...Q3...there is no work done when u drop the book..................u just drop the book on the floor ...............................because the place the book is on the table and not the floor................

Answer 7

W = Fuerza x distancia x cos (theta)

use this

1. Yes....

2. Not.

3. Not

Answer 8

1. Yes.... W=Fnet * d.... work done against gravity

2. No.... W=Fnet * d..... the force is perpendicular to the distance(force upward; distance sidward) W=0..... because Fnet=Fcos90°=0..... W=0d.... therefore. W=0J

3. Yes.... W=Fnet * d. work done with/by gravity....

Answer 9

Muscles are not machines you use energy lowering a book unless you let it fall, the slower you you lower it the more work you perform.

Answer 10

Good question