Question

A 20-Kg rock is at the edge of a 100 m height cliff.
a) What is the rock kinetic energy respect to the base of cliff?
b) if the rock is falling. What is its kinetyc energy right before crashing against the ground?

Answer 1

a) 0 J
b) 20000 J

Answer 2

can you show the work, please?

Answer 3

im really confused

Answer 4

see the rock is stationary on the cliff..so its kinetic energy is zero...get it?

Answer 5

you mean that no movement = kinetic energy =0?

Answer 6

yes

Answer 7

what about the second one?

Answer 8

see now when the rock falls, the only force acting on it is gravity, which is a CONSERVATIVE force...so the total mechanical energy remains conserved at all points

Answer 9

in that case
ME (initial) = ME(final)

Answer 10

ok

Answer 11

KE(i) + PE(i) = KE(f) +PE(f)
0 + mgh = KE +0
KE = 20(10)(100)

Answer 12

get it?

Answer 13

what does the "i" and "f" inside the brackets mean?

Answer 14

initial and final

Answer 15

does this equation works for all cases when something falls?

Answer 16

yes..coz only gravity acts..which is a conservative force

Answer 17

ill tell u one more concept which can be used....anywhere

Answer 18

should i?

Answer 19

sure, thank you so much. I really need that.

Answer 20

its called the work-energy theorem
The total work done by all the forces acting on a body equals the change in its kinetic energy

Answer 21

try doing this with the nw concept

Answer 22

I have to solve tree exercises. Can you help me with them first, please?

Answer 23

wts tree exercise?

Answer 24

three exercises. Dam Ecuadorian that cannot speak English very well lol

Answer 25

can you help me?

Answer 26

no prob
as long as the battery on my laptop allows...fire away quickly

Answer 27

Initially, a 2 Kg rock is at reast. It losses 400J of potential energy as it falls to the ground.
a) Find the kinetic energy that the rock gains as it falls
b)what is the speed of the rock right before it hits the ground

Answer 28

since energy is again conserved
Loss in Potential Energy = Gain in kinetic energy

Answer 29

400=KE=0.5mv^2
400=v^2
v=20

Answer 30

get it?

Answer 31

No 100% but today im not sleeping in order to undertand it. Help me with the last one

Answer 32

go on

Answer 33

Mary weights 420N and is sitting on a swing . The swing is 0.4 m from the ground. Thomas push the swing until it is at 1m from the ground.

Answer 34

a) what is the speed of the swing (mary) when it reaches the lowest position?

Answer 35

mg(1-0.4) = 0.5mv^2
g(0.6) = 0.5v^2
v=root(12)

Answer 36

b) if in its lowest position the speed of mary is 2m/s.. What work does do friction do over the swing?

Answer 37

now use Work-Energy Theorem
W(friction) + W(gravity) = Change in KE
W(friction) + mg(0.6) = 0.5 x m x 2^2
W(friction) = 2m -6m
=-4m = -168 J

Answer 38

this theorem is very useful

Answer 39

Thank you very much. Im gonna review the math behind work and energy and then im going to try to understand the problems you helped me with. Thanks him1618 im becoming you fan :)

Answer 40

its very easy when u start playing around with calculus

Answer 41

calculus?

Answer 42

yes

Answer 43

understanding math in work and energy and the whole of mechanics is easy when u play with calculus

Answer 44

Im not so bad at calculus; but i dont understant that lol

Answer 45

for eg

Answer 46

dW = Fdx = mdvdx/dt (as F = mdv/dt)
= mvdv(as dx/dt=v)
= 0.5mv^2

Answer 47

so u c that the work done by all forces is the change in kinetic energy

Answer 48

ahhh got you. Do you think I can handle potential and kinetic energy in one week? Is that easy?

Answer 49

My maths are no so bad. Physic topics are more difficult for me to understand

Answer 50

physics and maths correlate

Answer 51

just do it topicwise

Answer 52

Work and its definitions Conservative and NonConservative Forces Potential Energy Conservation of Energy Work-Energy Theorem Power

Answer 53

Ok. By the way. Where are you from? People always help me and I dont even know where they are from . Im Luis and Im from Ecuador :)

Answer 54

Himanish from India

Answer 55

Nice. country!

So I really appreciate you help Himanish. Im more relaxed now because of your help. Now im gonna sleep coz it is 2 am here. Hope we get in touch another day. Thank you

Answer 56

welcome...12 pm here..gnite though

Answer 57

:)

Answer 58

hey

Answer 59

Hi Himanish!

Answer 60

what time is in your country now. Here is almost 2 PM.

Answer 61

11.45 pm

Answer 62

are goint to sleep now. Or you work like me, at nights?

Answer 63

night

Answer 64

haha. That is cool dude. I cannot concentrate in the day

Answer 65

me too

Answer 66

so u got thru planning for work and energy?

Answer 67

Kind of. I just started reading that. I had to help my mom in the morning. Now Im free. I was reading about maximun and minima of a function in the morning one hour ago...

Answer 68

are you teacher? or are you student?

Answer 69

student

Answer 70

hey him. can you help me with this exercise?

The radius of the earth is 6370Km and the radius of mars is 3440Km. If an object weights 200N on earth, what will be the gravity on mars? Mars has a mass of 0.11 times the mass of the earth.