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lil_agelu4life

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**HELP! I almost failed a test... :(**

Well I had problems with one physics test...got 26.5 outta 43...

I got some problems rite tho??

Here are some that I had trouble figuring out.

(1) A 2.4 m tall weightlifter lifted an 80kg mass off the ground to a height 0.5 m aobve his head. Calculate the work done by the weightlifter on the weight.

I didnt get full marks for my working out tho...

F=mg

=80*10

=800N

d=2.4+0.5

=2.9m

W=Fd

=800*2.9

=2320 J

(2)Tina does 84J of work to move a 3.0 kg mass distance of 4.0m from rest.

a) Calculate the size of the force she applied.

W = Fd

84J = F*4.0

DIVIDE BOTH SIDES BY 4.0

F = 21 N

b)What is the acceleration of the mass?

I know that what I did was wrong...Plz don't laff...i'm still learning

Ek = 1/2 mv^2

84 = 1/2 * 3.0 * v^2

84 = 1.5 * v^2

DIVIDE BOTH SIDES BY 1.5

126=v^2

FIND THE SQUARE ROOT OF BOTH SIDES

v=11.224972

v = dt

11.22497216 = 4.0*t

DIVIDE BOTH SIDES BY 4.0

t = 2.80624304 s

a = vt^2

a = 11.224792 * 2.80624604^2

a = 88.39665576 ms^-2

(3)A piano of mass 250kg is moved 10m along the floor by two people with a total force of 400N.

a)What is the work done in shifting the piano the 10m?

W=Fd

W=400*10

W=4000J

b)What force is needed to life the piano?

F=mg

=250*10

=2500N

c)How much work is done to life the piano?

W=Fd

=2500*250

=625,000J

d)What is the total amount of work required to shift the piano 10cm along the floor and up the stairs?

W=4000+325,000

=629,000J

(4)A crane lifts 800kg of concrete a height of 25m in 20s

a)What is the weight of the concrete?

Fw=mg

=800*10

=8000N

b)What is the work done by the crane?

W=Fd

=8000*25

=200,000J

c)What is the power of the crane?

P=W/t

=200,000/20

=10,000 Watts

(5)A motor boat engine produces a useful power of 4.2 kW and is able to drive the boat through the water to a steady speed of 24km/hr

Calculate the force opposing the motion of the boat thru the water.

I got lost after converting the given measurements to the right units.

4.2 kW -> W

=4.2 kW * 1000

=4200W

...help please? I can't think of a formula that applys. Can I use the Ek=Ep formula??

(6) Rizza rides her skateboard - the combined mass of her and her skateboard is 60kg. She moves down a ramp at a constant speed of 3ms^-1. What is the kinetic energy of Rizza and her skateboard??

Ek= 1/2mv^2

=1/2 * 60 *3^2

=30 * 3^2

=270 J

Is my answer right??

(7)Lani pushes her skateboard with a force of 35N . Lani and her skateboard have a combined mass of 70kg. There is a friction of 5N. What is Lani's acceleration?

omg... ...i couldn't figure this one out.

I tried working it out like (2) b) but it still didnt work...HELP!

well this is my failed attempt:

Ek = 1/2 * 70 * v^2

= 45* v^2

well now I'm totalli stuck...how is the friction associated with figuring out the acceleration??

(8) Calculate the pressure of an elephant weighing 4000kg with surface area of 0.42cm^2; and a woman wearing stilletoes weighing 25kg with surface area 0.006cm^2. Find the difference of their pressures.

WOMAN

Fw=25kg*10

=250N

A=0.006*2

=0.012 m^2

P= F/A

= 250 / 0.012

=20833 1/3 Pa

ELEPHANT

Fw=4000kg*10

=40,000N

A=0.42*4 feet

=1.68 m^2

P=F/A

=40,000 / 1.68

=23,809.52381 Pa

I think I should've divided the surface area by 4 not multiply by 4...I did this as there are 4 feet...I thought the given surface area was the surface area of one foot...

Is this correct then??

Corrections:

WOMAN

A=0.006/2

=0.003

P=F/A

=250 / 0.003

=83,333 1/3 Pa

ELEPHANT

A=0.42/4

=0.105 m^2

P=F/A

=40,000 / 0.105

=380,952.38 Pa

OOPS i think its even more wrong...the woman should have a greater pressure...help me out somebody!

Well thanks to whoeva helps me out...

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