Why Did I Almost Fail My Physics Test?

[lil_agelu4life]

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...
From:
<< !De LiL AgeLu 4 Life! >>

 

[Hootenanny]

Hi there lil_agelu4life, welcome to PF.

Question One

Correct.

Question Two

(a)
Correct.

(b)
This is indeed incorrect. You have calulated the final velocity of the block. With the information given you need to use Newton's second law (F=ma) to calculate the acceleration of the object.

Question Three

(a)
Correct.

(b)
Correct.

(c)
I cannot answer this question as you have not specified the height to which the piano was raised.

(d)
Again, I cannot answer this question as you have not specified the height to which the piano was raised.

Question Four

(a)
Correct.

(b)
Correct.

(c)
Correct.

Question Five

A useful equation to remember (although it can be derived) is P = Fv. Power is equal to the product of force and velocity.

Question Six

Correct.

Question Seven

Again, this is a question where you would use Newton's Second law. The vector sum of the forces is equal to the product of mass and acceleration;

$$\sum\vec{F} = m\vec{a}$$

Note that the frictional force is acting in the opposite direction to the driving force.

Question Eight

I haven't thoroughly checked this question yet as I haven't got time. However two things that do strike me immediatly is that yes, you are correct you need to divide by the number of legs as opposed to multiplying (that is ofcourse assuming the surface are is of a single leg). Also the surface area is given in cm², yet you state that they are in m², you should convert them to m² first. I'll take another look at this when I get more time.

~H

 

[al_201314]

Hello,

For question 8, your original steps are correct. But you have to work in SI units. You've converted them wrongly.

To convert,

1 cm = 0.01m

0.06cm = 0.00006m

(0.06cm)^2 = (0.00006m)^2 => 3.6 x 10^-7.

Doing the same for the elephant, you should be right on track :)