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Work

vivian-disabled
1-Newbie

Work

I have two problems I need help with!

14.[1pt]
Which of the following statements is true? / I do it but it's still wrong, so I would appreciate it if anyone can explain why?

A) The work done to raise a box onto a platform depends on how fast it is raised. /FALSE/
B) The kinetic energy can be positive or negative. /FALSE/
C) The kinetic energy depends on the reference frame of the observer. /TRUE /
D) The kinetic energy of an object is independent on the direction of the motion involved. /FALSE?/
E) If the Earth revolves around the sun in a perfectly circular orbit, then the sun does work on the Earth. /TRUE /
F) Spring A and B are identical except that A is stiffer than B; that is kA > kB. More work must be expended on spring A if both springs are stretched by the same amount. /TRUE /

15.[1pt]
With brakes fully applied, a 1560 kg car deccelerates from a speed of 83.0 km/hr. (a) What is the work done by the braking force in bringing the car to a stop? (b) What is the change in the kinetic energy of the car?

Known: initial velocity = 83.0 km/hr = 23.055 s/m
mass = 1560 kg

unknown: Work done by braking force & change in kinetic

KE = mv^2/2

KE final= 0 J since the final velocity = 0m/s when the ball is at the highest height.

KE initial = mv^2/2 = 1560 kg x 23.05 s/m ^2 / 2 = 414595.75 J

W done by brake = KE final - KE initial
thus W = 0 J - 414595.75 J = -41595.75 J

and the change in kinetic energy is 41595.75 J since the final is 0 ???


And the computer gives back that the answer is wrong, so what did I do wrong?
1 REPLY 1

On 10/8/2003 10:46:56 PM, vivian wrote:
>I have two problems I need
>help with!
>
>14.[1pt]
>Which of the following
>statements is true? / I do it
>but it's still wrong, so I
>would appreciate it if anyone
>can explain why?
>
>A) The work done to raise a
>box onto a platform depends on
>how fast it is raised. /FALSE/
>B) The kinetic energy can be
>positive or negative. /FALSE/
>C) The kinetic energy depends
>on the reference frame of the
>observer. /TRUE /
>D) The kinetic energy of an
>object is independent on the
>direction of the motion
>involved. /FALSE?/
>E) If the Earth revolves
>around the sun in a perfectly
>circular orbit, then the sun
>does work on the Earth. /TRUE
>/
>F) Spring A and B are
>identical except that A is
>stiffer than B; that is kA >
>kB. More work must be expended
>on spring A if both springs
>are stretched by the same
>amount. /TRUE /

E is false; if there is no change in gravity altitude, there is no work done


>15.[1pt]
>With brakes fully applied, a
>1560 kg car deccelerates from
>a speed of 83.0 km/hr. (a)
>What is the work done by the
>braking force in bringing the
>car to a stop? (b) What is the
>change in the kinetic energy
>of the car?
>
>Known: initial velocity = 83.0
>km/hr = 23.055 s/m
>mass = 1560 kg
>
>unknown: Work done by braking
>force & change in kinetic
>
>KE = mv^2/2
>
>KE final= 0 J since the final
>velocity = 0m/s when the ball
>is at the highest height.
>
>KE initial = mv^2/2 = 1560 kg
>x 23.05 s/m ^2 / 2 = 414595.75
>J
>
>W done by brake = KE final -
>KE initial
>thus W = 0 J - 414595.75 J =
>-41595.75 J
>
>and the change in kinetic
>energy is 41595.75 J since the
>final is 0 ???
>
>
>And the computer gives back
>that the answer is wrong, so
>what did I do wrong?

Perhaps you have the signs backwards, since the change in kinetic energy of the car must be negative, since it had positive energy.

TTFN,
Eden
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