Physics Work and Energy help?

Question

i Cant figure out how to solve these 3 questions please help.Please help i dont understand and please dont just give me the answer. try to explaine the steps to me thank you very much

A force of 825NIS NEEDED FOR TWO PEOPLE TO PUSH A CAR 35 M. How much work is done? HOW MUCH ENERGY DID THE PEOPLE EXPEND?

Sarah pulls her little red wagon along a path a distance of 16 m. She exerts a force of 95 N at an angle of 35 degrees with respect to the direction of motion. How much work does she do?

An electric motor develops a 65 KW of power as it lifts an elevator 17.5 m in 35 sec. What average force does the motor ecert?
Please help i dont understand and please dont just give me the answer. try to explaine the steps to me thank you very much

Answer 1

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work = W

force = F

displacement = s

Angle between force and displacement = O

W = Fs cos O

When force and displacement are in same direction ,W = Fs

A force of 825 N is needed to push a car 35 m

W=825*35=28875 J

Energy expanded = work done by people =28875 J
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Sarah pulls wagon through a distance =s=16 m.

She exerts a force=F = 95 N

Angle= O = 35 degrees

work =W=Fs cos O =95*16*cos35=1245.11 J

Work done by Sarah is 1245.11 J
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An electric motor develops a 65 KW of power as it lifts an elevator 17.5 m in 35 sec

Power= P = 65 kW= 65000 watt

time = t = 35s

distance = s = 17.5 m

force = F =?

work = force*distance

work = power* time

force*distance = power*time

F = P t / s

F= 65000*35 / 17.5 = 130000 N

Average force exerted by the motor is 130000 N
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Answer 2

These can all be addressed just by focusing on the definitions of work and power. Work done by a force can be calculated by multiplying the force component in the direction of motion times the displacement undergone by the point of application of the force during the motion. (I know that's a mouthfull when you are just starting out.)

In the first problem you are simply given the displacement of the car. It isn't explicitly stated that the people push the car directly in the direction that it actually moves, but I'd assume that anyway since they'd try to push it in the easiest way. All you need to do to find the work is multiply the force applied by the displacement.

In the second problem the force and displacement are not aligned so you need to find the component of the force that acts in the direction of motion and then multiply that by the displacement.

For the third problem the force the motor applies and the displacement are aligned vertically so straight multiplication of force exerted and displacement will again give the work done. Since you are told about power instead of work you first need to use the definition of power -- work done per unit time -- to compute the work done by the motor. Then you can work backward to find the force.


Work and energy are complicated issues when you dig deeper. When the force and displacement are "backwards" from each other the work comes out negative. When more than one force acts at the same time each force does a certain amount of work, but the total of all the work done by all the forces acting on a particle turns out to be equal to to the change in the particle's kinetic energy during the time interval when the forces were acting.

When you go further and define potential energies in addition to kinetic energy you get even more relationships. And when you move from particles to extended bodies like cars and people things get a lot more complex. Take your time--this is tough stuff.

Answer 3

For the first one, work is equal to force times displacment. So, W=(825N)(35m)=( )Joules. Work is measured in Joules and so is already the amount of energy expended.

For the second, the force component responsible for the motion is cos(35)*95N=( )N.(Use the trig rules.) So the work done is W=(77.82N)(16m)=( )J.

For the third, watts is equal to a joule*second. So the work done by the motor is 65000w/35s=1857.143J. Then the Force=(1857.143J/17.5m)=( )N.

Answer 4

Work is defined as ∆E (The change in energy, which is E2-E1)

Oh no. Now i feel like a moron. I don't know what energy is. Sorry & i hope that what i did have helped :)

Answer 5

W=F*d*cos(angle)
P=W/t

1) 825N*35m=28875J
2) 95N*16m*cos35=1245.11J
3) 65J/s=17.5m*F/35s --> F=130n