elastic potential energy question

[F.B]

I really need help because i have a test tomorrow.

 

A student uses a compressed spring of force constant 22 N/m to shoot a 0.0075 kg eraser across a desk. The magnitude of the force of friction on the eraser is 0.042 N. How far along the horizontal desk will the eraser slide if the spring is initially compressed 3.5 cm. Use the law of conservation of energy.

 

I know i have to find the speed and acceleration but i dont get the right answer.

 

For speed i did

 

Ee=Ek

and then the answer i get is 1.89 m/s i think thats right. But for acceleration im not sure.I did this:

 

Fnet=Fe-Ff

ma=kx-Ff

and then i end up getting 97 m/s^2 i think thats wrong.

 

Oh can you also help me with this question, it doesnt involve calculations.

 

Which has more kinetic energy a slow moving car or a fast moving truck.

 

I think its the truck because the formula is Ek=1/2mv^2 but there has to be a catch to it.

[timo]

I know i have to find the speed and acceleration [...']

No, you don´t. You have to "Use the law of conservation of energy". In this case: Energy initially stored in the spring = energy "destroyed" by friction.

 

Oh can you also help me with this question, it doesnt involve calculations.

Which has more kinetic energy a slow moving car or a fast moving truck.

I think its the truck because the formula is Ek=1/2mv^2 but there has to be a catch to it.

I wouldn´t see any, except if trucks in your area are lighter than typical cars.

 

EDIT: If you like a mathematical proof for that the truck has more kinetic energy, try this one:

Let v, V and m, M be the speeds and the masses of the car and the truck, respectively (capital letter for the bigger one), then:

E(car) = 0.5 mv² < 0.5Mv² < 0.5MV² = E(truck)

However, I don´t think such an inequality would be asked from you in a test.

[F.B]

if i do Ee=Ek and determine the speed. What do i do with how can i find the distance

[insane_alien]

use the equations of motion which i can't quite remember just now.

[timo]

if i do Ee=Ek and determine the speed. What do i do with how can i find the distance

Well, guess the test is over now so my answer probably comes a bit late. Nevertheless: You don´t need to know velocity or acceleration at all in this problem. You have some kinetic energy. This kinetic energy will be completely lost due to friction. The force due to friction is the force of gravity times the friction coefficient. And finally the point most likely to be the one you missed: Energy is force integrated over distance (force times distance for the spimples problems like this one). That´s enough to solve the problem.