# Homework Help: Really need help with this one.

1. Nov 20, 2005

### F.B

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 think you have to find acceleration first but it doesnt make sense when i do it. I end up 97 m/s.

Heres what i did.

Fnet=Fe-Ff
But i think thats wrong.

so can anyone please help me.

2. Nov 20, 2005

### Fermat

What work is done by the spring as it decompresses by 3.5 cm ?
That work done/energy lost is transferred to the eraser as kinetic energy.
Use that to find the initial velocity of the eraser.

The friction force is what provides the deceleration.

3. Nov 20, 2005

### runicle

fnet = fapp + f
force of friction is opposite to f so
fnet = fapp + ( - f)

ma= .... you know (hint look below an your units)

if not

force is known as kg*m/sy*2 (y* means to the power of) and by the way don't forget to convert your units to standard SI untis

4. Nov 20, 2005

### F.B

if i do Ee=Ek i get 1.89 m/s as speed. What do i do now with it.

Whats Fapp you get figure that out unless its a constant velocity

5. Nov 20, 2005

### runicle

Fapp is "applied force" and use the 6 basic equations to find your displacement

6. Nov 20, 2005

### F.B

I know that Fapp is the applied force but there isnt an applied force in this question.

7. Nov 20, 2005

### Fermat

Ok, you used conservation of energy to get the corrrect initial velocity.

Now use the kinematic eqns to find out how far the eraser travels under a constant decelerating force of 0.042N with an initial velocity of 1.89m/s.

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