# A couple Force, work, KE, and spring problems I'm working on.

1. Oct 25, 2008

### raven2783

I'm real rusty on my physics, but here are five simple sample problems we were given, I'd appreciate your help in helping me solve these. Thanks!

1. The problem statement, all variables and given/known data

(1) A single horizontal 10.0 N force, F acts on a 2.0 kg block which is initially at rest at x=0 m. In response to the force, the block accelerates in the direction of the force and some time later has travelled 20.0 m. Use this information to answer the following three questions:

a. In moving through the 20.0 m displacement, the force F does an amount of work given by?
b. What was the horizontal acceleration of the block in m/s2?
c. What was the speed (in m/s) of the block when it passes the x=20.0 m mark?

(2) Suppose that a computer hard drive has a disk that rotates at a constant speed of 7200 rpm (revolutions per minute).

a. What is the direction of acceleration of a point 2.0 cm from the axis of this rotating disk?
b. What is the magnitude of the acceleration (in m/s2) of a point 2.0cm from the axis of rotation?

(3) Suppose the mass of a 1.0 g match was converted entirely to energy. How many Joules of energy would this create?

(4) A spring with constant 2.0 N/m stores 16.0 J of energy when it is compressed by what distance?

(5) A block of mass M falls a distance D near the surface of the earth and reaches a speed V. What is the final speed of an identical block which falls a distance 16D?[/B]

2. Relevant equations

F=ma
KE=1/2mv²
a=v²/r
W=FD
W=1/2kx²
s=d/t
yf=yi+vit+1/2at²

3. The attempt at a solution

1) A single horizontal 10.0 N force, F acts on a 2.0 kg block which is initially at rest at x=0 m. In response to the force, the block accelerates in the direction of the force and some time later has travelled 20.0 m. Use this information to answer the following three questions:

a) In moving through the 20.0 m displacement, the force F does an amount of work given by?

W=FD
200N·M=10N*20m

b) What was the horizontal acceleration of the block in m/s2?

f/m=a
10N/2kg=5m/s²

c) What was the speed (in m/s) of the block when it passes the x=20.0 m mark?

KE=1/2mv² and I need to think about conservation of energy, and I know s=d/t but I need time... t=d/s but I need speed... hrrm.

2) Suppose that a computer hard drive has a disk that rotates at a constant speed of 7200 rpm (revolutions per minute). What is the direction of acceleration of a point 2.0 cm from the axis of this rotating disk?

This must be toward the axis of rotation?

b) What is the magnitude of the acceleration (in m/s2) of a point 2.0cm from the axis of rotation?

a=v²/r
2cm=.02m
2πr?? involved I really don't know how to approach this one.

3. Suppose the mass of a 1.0 g match was converted entirely to energy. How many Joules of energy would this create?

A lot... I know that..

4. A spring with constant 2.0 N/m stores 16.0 J of energy when it is compressed by what distance?

x=f/k

8m=16/2 ...right?

5. A block of mass M falls a distance D near the surface of the earth and reaches a speed V. What is the final speed of an identical block which falls a distance 16D?

iv+at for the vf of free-falling objects
xi=xf+vit+1/2at² for the position of free-falling objects.

so 16 times the speed would be 16 times the distance.. I think.

Last edited: Oct 25, 2008
2. Oct 25, 2008

### raven2783

help?

3. Oct 27, 2008

### raven2783

It's been more than a day could anyone help me?

4. Oct 27, 2008

### heth

If you posted an individual question and gave an indication of your thought process, rather than an equation plucked out of the air that may or may not be relevant with some numbers stuck in it, you might get a better response.