# Point Particle System ?-> help please

1. Oct 18, 2007

### quickclick330

Point Particle System ?--> help please!!

Two identical 0.17 kg blocks (labeled 1 and 2) are initially at rest on a nearly frictionless surface, connected by an unstretched spring whose stiffness is 97 N/m, as shown in the upper diagram. Then a constant force of 7 N to the right is applied to block 2, and at a later time the blocks are in the new positions shown in the lower diagram. At this final time, the system is moving to the right and also vibrating, and the spring is stretched.

What was the change in the translational kinetic energy of the point particle system?
Ktrans = ________ joules

I've already figured that
deltax = 0.07 m
W = .49 joules

Ktrans = Fnet*deltat

I tried this...

Fnet = Fpush - Fspring
= 7 - (97 n/m * 0.07 m) = 47.53 N

47.53 N * 0.07 m = 3.327 joules

What am I doing wrong? thanks for the help!!!

2. Oct 19, 2007

### learningphysics

Your 0.49J is the change in translational kinetic energy I believe...

Force*(distance moved by the center of mass) gives the change in translational KE...

But remember that this is not the work done by the force... just the part of the work that becomes translational KE...

3. Mar 4, 2008

### mpeake

Delta KE = The Integral of F(net,sum) * dr(cm) = 7 * 0.7 = 0.49 J

: )

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