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**1. Homework Statement**

i've been stuck on this problem for about 45mins.

A constant external force P = 160 N is applied to a 20 kg box,which is on a rough horizontal surface. The force pushes the box a distance of 8.0 m, in a time interval of 4.0 s, and the speed changes from v1 = 0.5 m/s to v2 = 2.6 m/s. Find the work done by friction. The work done by friction is closed to:

There is a force applied at 30degree to the horizontal of the box.

the answer is -1043 J but idk how to get that answer.

**2. Homework Equations**

Ff = µFn

W=Fd

W=mgd

**3. The Attempt at a Solution**

W=mgd

W=20kg*9.8*cos(30)*8=1357.9J

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