# Homework Help: Work done on a block

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1. Feb 26, 2016

### x2017

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

A 100 N net force acts on a 100 kg block (initially at rest) for 4s
a. How much work was done on the block? (1)
b. What is the block’s velocity after the 4 s? (1)
c. How much energy does the block have after this time?(1)
d. The 100 N net force is no longer acting on the block. If the block slides an additional 3 m before coming to rest, what average force must have acted on the block over those 3 m? (1)

2. Relevant equations
P=W/t
work done = change in energy
Hooke's Law: Fs=kΔx
Ws=(1/2)kΔx2
KE=(1/2)mv2
PEg=mgh
SE=(1/2)kΔx2
Total energy = KE+PE+SE
W=FΔd

3. The attempt at a solution
The only information I have is:
F=100N
m=100kg
Δt=4s

All of the equations I have require information I don't have so I am very confused as to how to attempt this question.

W=FΔd was what I thought I should be using, but I don't have Δd. Then I thought maybe this one: Ws=(1/2)kΔx2, but I don't have k OR Δx....

2. Feb 26, 2016

### cnh1995

You can calculate acceleration from this. Use kinematical equations to find the displacement.

3. Feb 26, 2016

### x2017

The kinematic equation I have for acceleration is
a=(vf-vi)/Δt
a=(vf-0)/4

I don't have a final velocity though, just F, m and Δt.

4. Feb 26, 2016

### cnh1995

Remember Mr. Newton..

5. Feb 26, 2016

### x2017

Like the three laws?

f=ma
for every action there's an equal and opposite reaction.
an object in uniform motion will stay in moth in until a force act on it.

EDIT: Okay, I found this page on here https://www.physicsforums.com/threads/find-velocity-given-force-and-mass.300413/ and now understand

vf=Ft/m
vf=(100)(4)/100
vf=4m/s

6. Feb 26, 2016

### cnh1995

That is true because initial velocity=0.
F=m(vf-vi/t.
In general, F=ma
You have m=100kg, F=100N.
This gives a=1m/s2...
Since
v=u+at, and u=0,
v=at=1*4=4m/s...