Physics Problems

Test Preparation
1

<p>Hey…I have some physics problems to do for homework and Im having some problems. Do you think you could help me? Thanks so much!</p>

<li><p>At point A, a 3 kg block with initial velocity 0 m/s is moved 10 meters to point B with a force of 100 N. Using energy methods, find the speed of the block at point B.</p></li>
<li><p>A .5 kg bullet at 100 m/s goes through a 4 kg block with initial velocity 0 m/s. Afterwards, the bullet is traveling 60 m/s. What is the recoil velocity of the block?</p></li>
</ol>

2

<ol>
<li>this is the work energy thereom. the work done by the object is equal to the change in kinetic energy</li>
</ol>

<p>W = delta KE
Force * distance moved = 1/2 m v^2</p>

<p>plug in numbers and solve for v. I think you can do that by yourself.</p>

3

<p>Edit: athlon's got it. ;)</p>

4

<ol>
<li>conservation of energy.</li>
</ol>

<p>KE initial of bullet + KE initial of block = KE final of bullet + KE final of block</p>

<p>but the block has an initial velocity of 0 so it has no initial KE
so</p>

<p>KE initial of bullet = KE final of bullet + KE final of block
1/2<em>(mass of bullet)</em>(initial speed of bullet)^2 = 1/2<em>(mass of bullet)</em>(final speed of bullet)^2 + 1/2<em>(mass of block)</em>(final speed of block)^2</p>

<p>solve for the final speed of the block</p>

5

<p>thanks guys!</p>

6

<p>Actually, #2 is an example of the conservation of momentum. When a bullet crashes into a block, this colllusion is what is called inelastic, meaning that KE is not conserved (much of the KE instead become heat). Thusly:</p>

<p>Initial momentum of bullet + Initial momentum of block = Final momentum of bullet + final momentum of block.....</p>

<p>...work from here.</p>

7

<p>oh my bad, haha yeah it's momentum, KE isn't conserved because it's probably not perfectly elastic</p>