course Phy 231 ?E?????????????assignment #009
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09:01:47 Introductory prob set 3 #'s 1-6 If we know the distance an object is pushed and the work done by the pushing force how do we find the force exerted by the object?
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RESPONSE --> We know that work = force x distance (W = F x D), so force = work / distance. confidence assessment: 2
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09:03:24 ** Knowing the distance `ds and the work `dW we note that `dW = F * `ds; we solve this equation and find that force is F=`dw/`ds **
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RESPONSE --> OK I also see that you used 'dw and 'ds. I will add this to my notes. self critique assessment: 3
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09:12:41 If we know the net force exerted on an object and the distance through which the force acts how do we find the KE change of the object?
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RESPONSE --> We can find the KE change by first determining the product of force and distance on the object. Assuming all other things equal, the work done on the object will be equal to the calculation of Force x 'ds. confidence assessment: 3
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09:15:56 **`dW + `dKE = 0 applies to the work `dW done BY the system and the change `dKE in the KE OF the system. The given force acts ON the system so F `ds is work done ON the system. The work done BY the system against that force is `dW = -F * `ds. When you use the energy equation, this is the work you need--the work done BY the system. **
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RESPONSE --> OK that makes sense. So if we use 'dW + 'dKE = 0, and then plug in 'dW = - (F * 'ds), then we can solve for 'dKE and get: 'dKE = F*'ds self critique assessment: 3
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09:20:44 Why is KE change equal to the product of net force and distance?
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RESPONSE --> If using the equation 'dW + 'dKE = 0, then we can solve for 'dKE and get 'dKE = - 'dW. This makes sense, because we know that 'dW = F * 'ds. We can then plug this into our original equation and get: 'dKE = - (F * 'ds) confidence assessment: 3
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09:25:07 ** It comes from the equation vf^2 = v0^2 + 2 a `ds. Newton's 2d Law says that a = Fnet / m. So vf^2 = v0^2 + 2 Fnet / m `ds. Rearranging we get F `ds = 1/2 m vf^2 - 1/2 m v0^2. Defining KE as 1/2 m v^2 this is F `ds = KEf - KE0, which is change in KE. **
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RESPONSE --> OK So using Newton's 2nd law, which is Fnet = m * a; or, solved for a is: a = Fnet / m, we can plug that into the equation we are already familiar with: vf^2 = v0^2 + 2 a * 'ds; this gives us: vf^2 = v0^2 + 2 (Fnet / m) * 'ds. Then, solving for F * 'ds (which is Work), we get: 1/2m vf^2 - 1/2mV0^2. Since KE = 1/2 mv^2, we can say F*'ds = KEf - KE0, which is change in KE. self critique assessment: 2
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09:31:09 When I push an object with a constant force, why is KE change not equal to the product of the force I exert and the distance?
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RESPONSE --> Because we also have to take into account the work done by the object and any forces acting on that object. confidence assessment: 1
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09:32:48 ** Change in KE is equal to the work done by the net force, not by the force I exert. When I push an object in the real world, with no other force 'helping' me, there is always at least a little force resisting my push. So the net force in this case is less than the force I exert, in which case the change in KE would be less than the product of the force I exert and the distance. If another force is 'helping' me then it's possible that the net force could be greater than the force I exert, in which case the change in KE would be greater than the product of the force I exert and the distance. It is actually possible for the 'helping' force to exactly balance the resisting force, but an exact balance would be nearly impossible to achieve. ANOTHER WAY OF LOOKING AT IT: If I push in the direction of motion then I do positive work on the system and the system does negative work on me. That should increase the KE of the system. However if I'm pushing an object in the real world and there is friction and perhaps other dissipative forces which tend to resist the motion. So not all the work I do ends up going into the KE of the object. **
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RESPONSE --> OK - I think I was on the right track. It's quite possible that there were additional forces either helping you or acting against you that would influence the amount of Net Force, which is different than just the amount of force you exerted in moving the object. self critique assessment: 2
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