course Phy 121 \½šáå³[ËIÖ¥ü‡w“ÝÖá¼üŠ«Ò«z|Àâassignment #015
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21:16:44 `q001. Note that this assignment contains 5 questions. . Suppose that a net force of 10 Newtons acts on a 2 kg mass for 3 seconds. By how much will the velocity of the mass change during these three seconds?
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RESPONSE --> To obtain this answer we use the equation Fnet * `dt = m vf - m v0: 10 Newtons * 3 Seconds = mvf - 0 30 Newton Seconds = mvf confidence assessment: 1
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21:18:11 The acceleration of the object will be accel = net force / mass = 10 Newtons / (2 kg) = 5 m/s^2. In 3 seconds this implies a change of velocity `dv = 5 m/s^2 * 3 s = 15 meters/second.
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RESPONSE --> The acceleration of the object will be accel = net force / mass = 10 Newtons / (2 kg) = 5 m/s^2. In 3 seconds this implies a change of velocity `dv = 5 m/s^2 * 3 s = 15 meters/second. GOT IT! I didn't believe it was the correct answer due to the fact that I had Newton Seconds which does not make any sense at all. self critique assessment: 3
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21:24:00 `q002. By how much did the quantity m * v change during these three seconds? What is the product Fnet * `dt of the net force and the time interval during which it acted? How do these two quantities compare?
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RESPONSE --> m * v = 2kg * 15m/s = 30m/s kg Fnet * 'dt = 10N * 3s = 30N Seconds They look as if they compair, but the m*v is Meters per Second Kg while the other is equal to Meters per Seconds Cubed Kg. confidence assessment: 1
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21:24:50 Since m remained constant at 2 kg and v changed by `dv = 15 meters/second, it follows that m * v changed by 2 kg * 15 meters/second = 30 kg meters/second. Fnet *`dt is 10 Newtons * 3 seconds = 30 Newton * seconds = 30 kg meters/second^2 * seconds = 30 kg meters/second. The two quantities m * `dv and Fnet * `dt are identical.
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RESPONSE --> My arithmetic was incorrect. I understand. self critique assessment: 3
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21:27:10 `q003. The quantity m * v is called the momentum of the object. The quantity Fnet * `dt is called the impulse of the net force. The Impulse-Momentum Theorem states that the change in the momentum of an object during a time interval `dt must be equal to the impulse of the average net force during that time interval. Note that it is possible for an impulse to be delivered to a changing mass, so that the change in momentum is not always simply m * `dv; however in non-calculus-based physics courses the effective changing mass will not be considered. If an average net force of 2000 N is applied to a 1200 kg vehicle for 1.5 seconds, what will be the impulse of the force?
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RESPONSE --> The impulse of the force in thsi problem is Fnet * 'dt, which is equal to the following: 2000N * 1.5s = 3000kg m/s confidence assessment: 3
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21:27:42 The impulse of the force will be Fnet * `dt = 2000 Newtons * 1.5 seconds = 3000 Newton*seconds = 3000 kg meters/second. Note that the 1200 kg mass has nothing to do with the magnitude of the impulse.STUDENT COMMENT: That's a little confusing. Would it work to take the answer I got of 3234 N and add back in the weight of the person at 647 N to get 3881? INSTRUCTOR RESPONSE: Not a good idea, though it works in this case. Net force = mass * acceleration. That's where you need to start with problems of this nature.Then write an expression for the net force, which will typically include but not be limited to the force you are looking for. *&*&
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RESPONSE --> self critique assessment: 3
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21:35:41 `q004. If an average net force of 2000 N is applied to a 1200 kg vehicle for 1.5 seconds, what will be change in the velocity of the vehicle?
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RESPONSE --> a = Fnet / mass a = 2000N/ 1200kg = 1.67m/s^2 To get 'dv you must multiply 1.67m/s^2 by 1.5s which equals 2.5m/s. This is your change in velocity. confidence assessment: 3
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21:35:57 The impulse of the 2000 Newton force is equal to the change in the momentum of the vehicle. The impulse is impulse = Fnet * `dt = 2000 Newtons * 1.5 seconds = 3000 Newton*seconds = 3000 kg meters/second. The change in momentum is m * `dv = 1200 kg * `dv. Thus 1200 kg * `dv = 3000 kg m/s, so `dv = 3000 kg m/s / (1200 kg) = 2.5 m/s. In symbols we have Fnet * `dt = m `dv so that `dv = Fnet * `dt / m.
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RESPONSE --> self critique assessment: 3
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21:47:20 `q005. Use the Impulse-Momentum Theorem to determine the average force required to change the velocity of a 1600 kg vehicle from 20 m/s to 25 m/s in 2 seconds.
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RESPONSE --> I believe we now use this equation to find the Fnet: Fnet * `dt = m vf - m v0, now we substitue: To find the m at initial velocity and final velocity we have to multiply the mass by the velocity. Thus: v0 = 1600kg * 20m/s vf = 1600kg * 25m/s Now we use the equation Fnet * `dt = m vf - m v0: Fnet * 2s = 40000kg m/s - 32000kg m/s Fnet = 8000kg m/s / 2s Fnet = 4000 Newtons confidence assessment: 3
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21:48:10 The vehicle changes velocity by 5 meters/second so the change in its momentum is m * `dv = 1600 kg * 5 meters/second = 8000 kg meters/second. This change in momentum is equal to the impulse Fnet * `dt, so Fnet * 2 sec = 8000 kg meters/second and so {} Fnet = 8000 kg meters/second / (2 seconds) = 4000 kg meters/second^2 = 4000 Newtons. In symbols we have Fnet * `dt = m * `dv so that Fnet = m * `dv / `dt = 1600 kg * 5 m/s / ( 2 s) = 4000 kg m/s^2 = 4000 N.
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RESPONSE --> GOT IT!! self critique assessment: 3
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