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Homework Help: Force / Acceleration help

  1. Apr 13, 2004 #1
    Can someone help me with these two questions:

    1. a body of mass 2.0 kg is acted on by a downward force of gravity and a horizontal force of 40N. Find it's acceleration and velocity as a function of time, assuming it starts from rest.

    2. Find the resistance of the air to the motion of a body of 4.0kg in free fall with a gravitational acceleration of 9.2m/s^2.

    I feel there is not enough information to answer these.
  2. jcsd
  3. Apr 13, 2004 #2


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    You should assume some value for the gravitational acceleration. Usually, the standard value is 9.8 m/s2. You can use the kinematical equation to get the velocity.

    This sounds like one of two things: 1) not enough info., or 2) a trick question. Do you usually get trick questions?
  4. Apr 13, 2004 #3


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    1. The block goes 80ms^-2 to the right (let's say), and goes 9.8ms^-2 downwards. Use the pythagorian thereom to find its resulting acceleration. By finding the velocity in function of time they might want you to draw a graph... good luck, I hate doing those.

    2. Objects in free fall eventually stop accelerating downwards when they reach a high enough velocity. So, at that point the resistance would be 9.2ms^-2 times 4kg.

    Unless I'm missing something, that should be it.
  5. Apr 13, 2004 #4
    Do you usually get trick questions?[/QUOTE]

    Sometimes. But I don't see how it can be a trick question.
  6. Apr 13, 2004 #5


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    I don't see anything very tricky about them. Number 1 is a straightforward vector problem. The only thing "tricky" is the wording: "a gravitational acceleration of 9.2m/s^2." Did the problem really say that or did it say something like "falling under gravity the acceleration is 9.2 m/s2? The point is that gravitation acceleration (on the surface of the earth) approximately 9.8 m/s2 because the force of gravity is 9.8 times the mass. If the body is falling with acceleration 9.2 m/s2 then there is a net force of 9.2 times the mass. The air resistance force is (9.8- 9.2)= 0.6 times the mass. Since the mass is given as 4.0 kg, the force of air resistance is 0.6(4)= 2.4 Newtons.
  7. Apr 14, 2004 #6
    But I still don't understand how to get the velocity.
  8. Apr 14, 2004 #7


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    Newtons 2nd:
    F = ma

    Constant acceleration motion:
    V(t) = V0 + a*t

    Give it a shot and let us know if and where you get stuck.
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