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Force Problems.

  1. Jul 23, 2006 #1
    The force of air resistance on a raindrop is 5 x 10^-5 N when it falls with a terminal velocity of 4.2 m/s.
    The acceleration of gravity is 9.8 m/s2.
    What is the mass of the raindrop? Answer in units of kg.

    How would I do this question? My understanding is to divide the Force of air resistance on a raindriop by the acceleration of gravity to get the mass of the raindrop.

    and the other question is,
    007 (part 1 of 2) 5 points
    A block weighing 6.9 N requires a force of 2.4 N to push it along at constant velocity.
    What is the coefficient of friction for the surface?

    I'm new to friction so I don't know what to do here..

    Any ideas would be appreciated. :)
  2. jcsd
  3. Jul 23, 2006 #2


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    Sounds good to me, since at the terminal velocity the net force on the raindrop is zero the drag force is equal in magnitude to the force of gravity.
    Do you know an equation which involves the applied force, normal force and the coefficient of friction?
  4. Jul 23, 2006 #3
    1) First you need to understand the question well. Terminal velocity means means the sum of forces is zero. They've given you one force, which is air resistance. What's the other force? Be careful with the directions, forces are vectors.

    2) Three things you need to know about friction:

    i) Friction is a force.
    ii) Friction always acts in the direction opposite of motion
    iii) The formula for friction is (coefficient of friction) * (normal force).

    In every physics problem, you MUST identify all the forces. There are 4 forces in your problem. The first one is weight, the second one is the normal force exerted on the block due to the weight, the third one is the force you apply to push it, and the fourth one is friction.

    Since the block is not moving up or down, you know that the normal force equals to the weight.

    But the block is moving along the surface, with a constant velocity. What does "constant velocity" tell you about the forces?
  5. Jul 23, 2006 #4
    Okay I got it...

    I have 2 more lol

    Question #1
    A 7kg block rests on a horizontal table, attached to a 6kg block by a light string as shown in the figure. The acceleration of gravity is 9.81 m/s^2. What is the minimum coefficient of static friction such that the objects remain at rest? The answer is 0.857142857 N.

    The second part of the question is...
    If the coefficient of static friction is less than that found above, and if the coefficient of kinetic friction between the block and the table is 0.3, find the time it takes for the 6kg mass to fall 10m to the floor if the system starts from rest. Answer in units of s.

    Question #2
    Pam has a mass of 40.9 kg and she is at rest on smooth, level, frictionless ice. Pam straps on a rocket pack. The rocket supplies a constant force for 18.2 m and Pam acquires a speed of 60.2 m/s.
    What is the magnitude of the force? Answer in units of N.

    So for this question, I first divided 18.2m / 60.2m/s to get the time. I then divided 60.2m/s / the time to get the acceleration. I then multiplied the acceleration and the mass to get the force. But it's wrong, where did I go wrong?
  6. Jul 23, 2006 #5
    For Question #2, note that 60.2 m/s is the speed Pam acquires after travelling 18.2 m.

    What are your thoughts regarding Question #1? Did you manage to solve the first part?
  7. Jul 23, 2006 #6
    I'm thinking that I need to find the average velocity.

    But I'm not sure what to do after.

    Edit: and for Question 1, i dont know how to do the 2nd part.
    Last edited: Jul 23, 2006
  8. Jul 23, 2006 #7
    Question #2

    By Newton's 2nd Law, net force = mass x acceleration. Now, we consider forces in the horizontal direction since Pam is moving horizontally.

    What is the net force acting on Pam? Is it constant? If it is, then Pam's acceleration is constant too. Are you able to find the value of this constant acceleration by using the information provided?

    Question #1

    Sometimes, the concepts of Kinematics and Dynamics can be applied in a single problem. Usually, acceleration will be the quantity that links these 2 topics. So, use Newton's 2nd Law to find the acceleration of the 6kg mass, and with the acceleration, you can apply a suitable Kinematics equation.
    Last edited: Jul 23, 2006
  9. Jul 23, 2006 #8
    Yes you can find the constant acceleration.
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