• Describe the motion of the cyclist if:

    1. the resistive force is greater than the driving force.

    2. the driving force is greater than the resistive force.

    3. the driving force has the same magnitude as the resistive force.

    4. Suggest two ways in which the cyclist could reduce the resistive force.

  • Why does the propulsive force from the car engine will need to be greater than the net force (that causes acceleration)?

For bullet point 1's...

(1) Will the cycle deccelerate or move in the opposite direction?
(2) Will the cycle speed up or simply move?
(3) Will the cycle keep moving in constant speed or stop moving at all?
(4) One way can be reducing the amount of friction. Is there another way?

For bullet point 2...

Is it because of the presence of the resistive forces? So does this make the answer to 1's (3) the second option?

  • 1
    $\begingroup$ Use Newton's Second Law of Motion. $\endgroup$ – electronpusher May 15 '17 at 18:33

Newton's 2nd Law answers it all:

$$\sum F=ma$$

And as you see in this law, no velocities are involved. The speed in whatever direction is not connected to the accelerations that might happen.

  1. Net force is negative: $\sum F=ma<0$. In other words, the net force is pointing backwards. The acceleration is in the same direction as the net force and is so also backwards.

  2. Net force is now positive: $\sum F=ma>0$. Acceleration is too and is thus forward.

  3. Net force is zero: $\sum F=0=ma$. Nothing accelerates. Whatever motion it has is not change.

All these three descriptions never mentioned speed. In all cases the speed could be either forwards or backwards, we don't know. For example, in the first bullet point, if the speed is forward, then is is slowing down; is the speed backwards, then it is speeding up backwards.

  1. The resistive force is the friction. The question is asking how you can reduce that. For example by pumping your tires harder and by not biking on a sand beach. Other resistive forces could be frictions in joints and gears and alike, but I assume that this question assumes these things ideal.

For bullet point 2... Is it because of the presence of the resistive forces?

The question doesn't really make sense. Is the propulsion force larger than the net force? Yes of course, since the net force could for example be zero.

If you mean that the propulsion force must be bigger than the resistive force, then your answer is fine and correct.

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