A girl on a sled is pulled along on a flat surface by her father at constant vel

Question

A girl on a sled is pulled along on a flat surface by her father at constant velocity. See the diagram below. Which of the following statements is true for this situation? (Select all that apply.)

SEE FIGURE***http://www.webassign.net/ncsuplsemech2/3-pre-002-v1.gif

The net force acting on the sled is zero.
The friction force between the sled and the surface is equal in magnitude and opposite in direction to the pulling force exerted by her father.
The acceleration of the sled is in the same direction as the pulling force.
The friction force between the sled and the surface is negligible.
The friction force between the sled and the surface is equal in magnitude and opposite in direction to the horizontal component of the pulling force exerted by her father.

Explanation / Answer

The first is true because there is constant velocity, therefore any and all force on the sled is in balance, and the only motive force is the father, and the retarding force is entirely friction. The second might be true, but not necessarily, and probably not. So no. Third is not true because the sled is not accelerating (trick question - velocity is in that direction and acceleration initially was) fourth Not true because the pulling force is presumably exerted from above the sled, so there's a vertical component (see #1) to the force that is not acting to pull the sled, it's acting to lift the sled, which reduces the force of gravity that creates the friction. The last is true because the sled is at constant velocity. If there was a net force, there would be an acceleration.

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