You’ve just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these forces are forces acting on the rock?

Question

You’ve just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these forces are forces acting on the rock?

1. Gravity, acting downward
2. The normal force, acting upward
3. The force of the kick, acting in the direction of motion.
4. Friction, acting opposite the direction of motion.
5. Air resistance, acting opposite the direction of motion.

Answer 1

Hi Alizee:

Let's tackle this one. The forces acting on the rock are:

1. Yes, gravity always acts on bodies within the earth's gravitational field. The force is mg for a mas, m.
2. Yes, the normal force counters the gravitational force, so it, too, equals mg.
3. No, the kick force is ended and now the body moves under the et effect of the other forces.
4. Yes, the force of kinetic (i.e., moving) friction acts opposite to the direction of motion and tends to cause the body to stop moving. This force depends on the "roughness" of the rock and of the ground across which it's moving.
5. Yes, air resistance operates opposite to the direction of motion and is usually proportional to the object's speed. Since the rock moves relatively slowly this force is likely to be small, compared to the frictional force.