I am just going over my class notes and I just wanted to clear up a few concepts regarding forces and free-body diagrams -- i'm either missing some forces or adding one too many:
Identify the forces acting on objects in each of the following situations and draw a free-body diagram (assume no air resistance present):
1. A tennis ball flies over a net. At the instant we are examining it, it is travelling horizontally.
2. An elevator is moving vertically upwards and is coming to a stop.
3. A skier is sliding down the slope at constant speed.
Fnet=ma, or Fg=mg
The Attempt at a Solution
1. There is Fg pointing downwards -- but shouldn't there be an applied force somewhere? What keeps the ball moving and why do we neglect this part?
2. The instructor's notes show that there is an Fnet force point downwards. If the elevator is moving upwards, why is Fnet pointing downwards? Is this just the force of gravity/weight of the elevator?
3. I drew it as Fg pointing directly downwards from the skier and the normal force as perpendicular to the slope -- but I missed the force of kinetic friction which is supposed to point upwards (opposite to the skier's movement) and parallel to the slope's surface; why should any kind of friction be present if the skier is sliding at constant speed?
I appreciate any kind of feedback/help! Thank you