Drawing free-body diagrams

  • #1
mirs08

Homework Statement


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.

Homework Equations


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
 

Answers and Replies

  • #2
kuruman
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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?
1. A key part of FBDs is identifying all the pieces of the Universe that exert a significant force on the system. Here the system is the tennis ball and the only piece of the Universe acting a significant force on it is the Earth. I say "significant", because the Sun and the Moon and Jupiter and ... so on also exert forces on the tennis ball, but they are insignificant relative to the force exerted by the Earth. The direction of the velocity is irrelevant in this case. The Earth will exert the same force on the tennis ball whether it moves horizontally or vertically or at an angle. That force is equal to the weight of the tennis ball.
2. I assume the question is about the elevator slowing down while it's moving up. You need a force (and an acceleration) opposite to the direction of the velocity in order to reduce the object's speed.
3. Good question. The acceleration has to be zero because the skier is moving at constant speed. There is a component of gravity parallel to the slope giving rise to a downhill acceleration. If there were no friction to oppose it, the skier would move downhill faster and faster.
 
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  • #3
mirs08
1. A key part of FBDs is identifying all the pieces of the Universe that exert a significant force on the system. Here the system is the tennis ball and the only piece of the Universe acting a significant force on it is the Earth. I say "significant", because the Sun and the Moon and Jupiter and ... so on also exert forces on the tennis ball, but they are insignificant relative to the force exerted by the Earth. The direction of the velocity is irrelevant in this case. The Earth will exert the same force on the tennis ball whether it moves horizontally or vertically or at an angle. That force is equal to the weight of the tennis ball.
2. I assume the question is about the elevator slowing down while it's moving up. You need a force (and an acceleration) opposite to the direction of the velocity in order to reduce the object's speed.
3. Good question. The acceleration has to be zero because the skier is moving at constant speed. There is a component of gravity parallel to the slope giving rise to a downhill acceleration. If there were no friction to oppose it, the skier would move downhill faster and faster.
That makes so much sense! Thank you!
 
  • #4
haruspex
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That makes so much sense! Thank you!
Not sure exactly where your difficulties lay with the first two, so I'll try to cover a bit more.
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?
The downward Fg provides acceleration. Do not confuse acceleration with velocity. Acceleration is the rate of change of velocity.
At the highest point of the ball's trajectory, its velocity is entirely horizontal, but that is an instantaneous state. Before that it was moving up a bit, and afterwards it moves down. The net change is a downward velocity, so that is a downward acceleration.
It keeps moving because there is no horizontal force, so no horizontal acceleration, so no change in horizontal speed.
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?
Fnet is the net force. It is not an applied force, but rather the vector sum of the applied forces.
The applied forces are weight, Fg, and the normal force, N, from the floor of the elevator. The net force is N+Fg (these will have opposite sign, so partly cancel). The net force results in the acceleration. Which way is the acceleration?
 
  • #5
CWatters
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Points to remember..

a) The "F" in Newton's F=ma is the net force.
b) it works for negative forces and negative acceleration.
c) F=ma says nothing about the sign of the velocity, you can have a velocity in the opposite direction to an acceleration.
 
  • #6
Stephen Tashi
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-- but shouldn't there be an applied force somewhere? What keeps the ball moving and why do we neglect this part?
As the others have indicated, no force is required to sustain a constant velocity. This fact seems to contradict our daily experience. For example, if we are pushing a wheelbarrow along, we must continually exert force on it when our goal is to keep it moving at a constant rate. This is because there are other forces acting upon the wheelbarrow due to the unevenness of the ground, friction etc.

If a real tennis ball is moving horizontally over a net, there is a force of "air resistance" acting on it and it would not move at a constant velocity unless some force counteracted that. The problem doesn't say the tennis ball is moving at a constant velocity. An object that is deaccelerating can "keep moving" as it slows down. The fact that there is a force acting on a tennis ball that tends to slow it down doesn't mean that another force is needed to prevent the tennis ball from suddenly stopping.


It's common for textbook problems to ignore air resistance, friction, uneven surfaces etc. You have to guess if your text materials intend for you to do this when they describe real life situations.
 

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