Determining net force and acceleration

In summary: It's starting to make more sense now. So essentially, deceleration is just a more general term that encompasses both negative acceleration and slowing down in everyday language, but in physics, we use the term negative acceleration to describe a decrease in speed.
  • #1
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Homework Statement
The diagram (attached) shows a parachutist of mass 82 kg falling towards
the Earth. In each case, determine the net force and the acceleration
of the parachutist
Relevant Equations
F=Ma
Hi All,

Please see attached photo of the question.

It is asking for net force and acceleration. Taking the forces acting upwards and downwards on the
parachutist as vectors: for A the net force would be 800-300=500; B would be 0 and C; would that be 800-1500=-700? And if so would that imply that this is a negative acceleration of -8.5ms^-2 or is this a deceleration?

Thanks
 

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  • #2
Shaye said:
Homework Statement:: The diagram (attached) shows a parachutist of mass 82 kg falling towards
the Earth. In each case, determine the net force and the acceleration
of the parachutist
Relevant Equations:: F=Ma

Hi All,

Please see attached photo of the question.

It is asking for net force and acceleration. Taking the forces acting upwards and downwards on the
parachutist as vectors: for A the net force would be 800-300=500; B would be 0 and C; would that be 800-1500=-700? And if so would that imply that this is a negative acceleration of -8.5ms^-2 or is this a deceleration?

Thanks
Welcome to PhysicsForums :smile:

You are on the right track (although I would include units with each of your equations and answers).

Think about what's going on in each of those 3 stages:
  1. Initially the force of gravity is greater than air resistance, so the parachutist is speeding up in the fall
  2. At terminal velocity, the forces of gravity and air resistance are equal
  3. Right after pulling the ripcord, the force up is greater than the force of gravity down, so the parachutist decelerates
  4. (Eventually the parachutist+parachute reach a new, slower terminal velocity)
Makes sense?
 
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  • #3
Shaye said:
Taking the forces acting upwards and downwards on the
parachutist as vectors
I think you mean "taking down as positive for forces and accelerations ".
Shaye said:
would that imply that this is a negative acceleration of -8.5ms^-2 or is this a deceleration?
Since you have taken down as positive, this is an upward acceleration.
The term 'deceleration' is not much used in physics. In every day usage it means the speed is decreasing, i.e. the velocity and acceleration have opposite signs. So, yes, it is also a deceleration.
 
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  • #4
berkeman said:
Welcome to PhysicsForums :smile:

You are on the right track (although I would include units with each of your equations and answers).

Think about what's going on in each of those 3 stages:
  1. Initially the force of gravity is greater than air resistance, so the parachutist is speeding up in the fall
  2. At terminal velocity, the forces of gravity and air resistance are equal
  3. Right after pulling the ripcord, the force up is greater than the force of gravity down, so the parachutist decelerates
  4. (Eventually the parachutist+parachute reach a new, slower terminal velocity)
Makes sense?

Thanks! My point of confusion has always been around the difference between deceleration and negative acceleration. Deceleration as you said is slowing down and negative acceleration just depends on what we take as positive or negative direction I guess
 
  • #5
haruspex said:
I think you mean "taking down as positive for forces and accelerations ".

Since you have taken down as positive, this is an upward acceleration.
The term 'deceleration' is not much used in physics. In every day usage it means the speed is decreasing, i.e. the velocity and acceleration have opposite signs. So, yes, it is also a deceleration.
Cool! Thanks for this. Always needed to clarify in my head deceleration and negative acceleration
 
  • #6
Shaye said:
... My point of confusion has always been around the difference between deceleration and negative acceleration.
The direction of the net force respect to the existing direction of movement is the key.
Negative or positive is just a convention of signs that you are free to establish.
Please, see:
http://hyperphysics.phy-astr.gsu.edu/hbase/Mechanics/motgraph.html#c2 :cool:
 
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