Why Doesn't My Power Calculation Match Real-World Scenarios?

  • Thread starter Thread starter Aeronautician
  • Start date Start date
  • Tags Tags
    Accelerate Power
Click For Summary
The discussion centers on discrepancies between calculated power requirements for a motor and real-world scenarios. The user calculated that a motor needs approximately 8.8 horsepower to accelerate a 1633 kg object over 2 meters in 1 second, but noted that a much larger 215 hp system is used in a different scenario. Participants suggest that the user's calculation assumes constant acceleration, while real-world applications often involve varying power outputs and maximum power at peak velocity. They recommend using kinematic equations and clarifying whether the motor operates at constant power or has a specific power versus velocity curve. Understanding these dynamics is crucial for accurate power calculations in practical applications.
Aeronautician
Messages
2
Reaction score
0

Homework Statement


Hi, guys. I'm in a bit of a quandary. I'm trying to calculate the power a motor should have in order to move a mass a certain distance. My calculation keeps telling me that the horsepower output is much lower than my reference scenario. Here's the situation. I'm trying to figure out how to accelerate a 1633 kg object a distance of 2 m in 1 sec.
m = 1633 kg
d = 2 meters
t = 1 sec

Homework Equations


F=ma
P=F(d/t)

The Attempt at a Solution


Acceleration is thus 2 m/(s^2). According to F=ma, F = 3266 Newtons. Plugging this into P=F(d/t), where d=2m, t=1 sec, Power needed = 6532 Watts or approx 8.8 hp. The problem is in a similar situation, a 215 hp propulsion system is needed to provide the thrust to accelerate and sustain a mass of 95 kg above ground. Where is this discrepancy coming from? My calculations clearly do not correlate with the already established real world scenario. Can I get some direction in this issue?
 
Physics news on Phys.org
Use the kinematic motion equations
to solve for acceleration. Your formula for power yields average power, not max power at the 2 meter mark
 
  • Like
Likes Zachary Kempler
+1. The required acceleration is not 2m/s^2.

If the acceleration is constant the power isn't constant and the max power will occur at max velocity.
 
Aeronautician said:
trying to calculate the power a motor should have
Turning around the preceding responses, your situation is constant power, so not constant acceleration.
The SUVAT equations will therefore not apply. You need to work instead with the more general differential forms.
Work done in time t ##=Pt=\frac 12 mv^2##. Can you solve to find position as a function of time?
 
Perhaps we need the OP to clarify...

Does the motor generate a constant power? Does it have a power vs velocity or rpm curve?
 
The book claims the answer is that all the magnitudes are the same because "the gravitational force on the penguin is the same". I'm having trouble understanding this. I thought the buoyant force was equal to the weight of the fluid displaced. Weight depends on mass which depends on density. Therefore, due to the differing densities the buoyant force will be different in each case? Is this incorrect?

Similar threads

  • · Replies 25 ·
Replies
25
Views
2K
  • · Replies 42 ·
2
Replies
42
Views
6K
  • · Replies 4 ·
Replies
4
Views
1K
  • · Replies 41 ·
2
Replies
41
Views
5K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 13 ·
Replies
13
Views
4K
Replies
7
Views
2K
  • · Replies 20 ·
Replies
20
Views
2K