Calculating Engine Power Output for 4000kg Car

AI Thread Summary
To calculate the engine power output for a 4000kg car traveling at 10m/s with a frictional force of 400N, the correct approach involves recognizing that the net force must equal zero for constant speed. The applied force from the engine must balance the frictional force, which means the engine's force is 400N, not 4000N. Using the formula P=Fv, the power output can be calculated as 400N multiplied by 10m/s, resulting in 4000W. Initial conditions, such as starting from rest, do not affect the calculation since the car is already moving at a constant speed. Understanding the problem thoroughly and applying the correct equations is crucial for accurate results.
AllenHe
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Homework Statement


In order for a car of 4000kg to travel at 10m/s with a frictional force of 400N, find the power output of the engine.


Homework Equations


P=Fv


The Attempt at a Solution


I found
40000*10+400*10=400400W,
is this right?
 
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AllenHe said:

Homework Statement


In order for a car of 4000kg to travel at 10m/s with a frictional force of 400N, find the power output of the engine.


Homework Equations


P=Fv


The Attempt at a Solution


I found
40000*10+400*10=400400W,
is this right?

No, not really.

For the car to be traveling at a constant speed, Newton's second law says that the net force on it must be 0. We know that there are two forces acting on the car: the friction force and the force from the engine. For the net force to be 0, these two must balance each other out. That tells you what the applied force from the engine must be. You can then use that applied force in the equation you posted above in order to determine the engine's power.
 
oh. SO it's 4000N?
But what if the question did not state whether the car started from rest or with an initial velocity of 10m/s?
 
AllenHe said:
oh. SO it's 4000N?

No, it's not 4000 N. Write the force balance equation for the horizontal direction:

sum of forces in horizontal direction = 0

and use this equation to solve for the applied force from the engine.

But what if the question did not state whether the car started from rest or with an initial velocity of 10m/s?

The initial condition is irrelevant here. You're told that, right now the car is moving at a constant rate of 10 m/s. This, along with the applied force, is enough info to determine the power output right now.
 
oh.. mistake I wanted to type 4000w.
do you have any pdf book which can help me at as level?
Sometimes I just use the formula to answer the question, without looking at the question carefully?Do you have any tips?
 

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