Problem with the power of car

[rickyw2777]

Homework Statement

Some of the power performance parameters of an electric car are shown in Table 7-1. If the car starts from rest and moves along a straight road with a uniform acceleration of a = 1.5 m/s2, when the speed reaches v1 = 72 km/h, the motor works at the rated power, and the power of the motor remains unchanged. Assume that the resistance of the car during driving is constant.

(1) Analyze and explain the changes in the acceleration and speed of the car after the motor reaches the rated power.

(2) What is the maximum speed of the car when the motor is working at the rated power?

(3) After the car is fully charged, how long does it take to move in the above manner?

Relevant Equations

P=Fv P=W/t

Some of the power performance parameters of an electric car are shown in Table 7-1. If the car starts from rest and moves along a straight road with a uniform acceleration of a = 1.5 m/s2, when the speed reaches v1 = 72 km/h, the motor works at the rated power, and the power of the motor remains unchanged. Assume that the resistance of the car during driving is constant.

(1) Analyze and explain the changes in the acceleration and speed of the car after the motor reaches the rated power.

(2) What is the maximum speed of the car when the motor is working at the rated power?

(3) After the car is fully charged, how long does it take to move in the above manner?

I don't quite understand why the driving force decreases after the car reaches the rated power? Why does the car still accelerate after reaching the rated power? Why does the resistance equal the power when the car maintains the rated power?

 

[phinds]

I don't quite understand why the driving force decreases after the car reaches the rated power? Why does the car still accelerate after reaching the rated power? Why does the resistance equal the power when the car maintains the rated power?

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[erobz]

The questions seem half baked…but give them a try so we can see what you know.

 

[haruspex]

Start by thinking through what is happening before it reaches $v_1$.
If the resistance is $F_r$, what is the thrust from the engine during that time?
What, therefore, is the thrust when it reaches $v_1$? How does that relate to the rated power?