How Do You Calculate Car Weight and Required Motor Force?

[Jessi]

I was just hoping someone could check the work I did on this two part problem? :)

18a. A car has a mass of 1200 kg. What's the weight of the car?

Answer: 1200 kg*9.8 m/s^2 = 11760 N.

b. Disregarding friction, what force must the car motor apply to accelerate the car along a level highway at the rate of 4.0 m/s^2?

Answer: 11760 N/4.0 m/s^2 = 2940 kg.

Thanks so much in advance!

 

[Doc Al]

18a. A car has a mass of 1200 kg. What's the weight of the car?

Answer: 1200 kg*9.8 m/s^2 = 11760 N.

Right!

b. Disregarding friction, what force must the car motor apply to accelerate the car along a level highway at the rate of 4.0 m/s^2?

Answer: 11760 N/4.0 m/s^2 = 2940 kg.

No. You need to use F = ma. (Note that force must have the units of force, which is N. Mass (measured in kg) is not a force.)

 

[Jessi]

Oh, okay! :) Thank you for helping me.

So is it 11760 N*4.0 m/s^2 = 47040? I don't understand what the unit is, though - I thought I'd already used N in the problem, so it had to be mass. Does that not matter?

 

[Doc Al]

So is it 11760 N*4.0 m/s^2 = 47040?

No! This problem has nothing to do with the weight of the car.

Use F = ma, which means:
Force needed to accelerate car = (mass of car) x (acceleration).

The answer will be in N, the unit of force.

 

[Jessi]

Alrighty!

1200 kg (mass of car)*4.0 m/s^2 (acceleration) = 4800 N?

*hopeful*

 

[Gokul43201]

Yes, that's right.

 

[Jessi]

Wonderful! Thanks, both of you.