ap physics-level question: electric wheelchair

[Audentes]

HI, i'm having trouble with this question. i know it's physics, but its all math-involved and i hope someone can help me... i have answers down but i am very unsure about them because i think i may not have used the right formulas the whole way
thank you
FR

 

[topsquark]

HI, i'm having trouble with this question. i know it's physics, but its all math-involved and i hope someone can help me... i have answers down but i am very unsure about them because i think i may not have used the right formulas the whole way
thank you
FR


View attachment 34942

In the future, please present your work in a more organized manner.

Your method in part a) is correct. (I didn't check the numbers.)

But how did you do part b) that way without a coefficient of friction?

Here's how I would approach it. You were given a speed for the wheelchair. That hasn't changed. So how much power is going into motion? It's (1 - 0.000023)P. Thus, the actual force provided to make the wheelchair move is
F' = (1 - 0.000023)Pv.
The drag force is then Fd = F - F' = 0.000023Pv.

-Dan

 

[Audentes]

In the future, please present your work in a more organized manner.

Your method in part a) is correct. (I didn't check the numbers.)

But how did you do part b) that way without a coefficient of friction?

Here's how I would approach it. You were given a speed for the wheelchair. That hasn't changed. So how much power is going into motion? It's (1 - 0.000023)P. Thus, the actual force provided to make the wheelchair move is
F' = (1 - 0.000023)Pv.
The drag force is then Fd = F - F' = 0.000023Pv.

-Dan

Thank you for letting me know! I will make sure to follow your suggestion next time.

However, I do not understand why you used the final equation:

The drag force is then Fd = F - F' = 0.000023Pv.

This means the drag force is 0.12 N. That is a very low number which leads me to doubt it is the answer. I tried using Power = Force * Velocity and got 310 = 4.72 * F . Wouldn't the answer therefore be 65.68 N?

Thanks
FR

 

[topsquark]

Thank you for letting me know! I will make sure to follow your suggestion next time.

However, I do not understand why you used the final equation:


This means the drag force is 0.12 N. That is a very low number which leads me to doubt it is the answer. I tried using Power = Force * Velocity and got 310 = 4.72 * F . Wouldn't the answer therefore be 65.68 N?

Thanks
FR

(Looks like you need to check your force from part a).)

P = 310 W is the amount of power the battery supplies. How does that figure into the force due to drag? You need to find out how much power is being wasted by the drag force. That will be [imath]P_{ \text{wasted}} = F_{\text{drag}} v[/imath]. How much power is wasted?

Hopefully the drag force is small. Your calculation is the same size as the force used to move the wheelchair!

-Dan

 

[Audentes]

(Looks like you need to check your force from part a).)

P = 310 W is the amount of power the battery supplies. How does that figure into the force due to drag? You need to find out how much power is being wasted by the drag force. That will be [imath]P_{ \text{wasted}} = F_{\text{drag}} v[/imath]. How much power is wasted?

Hopefully the drag force is small. Your calculation is the same size as the force used to move the wheelchair!

-Dan

Got it, thank you!

FR