Why Does a Car Speed Up on a Sloped Driveway Despite Friction?

[imaqueen]

I am really having a hard time with this there is something I am missing can someone help
a 2100 kg car starts from rest at the top of a 5.0 m long driveway that is sloped at 20 Deg with the horizontal . if an average friction force of 4000 N impedes the motion find the speed of the car at the bottom of the driveway. The answer is 3.8m/s

 

[nbo10]

Use Newton's law F_total = m*a.

JMD

 

[Astrophysics]

That's not entirely correct nbo10, first u need to calculate the Gravitational force on the truck, then you calculate the Fx, which will help you further.

p.s. don't forget the 20 degrees

 

[nbo10]

Originally posted by Astrophysics
That's not entirely correct nbo10, first u need to calculate the Gravitational force on the truck, then you calculate the Fx, which will help you further.

p.s. don't forget the 20 degrees

No, I'm entirely correct. F_total includes gravitational foce and friction.

JMD

 

[Astrophysics]

I think, I think more like a physics teacher, since I don't think F_total = m*a is very precise. I think emaqueen already knew about this equation.

 

[HallsofIvy]

I'm puzzled by that. I will admit that I'm NOT a physics teacher so perhaps I don't think like one. But in a problem that talks about forces and asks for speed, I would think that F= ma would be key.


Perhaps you mean that we should do it by calculating the energy!
Okay, that's often a good way to do these problems. Initially, the car has 0 kinetic energy. Since the driveway is 5m long and sloped at 20 degrees, the height of the car at the top of the driveway is 5 sin(20)= 1.71 m and its potential energy is (2100)(9.8)(1.71)=