Forces acted on the car when a car moves

• Automotive
Hi everyone, good day.
when a car is moving (accelerating), what are the forces that actually acted on the car?
Is it possible to draw a free body diagram to illustrate the idea? (please draw one for me, i really need this in order to understand)
I am having problem in relating how the engine torque causing the car to move forward. i mean i can totally understand how engine convert chemical energy in petroleum to rotating output shaft, and how output shaft goes through gearing to the wheel (i understand the physical mechanism). but i failed to draw a free body diagram to analyze the system.
Also, i am quite confuse in the direction of friction acting on the car wheels. Say, the car is front wheel driven by engine, i heard people saying the friction force is the same direction as the car's forward motion on the front wheel (driven by engine), while is in the opposite direction with the car's forward motion on the back wheel (non driven wheel). is this true? if it is, can you please explain to me why it is so?
Thanks a lot for you reply!

russ_watters
Mentor
Can't you give the fbd a try yourself? Then we can help you fix the errors.

Hi, russ_watters, thanks for the reply.
attached is the fbd i tried. please comment on it. Thank you!
F_f = friction act on front wheel
F_b = friction act on rear wheel
N_f = normal force act on front wheel
N_b = normal force act on rear wheel
w = weight of the car
T = torque from the engine acted on front wheel

Attachments

• IMAG1133.jpg
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Also, i am actually quite confuse on one thing. let say, there are two cars, all features on them are the same except one, the mass of two cars. Now, if the two cars are accelerated to the same speed and then the engine is turned off, they will slide for some distance and then come to stop.
my question is, which car can slide further? and what theory explains the answer?
originally, i am thinking of the lighter car can slide farther because the friction force act on it is lesser (smaller normal force), but i recalled that the body with higher mass has higher inertia, which maybe allows the heavier car to slide further.
can anyone clarify this for me? this has been bugging me for few days. thanks a lot!

Ranger Mike
Gold Member
read post on last page of Jul24-12, 10:34 AM

Re: Race car suspension Class in this forum

Hi Ranger Mike, I have read the article. It does not really answer my question. It does not clear my confusion. Can you please elaborate further?

Mech_Engineer
Gold Member
What is your F_b force acting horizontally on the rear wheel? Aditionally, do you want to take into account wind resistance?

jack action
Gold Member
Look at the theory at the bottom of this page (Theory »» Longitudinal acceleration).

DaveC426913
Gold Member
Also, i am quite confuse in the direction of friction acting on the car wheels. Say, the car is front wheel driven by engine, i heard people saying the friction force is the same direction as the car's forward motion on the front wheel (driven by engine), while is in the opposite direction with the car's forward motion on the back wheel (non driven wheel). is this true? if it is, can you please explain to me why it is so?
The car has power to its front wheels.