Your car is accelerating to the right from a stop. Identify all forces acting on

Question

Your car is accelerating to the right from a stop. Identify all forces acting on the object and draw the free-body diagram.

NOTE: I understand the normal force will point straight up, and that the weight will point straight down. I also understand that the Drag will point to the left to oppose motion. What I am confused about is my teacher put on our key that the frictional force should point to the right (he is prone to error and not answering emails).

My question is: wouldn't the frictional force go left to oppose the car's right facing motion?

Explanation / Answer

First you have to understand what is friction. Friction always opposes relative motion. It always acts opposite to the direction of "relative intended motion'. So, it always opposes the tendency of two objects to slip over each other. Now coming to the car case. As your car is at rest initially, think about the moment you start and give it acceleration. What happens to the rear tire. It will try to spin(clockwise if you imagine the car moving right), and thus it tries to slip over the ground. Now friction sees that there are two surfaces(tyre and the road) which are trying to slip. you can deduce that the 'intended direction of the slip of tyre ' is bakcward, So friction will push the tyre and hence the car forward.
(PS: This is indeed a very confusing doubt and you'll learn more about it in details when you read rotational motion. There is a whole lot of theory and understanding behind friction direction on tyre in various cases. If you want to learn more about it in detail, you can google msg me at gml mentioned in the next line.
iamashishgour93)

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