Force Diagrams for a Combined Mass

[rmiller70015]

Homework Statement:

A truck that is towing a trailer is accelerating, draw the force diagram for the truck and the trailer

Relevant Equations:

F = ma

I know that the $F_{AonB} = -F_{BonA}$, but I just wanted to check something. If object A is the truck, then the x-direction should have a vector coming from the force of the engine driving the truck forward, a vector pointing in the negative direction for friction, and a vector in the negative direction for $F_{BonA}$. But for the FBD of the truck, does the driving force vector need to be larger than the combined $F_{BonA}$ and $F_k$ or just larger than the friction force?

 

[haruspex]

a vector coming from the force of the engine driving the truck forward, a vector pointing in the negative direction for friction,

What is this force the engine exerts on the truck? Isn't the engine part of the truck?
And what friction force do you have in mind opposing that? Try to be precise.

for the FBD of the truck, does the driving force vector need to be larger than the combined $F_{BonA}$ and $F_k$ or just larger than the friction force?

Would the truck go faster without the trailer?
Remember, NET force = mass x acceleration.

 

[Delta2]

What is this force the engine exerts on the truck? Isn't the engine part of the truck?

This is used in many introductory books in physics, that is that they resemble the role of the engine as something that supplies an external force to the vehicle. Of course what happens in reality is that the engine provides angular acceleration to rotate the wheels and the real external force that gives translational acceleration to the vehicle is the friction between the rotating wheels and the ground.

 

[haruspex]

This is used in many introductory books in physics, that is that they resemble the role of the engine as something that supplies an external force to the vehicle. Of course what happens in reality is that the engine provides angular acceleration to rotate the wheels and the real external force that gives translational acceleration to the vehicle is the friction between the rotating wheels and the ground.

Contrast that with what post #1 says about friction.

 

[Delta2]

Contrast that with what post #1 says about friction.

So you trying to say that friction is an accelerating force, not a decelerating one?

 

[haruspex]

So you trying to say that friction is an accelerating force, not a decelerating one?

It depends what the OP meant by friction in post #1, hence the question in post #2..