Force Diagrams for a Combined Mass

AI Thread Summary
In the discussion about force diagrams for a combined mass, participants explore the dynamics of a truck's motion, focusing on the forces acting on it. The engine's role is clarified as providing angular acceleration to the wheels, with the actual external force for translational acceleration being the friction between the wheels and the ground. Questions arise regarding whether the driving force must exceed both the combined forces of friction and the force exerted by object B on A, or just the friction force alone. The conversation also touches on the nature of friction, debating whether it can act as an accelerating force under certain conditions. Overall, the thread emphasizes the importance of accurately representing forces in free body diagrams to understand vehicle dynamics.
rmiller70015
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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?
 
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rmiller70015 said:
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.
rmiller70015 said:
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.
 
haruspex said:
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.
 
Delta2 said:
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.
 
haruspex said:
Contrast that with what post #1 says about friction.
So you trying to say that friction is an accelerating force, not a decelerating one?
 
Delta2 said:
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..
 
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