# Force Diagrams for a Combined Mass

• rmiller70015
In summary, the conversation discusses the relationship between different forces acting on a truck, including the force of the engine, friction, and other external forces. The question is raised about the relative size of these forces and their impact on the truck's speed. The concept of friction as an accelerating force is also briefly discussed.
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?

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..

## 1. What is a force diagram for a combined mass?

A force diagram for a combined mass is a visual representation of all the forces acting on a system of multiple masses that are connected or in contact with each other. It helps to analyze and understand the overall forces acting on the system.

## 2. How do you draw a force diagram for a combined mass?

To draw a force diagram for a combined mass, first identify all the masses involved and the forces acting on them. Then, draw each mass as a separate point and label them. Next, draw arrows representing the magnitude and direction of each force acting on each mass. Finally, make sure the forces are labeled and the diagram is neat and organized.

## 3. What are the types of forces that can be included in a force diagram for a combined mass?

The types of forces that can be included in a force diagram for a combined mass are gravitational force, normal force, tension force, frictional force, and applied force. These forces can act in different directions and magnitudes on the different masses in the system.

## 4. How does a force diagram for a combined mass help in solving problems?

A force diagram for a combined mass helps in solving problems by providing a visual representation of all the forces acting on the system. This allows for a better understanding of the forces involved and helps in applying the principles of Newton's laws of motion to solve the problem. It also helps in identifying any missing or unbalanced forces that may affect the system.

## 5. Can a force diagram for a combined mass be used for any system of masses?

Yes, a force diagram for a combined mass can be used for any system of masses as long as the masses are connected or in contact with each other. It is a universal tool that can be applied to various scenarios, such as objects on an inclined plane, pulley systems, or objects connected by ropes or springs.

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