Relation between Friction, Velocity, and Acceleration

[steff-23]

Homework Statement

Draw a free force diagram and its graphs of constant velocity and acceleration

Relevant Equations

net force=(mass)(Acceleration)

For reference I'm a high school senior in physics.

My thought process:
1) If the force of friction and applied force are equal the object "should" be at rest because the forces are equal (obviously this isn't correct because my professor tried explaining it and I'm not understanding the concept)
2) If there is friction in the free force diagram there's resistance in the object's motion meaning it'll eventually stop, thus acceleration is not constant
3) Friction is a "slowing down" force
4) Both diagrams have proper force of gravity and normal force, as well as applied forces with no force of friction (will upload photo of my diagrams when I can but anything helps)

What my diagrams look like:
Constant velocity would be a straight line with no slope starting at some positive velocity, constant acceleration would be an increasing line starting at the origin with equal slopes

If someone could explain the relationships between each of these forces that'd be great thanks!

 

[kuruman]

Homework Statement: Draw a free force diagram and its graphs of constant velocity and acceleration

A free force diagram of what? What is the system?

 

[haruspex]

If the force of friction and applied force are equal the object "should" be at rest because the forces are equal

Which of Newton's laws applies here? What does it say?

If there is friction in the free force diagram there's resistance in the object's motion meaning it'll eventually stop

Friction is a "slowing down" force

Can you accelerate (as in, go faster) when walking on the flat or up a hill? What force is responsible?

 

[steff-23]

A free force diagram of what? What is the system?

I just attached my version of the diagrams

 

[steff-23]

Which of Newton's laws applies here? What does it say?

First law, objects at rest will stay at rest unless a force acts on it

Can you accelerate (as in, go faster) when walking on the flat or up a hill? What force is responsible?

accelerating is easier on flat because there's less friction between your feet and the ground compared to up a hill

 

[steff-23]

A free force diagram of what? What is the system?

 

[haruspex]

First law, objects at rest will stay at rest unless a force acts on it

In post #1 you did not specify that the body is initially at rest.

accelerating is easier on flat because there's less friction between your feet and the ground compared to up a hill

That is not what I asked.
If you speed up while walking on flat ground, what force on your body causes the acceleration?

 

[kuruman]

Thank you for posting your free body diagrams (FBDs), but you did not really answer my question. Usually, when one draws a FBD there is a system that moves under the influence of one or more forces acting on it and the FBD is specifically drawn for that situation. Let me rephrase my question.

Does the statement of the problem as given to you specify what the system is and what its state of motion is? If "yes", then please tell us. If "no" there is a wide choice of FBDs to draw. Below are some possible examples, each having its own FBD.

• A small block at rest on a flat horizontal tabletop.
• A small block at rest on a flat tabletop inclined with respect to the horizontal.
• A small block being pushed up an incline at constant speed.
• A small block accelerating down flat tabletop inclined with respect to the horizontal under the influence of gravity only.
• A small block being pushed up an incline at constant acceleration.
• A small block accelerating down a curved slide.
• etc., etc.

Do you see where this is going?

 

[steff-23]

it's high school physics I'm just typing out what my teacher asked me to do. The diagrams are supposed to be the general FBD on 3 different objects if they were at rest, constant acceleration, and constant velocity. It wasn't specified if the object was on an incline in the question. What I have on my paper is exactly what I was told to write down.

 

[steff-23]

In post #1 you did not specify that the body is initially at rest.

That is not what I asked.
If you speed up while walking on flat ground, what force on your body causes the acceleration?

net force?

 

[kuruman]

it's high school physics I'm just typing out what my teacher asked me to do. The diagrams are supposed to be the general FBD on 3 different objects if they were at rest, constant acceleration, and constant velocity. It wasn't specified if the object was on an incline in the question. What I have on my paper is exactly what I was told to write down.

Thank you, that clarifies the task. The first posted FBD of the body at rest looks fine. The other two look the same when there should be a difference between them. What kind of difference? You may wish to redraw and repost to scale as best as you can.

 

[steff-23]

What kind of difference?

In my mind, the reason the FBD of acceleration/velocity looks the same is because if I'm changing position at a constant rate and the change in velocity is constant, there should be a greater applied force than the force of friction.

 

[kuruman]

In my mind, the reason the FBD of acceleration/velocity looks the same is because if I'm changing position at a constant rate and the change in velocity is constant, there should be a greater applied force than the force of friction.

Are you saying that, in both diagrams, the pulling force to the right must be greater than the force of friction to the left because the block is moving to the right?

 

[steff-23]

Are you saying that, in both diagrams, the pulling force to the right must be greater than the force of friction to the left because the block is moving to the right?

yes

 

[kuruman]

What does Newton's second law say?

 

[steff-23]

I've been thinking about this since noon it's currently 9 PM. I think I got it now. Constant velocity/acceleration just means I'm putting the same amount of force to move the object to exactly cancel out friction while still moving it forward. At rest just means no force going against friction, and friction does not have a force going against it. Did I get it??????

 

[haruspex]

net force?

Yes, of course, but when walking and accelerating on level ground there is only one horizontal force acting on you: what is it? Same for a car accelerating on level ground (ignoring air resistance), there is only one horizontal force acting on the car.
Another hint: would you have difficulty accelerating if the ground were smooth ice?