Forces and Motion, Free-body diagrams help

[FEMA]

Homework Statement

An object motion is described by the following equations:

deltax=(0m/s)(5s)+(1/2)(a_x)(5s²)

32N-18N=(12kg)(a_x)

Sketch a FBD, invent a situation which could be described by these equations. How far does the object move?

Homework Equations

displacement equation
?

The Attempt at a Solution

I sketched a box with two vertical vectors in opposite direction, one representing the weight and another the normal with equal magnitudes, and two horizontal vectors, one representing the friction against the box pointing left off the box and another representing the force accelerating the box pointing right. The magnitude of the vector pointing to the right is greater than the one pointing to the left. My situation: A rocket-propelled box starts at rest on a horizontal surface; it begins to accelerate. We are looking at the box 5s after it begins to accelerate.

After 5s, the box has moved deltax=(1/2)(1.17)(5²)=14.625m

Are all my answers consistent with the equations and with each other? And, when sketching displacement time and force time graphs, do I stop at 5 units on the x-axis (5s)?

Thank you very much.

 

[alphysicist]

Hi FEMA,

Homework Statement

An object motion is described by the following equations:

deltax=(0m/s)(5s)+(1/2)(a_x)(5s²)

32N-18N=(12kg)(a_x)

Sketch a FBD, invent a situation which could be described by these equations. How far does the object move?

Homework Equations

displacement equation
?

The Attempt at a Solution

I sketched a box with two vertical vectors in opposite direction, one representing the weight and another the normal with equal magnitudes, and two horizontal vectors, one representing the friction against the box pointing left off the box and another representing the force accelerating the box pointing right. The magnitude of the vector pointing to the right is greater than the one pointing to the left. My situation: A rocket-propelled box starts at rest on a horizontal surface; it begins to accelerate. We are looking at the box 5s after it begins to accelerate.

After 5s, the box has moved deltax=(1/2)(1.17)(5²)=14.625m

Are all my answers consistent with the equations and with each other? And, when sketching displacement time and force time graphs, do I stop at 5 units on the x-axis (5s)?

Thank you very much.

Depending on your teacher, you might have to be careful with the idea of a rocket. A rocket works by expelling mass, so you might have to explain how you have a system that is not losing mass during the 5s interval (or why such a loss is unimportant). Also, if you interpret one of the forces as friction, I would go ahead and calculate the coefficient since that is not difficult to do.

 

[thomate1]

I would say that your solution and reasoning are both valid and consistent with the given equations. Your free-body diagram accurately represents the forces acting on the box and your chosen situation is appropriate for the given equations. Your calculated displacement of 14.625m is also consistent with the given equations and makes sense in the context of the situation you described.

In terms of the displacement and force-time graphs, yes, it would make sense to stop at 5 units on the x-axis (representing 5 seconds) since that is the time interval given in the equations. However, it is always important to check and make sure that your graphs are consistent with the equations and the situation described, so it would be a good idea to double check your graphs to make sure they accurately represent the given information. Overall, your answers seem to be consistent and well thought out. Keep up the good work!