Forces and Motion, Free-body diagrams help

  • Thread starter FEMA
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  • #1
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Homework Statement



An object motion is described by the following equations:

deltax=(0m/s)(5s)+(1/2)(ax)(5s2)

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

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.
 

Answers and Replies

  • #2
alphysicist
Homework Helper
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Hi FEMA,

Homework Statement



An object motion is described by the following equations:

deltax=(0m/s)(5s)+(1/2)(ax)(5s2)

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

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.
 

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