Given some body with a constant force F acting on it upwards

In summary, the conversation discusses the effects of a constant upward force on a body subject to the gravitational force of the earth. The correct statement is that the net force on the body, including gravity, must be used in the "Work - KE" theorem. This means that the change in kinetic energy is equal to the work done by the net force minus the change in gravitational potential energy.
  • #1
xmjolx
4
0
Given some body with a constant force F acting on it upwards
(The body is subject to the gravitational force due to the earth)
What's the correct statement?

gif.gif


Or:

gif.gif


K - Kinetic Energy
 
Last edited:
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  • #2
Think about how the potential energy changes with [itex]\Delta{x}[/itex]. The change in total energy, which includes both kinetic and potential energy, will equal the total amount of work done on the body.

You might also find it helpful to consider the cases: F is only very slightly larger than mg; and F is so large relative to mg that we can ignore the effects of gravity.
 
  • #3
what if F is small?
how come gravity has no affect on the change in kinetic energy?
 
  • #4
xmjolx said:
Given some body with a constant force F acting on it upwards
(The body is subject to the gravitational force due to the earth)
What's the correct statement?

gif.gif


Or:

gif.gif


K - Kinetic Energy
If you want to apply the "Work - KE" theorem, you must use the net force on the body. That net force must include gravity.
 
  • #5
Doc Al said:
If you want to apply the "Work - KE" theorem, you must use the net force on the body. That net force must include gravity.

So
gif.gif
is the right statement?
 
  • #6
xmjolx said:
So
gif.gif
is the right statement?
Yes.
 
  • #7
Thank you very much! :)
i knew it was the right answer but i had an argument with my high school teacher(she claims to have Ph.D in physics) about that.
 
  • #8
Multiply out the rhs, then
[tex]\Delta K = F \Delta x - mg \Delta x.[/tex]
Identifying the terms on the right as work or energy:
[tex]\Delta K = work\;done\;by\;F - \Delta (grav\;potential\;energy).[/tex]
 

Related to Given some body with a constant force F acting on it upwards

1. What is the relationship between force and motion?

The relationship between force and motion is described by Newton's second law of motion, which states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This means that the greater the force applied to an object, the greater its acceleration, and the heavier the object, the less it will accelerate for a given force.

2. How does a constant force affect an object's motion?

A constant force acting on an object will cause it to accelerate at a constant rate. This means that the object's velocity will change by the same amount in each unit of time, resulting in a linear increase in its speed.

3. Does the direction of the force matter?

Yes, the direction of the force is an important factor in determining the object's motion. If the force is applied in the same direction as the object's motion, it will increase its speed. However, if the force is applied in the opposite direction, it will cause the object to slow down or even change direction.

4. What happens if there are multiple forces acting on an object?

If there are multiple forces acting on an object, the net force can be calculated by adding together all the individual forces. The resulting net force will determine the acceleration of the object. If the forces are equal and opposite, the object will remain at a constant velocity.

5. Can a constant force ever result in a change in an object's direction?

Yes, a constant force can result in a change in an object's direction if the direction of the force is not aligned with the object's initial motion. This is because the resulting acceleration will cause the object to change its velocity, resulting in a change in direction.

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