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Old Jul 11th 2014, 11:40 PM   #1
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An object does not always move in the direction of the net force?

I am confused on this statement. I am trying to find a scenario when an object doesn't move in the direction of Fnet. I think it does bc the direction of acceleration will determine the direction of Fnet and the acceleration will also determine the direction of the object.

The context is studying for physics with my physics book on dynamics. The book gives me another hint to figure my quandary on this concept out by telling me that Newton's second law tells us about the direction of an object's acceleration but does not define the direction of an object's velocity.

Even with the hint, I still do not understand the concept.

Shouldn't the direction of acceleration be also the same direction of velocity?

Thanks for all your help physics forum!

Sincerely,
Jonathan
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Old Jul 11th 2014, 11:51 PM   #2
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Ahhhh I figured it out. Velocity can be positive but decreasing, therefore acceleration will be negative and the objects direction will be positive though the force and acceleration will be negative in direction.
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Old Jul 12th 2014, 09:25 AM   #3
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Correct - the acceleration is in the direction of Fnet, but remember that acceleration is not velocity - it is the change on velocity per unit time. If the velocity is in the opposite direction of force (like for a ball thrown upward) the object's speed will decrease. Another example: a satellite in circular orbit about the Earth experiences gravitational force toward the Earth, but it moves perpendicular to that force. As it orbits in a circle its magnitude of velocity (speed) does not change, but its direction of velocity does - the rate of change of the direction of the velocity vector is equal to the magnitude of acceleration toward the center of the Earth.

Last edited by ChipB; Jul 12th 2014 at 11:16 AM.
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Old Jul 12th 2014, 10:36 AM   #4
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Thank you once again ChipB! You are too awesome. Thanks also for your patience and answering all my other questions. They make great sense!
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