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Definition of a Projectile:
A projectile is an object for which the only force acting upon it is gravity. The force of air resistance is negligibly small or nonexistent for a projectile.
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A force diagram is a diagram depicting all the forces acting upon an object. The forces are represented by arrows. Each individual force is represented by its own arrow directed in the direction of the force. There is only one force on a projectile - the force of gravity. Gravity pulls on an object in the downward direction. Thus, the only appropriate force diagram for a projectile is one which shows a single force, directed downwards.
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At the peak of its trajectory, a projectile's vertical velocity is 0 m/s for an instant in time. This tempts many students of physics to choose a diagram that does not show a vertical arrow. These students are confusing velocity vectors with force vectors. But don't be fooled! While a projectile has no vertical velocity at its peak, there is still a force of gravity acting upon it. In fact, gravity is the only force acting on a projectile throughout its motion. Thus, the only appropriate force diagram is the one that shows a single downward arrow representing the single downward force.
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