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Home Minds On Vectors and Projectiles Mission VP8 Velocity and Acceleration of a Projectile Help Help 8

Mission VP8 Velocity and Acceleration of a Projectile

The Question

Which of the following statements are true of the vertical velocity (vy) of a projectile? List all that apply ...

Define Help

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.

Minds On Time

Forces when unbalanced cause objects to accelerate; and the direction of the acceleration is in the same direction as the unbalanced or net force. For a projectile, the net force is the downward force of gravity; thus, the acceleration is downward. A downward acceleration could mean two things: the object is moving upward (has an upward velocity) and slowing down or the object is moving downward (has an downward velocity) and speeding up. Even at the peak of the trajectory, the acceleration is still downward. It is just that at the peak the projectile is at the single instant in time in which the vertical velocity is zero as it undergoes a change from an upward to a downward direction.
 
The approach to this question demands an understanding of the above ideas. The list of choices must be reflected upon to identify which choices are consistent with the vertical velocity of a projectile. The above paragraph describes which way it is directed and how it changes over the course of time.

Don't be Fooled

Many students confuse velocity and acceleration. It is common for a student to believe that both the acceleration of a projectile and its vertical velocity are always directed downward. But don't be fooled! The direction of the velocity is ALWAYS in the direction that an object is moving. The vertical component of velocity is either upward or downward depending on whether the projectile is rising towards its peak or falling from its peak. At the peak itself, the vertical velocity is momentarily 0 m/s.
 
Many students also become confused about the significance of 9.8 m/s/s. This value represents an acceleration value as is obvious from its units. It is referred to as the acceleration of gravity; it is the acceleration value of an object upon which the only force is gravity. Projectiles accelerate in the vertical direction at 9.8 m/s/s. Since acceleration is the rate at which the velocity changes, this acceleration means that the vertical velocity of a projectile changes by 9.8 m/s each second.

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