Which one of the following statements is true concerning the motion of an ideal projectile launched at an angle of 45° to the horizontal?
Correct answer: E. The vertical speed decreases on the way up and increases on the way down.
- A. The acceleration vector points opposite to the velocity vector on the way up and in the same direction as the velocity vector on the way down.
- B. The speed at the top of the trajectory is zero.
- C. The object's total speed remains constant during the entire flight.
- D. The horizontal speed decreases on the way up and also decreases on the way down.
- E. The vertical speed decreases on the way up and increases on the way down.
Explanation
The only external force acting on the object is the gravitational force vertically downwards. Thus the acceleration of the projectile is g downwards always. Thus there is no change in the speed of the object in the horizontal direction. The speed of an object in the vertical direction at the top of the trajectory is zero. However, the net speed at the point is equal to the constant horizontal component. Due to downward constant acceleration, the vertical speed decreases way up, becomes zero, and then increases way down. Thus option E is correct.
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