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In the video clip the ONLY forces acting on the football after it has lost contact with the hand of the quarterback and before it is intercepted are gravity and forces due to its interaction with air molecules. The dominant force is gravity. If we completely neglect the interaction between the football and the air, then we can predict its motion exactly, if we know the football's initial height above the ground, the launch angle, and its initial speed.
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Let us use a coordinate system with the y-axis pointing upward and the x-axis pointing towards the right. As the football leaves the hand of the quarterback, its initial velocity points upward and towards the right. The football has a vertical velocity component vy and a horizontal velocity component vx. The force of gravity points straight down. The acceleration a = F/m has the same direction as the force, it points straight down, in the negative y direction. Its magnitude is g = 9.8 m/s2 = 32 ft/s2. The force of gravity on an object near the surface of the earth has magnitude Fg = mg, it is proportional to the mass of the object m. When we divide Fg by m to find the acceleration, the mass cancels out. The acceleration due to gravity has the same magnitude for all objects near the surface of the earth.
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Acceleration is change in velocity / change in time. The change in the vertical velocity of the football is -9.8 m/s per second. Assume the football starts out with a positive vertical velocity of vy = 19.6 m/s. This vertical velocity decreases by 9.8 m/s every second.
A negative vertical velocity means that the football is moving down, towards the ground. The acceleration has no horizontal component, so the horizontal velocity does not change. It stays constant. |
