A hockey puck is hit on a frozen lake and starts moving with a speed of 13.3 m/s. Seven seconds later, its speed is 7.6 m/s. What is its average acceleration? The acceleration of gravity is 9.8 m/s^2 . Answer in units of m/s^2.

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Your question has some extra information that you do not need to solve the problem. Since the hockey puck is moving horizontally the effects of gravity (99.8 m/s/s) are zero.  The puck is slowing down because of friction between the puck and the ice.

Acceleration is the rate of change...

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Your question has some extra information that you do not need to solve the problem. Since the hockey puck is moving horizontally the effects of gravity (99.8 m/s/s) are zero.  The puck is slowing down because of friction between the puck and the ice.

Acceleration is the rate of change of velocity of any object over a period of time and is defined as:

a = (Vf - Vi)/t, where a is the acceleration; Vf is the final velocity, Vi is the initial velocity, and t is the time in seconds. Note that when an object is slowing the acceleration value will be negative. the negative sign tells you that it is slowing. If the acceleration value is positive, the object is speeding up.

Solving:

a = (7.6 - 13.3)/7 = 0.814 m/s/s

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