| Physics Talk Calculating Centripetal Force Newton's First Law states that an object at rest stays at rest and an object in motion stays in motion unless acted upon by an unbalanced force. A car with no net force will be at rest or travel at a constant speed in a straight line. Newton's Second Law states that accelerations require forces: Fnet = ma. Acceleration is defined as a change in velocity with respect to time.  This change in velocity can be a change in speed (the car can go faster or slower) or a change in direction (the car can move in a curve). Newton's Second Law states that any acceleration requires a force. Acceleration due to a change in speed is easy to calculate. When a car accelerates from 10 m/s north to 30 m/s north, its change in velocity is 20 m/s north. If the change occurred in 4 s, the acceleration is (20 m/s)/4 s = 5 m/s2. For acceleration due to a change in direction, the calculation is a bit more difficult. A car traveling at 10.0 m/s north that then travels 10.0 m/s east after 4.0 s also has acceleration. To determine the change in velocity, you have to calculate the change in the velocity by subtracting vectors vf – vi , where vf is the final velocity and vi is the initial velocity. This is mathematically equivalent to adding vf + (–vi).
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