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S E C T I O N 5 . 4 • Newton’s Second Law 117 and so on. According to this observation, we conclude that the magnitude of the acceleration of an object is inversely proportional to its mass. These observations are summarized in Newton’s second law: When viewed from an inertial reference frame, the acceleration of an object is di- Thus, we can relate mass, acceleration, and force through the following mathematical 1 (5.2) In both the textual and mathematical statements of Newton’s second law above, we #F acting on an object. The net force on an object is the vector sum of all forces acting on the object. In solving a Note that Equation 5.2 is a vector expression and hence is equivalent to three com- ponent equations: (5.3) # F x " ma x
# F y " ma y
# F z " ma z # F " ma ▲ PITFALL PREVENTION 5.2 Force is the Cause of Changes in Motion Force does not cause motion. Newton’s second law Quick Quiz 5.2 An object experiences no acceleration. Which of the follow- ing cannot be true for the object? (a) A single force acts on the object. (b) No forces act Quick Quiz 5.3 An object experiences a net force and exhibits an accelera- tion in response. Which of the following statements is always true? (a) The object Quick Quiz 5.4 You push an object, initially at rest, across a frictionless floor with a constant force for a time interval %t, resulting in a final speed of v for the ▲ PITFALL PREVENTION 5.3 ma is Not a Force Equation 5.2 does not say that the 1 Equation 5.2 is valid only when the speed of the object is much less than the speed of light. We treat the relativistic situation in Chapter 39. Newton’s second law— component form Definition of the newton Unit of Force The SI unit of force is the newton, which is defined as the force that, when acting on an object of mass 1 kg, produces an acceleration of 1 m/s 2 . From this definition and Newton’s second law, we see that the newton can be expressed in terms of the follow- (5.4) 1
N
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