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# lab3 - Lab 3 Newton's Second Law Introduction Sir Isaac...

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Lab 3 - Newton's Second Law Introduction Sir Isaac Newton put forth many important ideas in his famous book The Principia . His three laws of motion are the best known of these. The first law seems to be at odds with our everyday experience. Newton's first law states that any object at rest that is not acted upon by outside forces will remain at rest, and that any object in motion not acted upon by outside forces will continue its motion in a straight line at a constant velocity. If we roll a ball across the floor, we know that it will eventually come to a stop, seemingly contradicting the First Law. Our experience seems to agree with Aristotle's idea, that the "impetus" given to the ball is used up as it rolls. But Aristotle was wrong, as is our first impression of the ball's motion. The key is that the ball does experience an outside force, i.e., friction, as it rolls across the floor. This force causes the ball to decelerate (that is, it has a "negative" acceleration). According to Newton's second law an object will accelerate in the direction of the net force . Since the force of friction is opposite to the direction of travel, this acceleration causes the object to slow its forward motion, and eventually stop. The purpose of this laboratory exercise is to verify Newton's second law. Discussion of Principles Newton's second law in vector form is ( 1 ) F = ma or F net = ma This force causes the ball rolling on the floor to decelerate (that is, it has a "negative" acceleration). According to Newton's second law an object will accelerate in the direction of the net force. If F is the magnitude of the net force, and if m is the mass of the object, then the acceleration is given by ( 2 ) a = F m Since the force of friction is in the opposite direction to the direction of motion, this acceleration causes the object to slow its forward motion, and eventually stop. Notice that Eq. (1) and Eq. (2) are written in vector form. This means that Newton's second law holds true in all directions. You

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can always break up the forces and the resultant acceleration into their respective components in the x , y , and z directions. ( 3 ) F net,x = ma x ( 4 ) F net,y = ma y ( 5 ) F net,z = ma z Consider a cart on a low-friction track as shown in Fig. 1. A light string is attached to the cart and passes over a pulley at the end of the track and a second mass is attached to the end of this string. The weight of the hanging mass provides tension in the string, which helps to accelerate the cart along the track. A small frictional force will resist this motion. We assume that the string is massless (or of negligible mass) and there is no friction between the string and the pulley.
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