PHY 102 1st Edition Outline of Last Lecture I Conservation of Momentum a Experiment b Example II Numerical Problems a Example 1 b Example 2 c Example 3 III Newton s Second Law IV Second Law Reprise Outline of Current Lecture I Examples of Newton s Second Law a Car acceleration b Collision Example c Weight d Change in Velocity II Experience and Experiment a Experience shows that b Experiment Shows that III Newton s Third Law These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Current Lecture I Continued examples of Newton s Second Law a A car with the mass of 1500 kg accelerates at a rate of 2 m s2 under a constant force What are the magnitude and direction of that force F ma Force M 1500 kg A 2 m s2 F 1500 kg 2 m s2 3000 N N kg m s2 b How much force would it take to slow a 75 kg person riding in a car going 20 m s to a complete stop if the time was 03 seconds the typical time of an auto collision F ma M 75 kg V 20 m s T 03 s v t vf vi 0 m s 20 m s v 20m s 75 kg 20 m s 03 s a v t F t m v or F m c How much upward force does the ground apply to someone with a mass of 75 kg to counter balance the pull of gravity Note that g 9 8 m s 2 and that Fnet ma and that a g Wt mg Weight mass times 9 81m s2 F ma F M 75 kg A g 9 8 m s2 75 kg 9 8 m s2 740 N 740 N 740 N Weight of the person Force of the earth on the person d A force of 7 N is applied to a 3 5 kg mass for 2 seconds What is the change of velocity F ma F 7N v F m v t v M 3 5 kg T 2s II 7 0 N 2 s 3 5 kg 4 m s Experience Experiment a Experience shows that i A greater force for a given mass causes greater acceleration same force applied to two masses gives different results ii A greater mass for a given force gives lesser acceleration b Experiment shows that i Acceleration is directly proportional to force c F ma i A newton is the force required to accelerate a 1 kg mass at a rate of 1 m s2 III Newton s Third Law a Specific case of the second law i F ma the first law can be derived from this just means the acceleration is 0 b F12 F21 i Most misunderstood of the 3 laws ii If an object A exerts a force Fab on an object B then object B exerts a force Fba on object A such that Fab Fba iii The sum of the two forces vectors have to be 0 because the object is not accelerating Large mass small mass The change in velocity will be less in the large truck because it has a larger mass so the passenger will likely survive Forces are equal in the event of a collision iv Tug of war is possible because you have to consider all the forces on the rope not just the tension on the rope If the only force was the tension on the rope no one would ever win Centripetal force center seeking force
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