Lecture 8, Pre-Flight Questions 1&2 Consider the following situation: You are driving a car with constant speed around a horizontal circular track. On a piece of paper, draw a Free Body Diagram (FBD) for the car. How many forces are acting on the car? 1 2 3 4 5 correct FN f W HW: car Lecture 8, Pre-Flight Question 3 & 4 The net force on the car is 1. Zero 2. Pointing radially inward 3. Pointing radially outward FN correct f R W SF = ma = mv2/R If the car has constant velocity, there is no acceleration, no acceleration=no net force For the car to stay traveling around a horizontal circular track, there must be a net force pointing radially inward, toward the center of the circle. If there wasn't, the car would drive in a straight line. the car is akin to a ball on a string that is moving in a circle. If you let the ball go it will fly about a 90 degree angle from where you let it go away from the circle. Lecture 8, Pre-Flight Questions 5 & 6 Suppose you are driving through a valley whose bottom has a circular shape. If your mass is m, what is the magnitude of the normal force FN exerted on you by the car seat as you drive past the bottom of the hill 1. FN < mg 2. FN = mg 3. FN > mg correct a=v2/R R FN v mg There will be a centripetal force pointing up, reducing N=mg. SF = ma = mv2/R when level your normal force = your weight FN - mg = mv2/R Since there is centripetal acceleration, the normal force is greater than simply Mg FN = mg + mv2/R I don't know the real reason but I was thinking that when you are skating in a half pipe and you reach the very bottom it feels like you are being pulled down by more then just your weight. From personal experiences with hills, as you pass the bottom of the hill your body kind of scrunches down towards the seat. This extra force that you feel has to be exerted back on you according to newton's 3rd law, therefore the normal force must be greater than the gravitational force. If you are at the exact bottom of the hill, the ground is "flat". The car isn't being affected by acceleration due to gravity as a result of driving downhill or uphill so it's just like you're driving on regular land. The car isn't going up or down so the normal force and the force of gravity acting upon it (mg) are the same. According to newtons third law, forces are always equal and opposite Because when you go on a rollercoaster, and you go down a huge drop, your body feels really really heavy once you get to the bottom of the hill. It's like your body is being pushed into the seat. Therefore, your normal force is less then your weight.