![]() The normal force from the scale must be larger than your weight, so the scale will read a value larger than your weight. As the elevator starts up, your motion changes from still to moving upward, so you must have an upward acceleration and you must not be in equilibrium. For example, if you stand on a scale in an elevator as it begins to move upward, the scale will read a weight that is too large. When you stand on a scale and you are not in equilibrium, then the normal force may not be equal to your weight and the weight measurement provided by the scale will be incorrect. Image Credit: Italian V12 by Rob Oo via Wikimedia Commons The inverted airfoils (wings) on a shape of a formula one race car create a downward aerodynamic force, increasing normal force from the road and thus friction as well. ![]() Inserting our values for friction coefficient, g, and radius: Then we cancel the mass from both sides of the equation and solve for speed: Kinetic friction would be used if the tires were sliding.įor a typical car on flat ground the normal force will be equal to the weight of the car: Notice that we have used static friction even though the car is moving because we are solving the case when the tires are still rolling and not yet sliding. We want to know the maximum speed to take the curve without slipping, so we need to use the maximum static frictional force that can be applied before slipping: Next we recognize that the only force available to act on the car in the horizontal direction (toward the center of the curve) is friction, so the net force in the horizontal direction must be just the frictional force: What is the maximum speed that a car can have while rounding a curve with radius of 75 m without skidding? Assume the friction coefficient between tire rubber and the asphalt road is 0.7įirst, we recognize that as the car rounds the curve at constant speed the net force must point toward the center of the curve and have the value: The size of the acceleration experienced by an object undergoing uniform circular motion with radius at speed is:Ĭombined with Newton's Second Law we can find the size of the centripetal force, which again is just the net force during uniform circular motion: If the net force drops to zero (string breaks) the acceleration must become zero and the ball will continue off at the same speed in whatever direction it was going when the net force became zero (diagram on right above). As a result, that acceleration is called the centripetal acceleration. Due to Newton's Second Law, we know that the acceleration points toward the center of the circular motion because that is where the net force points. Therfore, the constantly changing direction of uniform circular motion constitutes a constantly changing velocity, and thus a constant acceleration, so all is good. How do we mesh this with Newton's Second Law, which says that objects with a net force must experience acceleration? We just have to remember that acceleration is change velocity per time and velocity includes speed and direction. Image Credit: Breaking String by Brews ohare via Wikimedia Commonįor both the ball and the satellite the net force points at 90° to the object’s motion so it can do no work, thus it cannot change the kinetic energy of the object, which means it cannot change the speed of the object. Right: The ball’s trajectory after the string breaks. Left: A ball on a string undergoing circular motion with uniform (constant) speed. For example, the centripetal force that keeps a satellite in orbit is just gravity and for a ball swinging on a string tension in the string provides the centripetal force. The centripetal force is not a new kind of force, rather the centripetal force is provided by one of the forces we already know about, or a combination of them. The object will undergo uniform circular motion, in which case we sometimes refer to the net force that points toward the center of the circular motion as the centripetal force, but this is just a naming convention. Therefore, the net force will only change the object’s direction of motion, change it’s kinetic energy) and the object must maintain a constant speed. We have seen that if the net force is found to be perpendicular to an object’s motion then it can’t do any work on the object. ![]() \).85 Weightlessness* Uniform Circular Motion
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