Phy 211 General Physics I Chapter 6 Force Motion II Lecture Notes Fluid Air Drag Forces When moving objects are in a fluid medium i e air the motion is impeded by interactions between the fluid and the object s surface This is referred to as Fluid Drag or Air Drag in the case of air Drag forces always oppose the direction of motion In the simplest model Drag forces are related to The object s relative motion within the fluid v The density of the fluid The effective cross sectional area of the object A Coefficient of drag C a constant that characterizes the interaction between the fluid and the surface of the object r 1 FDrag CAv 2 2 r terminal velocity v T roccurs when the drag force equals the forces that oppose it Fnet 0 Falling Objects with Air Drag Consider a ball 1 kg 0 20 m radius dropped from a helicopter and subject to air drag The coefficient of drag C is 0 5 and air is 1 21 kg m3 1 The effective cross sectional area for the ball is A pr 2 0 126 m2 2 As the ball descends its free body diagram is r FDrag r mg r r r FNet FDrag mg FDrag mg j r r CAv2 j FNet mg 2 3 When Fdrag mg the terminal velocity is r 2mg vT 16 0 ms CA Surface Frictional Forces When an object moves or tends to move along a surface there is an interaction between the microscopic contact points on the 2 surfaces This interaction results in a frictional force that is parallel to the surface opposite to the direction of the motion There are 2 types of surface friction Static sticking Kinetic sliding Static rFriction Static or sticking friction fs is the frictional force exerted when the object tends to move but the external force is not yet strong enough to actually move the object Increasing the applied force the static frictional force increases as well so the net force is zero The force just before breakaway is the maximum static frictional force The direction of the static friction force is always in opposition to the external forces s acting on the body The magnitude of the maximumr static friction force is rmax fs max FN s Where max s is the coefficient maximum of static friction FN is the normal force Kinetic Friction r Kinetic frictional force fk is the frictional force exerted by the surface on an object that is moving along the surface Kinetic frictional force always opposes the direction of the motion the direction is along the surface parallel to the surface The magnitude of the kinetic frictional force depends only on the normal force and the properties of the 2 surfaces in contact r r fk k FN Where k is the coefficient of kinetic friction FN is the normal force Notes Kinetic friction is independent to the rate of travel of the sliding body Kinetic friction is independent to the surface area of contact Centripetal Force Center seeking force that pulls an object into circular motion object is not in equilibrium r r r Fnet Fc mac The direction of the centripetal force is the same as the centripetal acceleration The magnitude of centripetal force is given by Fc mac mv2 r For uniform circular motion the centripetal force is the net r r r force Fnet Fc mac Notes the faster the speed the greater the centripetal force the greater the radius of motion the smaller the force Centripetal Force Gravitation Let s assume the Earth travels about the Sun in uniform circular motion SUN EARTH Questions 1 2 3 4 5 What force is responsible for maintain the Earth s circular orbit Draw a Free Body Diagram for the Earth in orbit Calculate gravitational force exerted on the Earth due to the Sun What is the centripetal acceleration of the Earth What is the Earth s tangential velocity in its orbit around the Sun Centripetal Force vs Centrifugal Force An object in uniform circular motion observes an outward force called centrifugal force due to the object s inertia Centrifugal force is a fictitious force it is not really a force it is only perceived as one is due to the body s inertia acting against its circular motion its rotating environment has a magnitude equal to the centripetal force acting on the body Artificial Gravity Objects in a rotating environment feel an outward centripetal force similar to gravity What the object really experiences is the normal force of the support surface that provides centripetal force The magnitude of effective gravitational acceleration is given by If a complete rotation occurs in a time T T is called the period then the rotational or angular speed is given v2 by ac gartificial r w 2p dq T dt Note there are 2p radians 360o in a complete rotation revolution 4p 2r Since v r rotational speed x radius 2 gartificial r T2 Thus the artificial gravitational strength is related to size of the structure r rotational rate and square of the period of rotation T2 Banked Curves Engineers design banked curves to increase the speed certain corners can be navigated Banking corners decreases the dependence of static friction between tires road necessary for turning the vehicle into a corner Banked corners can improve safety particularly in inclement weather icy or wet roads where static friction can be compromised How does banking work The normal force FN of the bank on the car assists it around the corner Vertical Circular Motion Consider the forces acting on a motorcycle performing a loop to loop Can you think of other objects that undergo similar motions