Friction and spring force Physics Mr. Maloney

© 2002 Mike Maloney Objectives You will be able to …  qualitiatively describe friction and what factors contribute to it.  differentiate between static and kinetic friciton.  describe and use a simple model of friction.  describe and use the Hookean model of a spring.  use models of friction and springs as part of force problems.

© 2002 Mike Maloney Friction  Friction is the force that opposes the motion between two surfaces. Friction  Friction always acts in a direction opposite the direction of motion, and is always parallel to the surfaces.

© 2002 Mike Maloney Friction Defined (wikipedia) Friction is not a fundamental force, as it is derived from electromagnetic force between particles. It cannot be calculated from first principles, but instead must be found empirically (in the lab). When contacting surfaces move relative to each other, the friction between the two surfaces converts kinetic energy into thermal energy, or heat. Contrary to earlier explanations, kinetic friction is now understood not to be caused by surface roughness but by chemical bonding between the surfaces.

© 2002 Mike Maloney Static Friction  Static Friction is the force that opposes the start of motion.  It is the friction that is at play before you get something moving.  If your applied force is greater than the maximum value of static friction, the object will move.

© 2002 Mike Maloney Sliding (Kinetic) Friction  Once something starts moving, the friction decreases.  The friction between surfaces that are in motion relative to one another is called SLIDING FRICTION (kinetic friction).  i.e., your car stops faster if it is not skidding.

© 2002 Mike Maloney Measuring Friction  What do you think friction depends on?  Both types of friction depend on the force that is pushing the surfaces together.  This is the NORMAL force {F N }  F N always acts perpendicular to the surfaces in contact.  It also depends on the surfaces.

© 2002 Mike Maloney Measuring Friction  Experimentally it has been found that  Friction = (coefficient of friction) x Normal Force  F f = µ∙F N  µ (pronounced mu) is the coefficient of friction  It is a constant that depends on the surfaces that are interacting.  It is the ratio of how much Friction Force you get from a certain Normal Force.  There is a static coefficient µ s  And a sliding coefficient µ k

© 2002 Mike Maloney Coefficient of Friction The coefficient of friction is always given for a set of surfaces, such as rubber on concrete. It does not make sense to say the coefficient of friction of rubber, or metal. The object needs a second surface material that it is interacting with.

© 2002 Mike Maloney Simple Friction Problem 1  A 100 kg mass is being pulled at a constant velocity by a 490 N force. What is the coefficient of friction between the object and the ground?  How do we solve this one? How do we solve this one

© 2002 Mike Maloney One last force the Spring Force  The SPRING FORCE is a little different from other forces.  The NORMAL force, GRAVITY, FRICTION and APPLIED forces we have talked about all have constant values.  The SPRING force changes

© 2002 Mike Maloney Spring Force  What happens when you (compress) squish a spring? Does it get harder or easier to the farther you push?  RIGHT, it gets harder.  What about when you pull a spring?  Correct again, it also gets harder to move.

© 2002 Mike Maloney Spring Force  The spring force is dependent on the displacement of the spring from its rest position.  The formula for the SPRING force looks like this  F sp = k sp ∙x  K sp = spring coefficient  X = displacement from rest

© 2002 Mike Maloney Spring Force  K sp varies from spring to spring.  The units of K sp are N/m  It is the measure of the amount of force needed to stretch a spring a unit distance, in our case the number of newtons needed to stretch a spring 1 meter.

© 2002 Mike Maloney Spring Force  Even though the SPRING force is a little different from other forces it can still be treated the same.  It can be added and combined just like any other force.  We will see this guy again in a couple chapters.

© 2002 Mike Maloney Spring Example  Suppose a 100 N weight is hanging from a spring. The spring is stretched 0.5 meters. What is the K sp of that particular spring.  F sp = K sp *x  K sp = F sp / x  K sp = 100 N / 0.5 m = 200 N/m

© 2002 Mike Maloney Spring Example 2  How far does a spring stretch that has a K sp of 300 N/m when a Force of 100 N is applied to it?  F sp = K sp *x  x = F sp /K sp  X = 100 N / 300 N/m = 0.33 m

© 2002 Mike Maloney Objectives Can you …  qualitiatively describe friction and what factors contribute to it.  differentiate between static and kinetic friciton.  describe and use a simple model of friction.  describe and use the Hookean model of a spring.  use models of friction and springs as part of force problems.