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What is a force? How does friction affect motion?Motion, Forces and Friction

MotionA change in position, also known as an objects displacement

Speed (rate)Rate of change in positionHow fast or how slowMeasured in m/s (meters per second)VelocitySpeed with a directionVector

Constant speedA speed that does not varyGraph a straight line, no turnsAccelerationRate of change in velocityChange in speed over timeChange in directionAccelerationUnits for acceleration m/sec2Acceleration refers to increasing speed, decreasing speed (deceleration, negative acceleration), or changing directionCalculating AccelerationAcceleration = Final speed Initial speed TimeA plane travels from 0 m/s to 24 m/s in 3 sec. What is its acceleration?Acceleration = 8 m/s2

Calculating Acceleration, alt.Force = mass x accelerationAcceleration = A force of 6 N is applied to an object that has a mass of 3 kg. What is the acceleration? Acceleration = 2 m/s2

What is a Force?A force is a push or a pull.All forces have two properties:Direction and Size

A Newton (N) is the unit that describes the size (magnitude) of a force.

The student is pushing down on the chair, but the chair does not move.The floor is balancing the force by pushing on the chair, which in turn pushes on the student.

A force between two surfaces that touch and are sliding, or trying to slide across one another, for example when you try to push a toy car along the floor. What is friction?Friction is a force that opposes motion between two surfaces that touch each other.Friction always works in the direction opposite from the direction the object is moving, or trying to move. It always slows a moving object down.

MovingFrictionThe surface of any object is rough.Even an object that feels smooth is covered with tiny hills and valleys.The contact between the hills and valleys of two surfaces causes them to stick, resulting in friction.

Friction converts kinetic (motion) energy into thermal (heat) energy.

Principles of FrictionFriction acts parallel to the surfaces that are in contact.The direction that friction acts is OPPOSITE the direction of the motion (or intended motion).FweightFnormalFappliedFfriction(k)What Affects Friction?The amount of friction depends on:Roughness of the surfacesForce pushing the surfaces together

What Creates More Friction?Rough SurfacesRough surfaces have more microscopic hills & valleys.The rougher the surface, the greater the friction will be.

Greater Force (caused by increased mass)The amount of force exerted by the object increases the amount of frictionIncrease the amount of force exerted by the object by increasing the mass of the object

What Does Not Affect Friction?Surface AreaChanging the amounts of the surfaces that touch does NOT change the amount of friction

4 N4 NWhat Does Not Affect Friction?2 N2 NVelocityFriction forces are independent of speed or velocityThe force of kinetic friction is the same at 5 m/s as it is for 20 m/s.5 m/s20 m/sTypes of FrictionThere are two types of friction:Static Friction the frictional force present just before motion begins. Kinetic Friction the frictional force present with motionNote: Static Friction is usually higher than Kinetic FrictionStatic FrictionStatic friction occurs when force applied to an object does not cause the object to move.In order to move something, you must first overcome the force of static friction between the object and the surface on which it is resting.

Kinetic friction occurs when force is applied to an object and the object moves.Examples:Sliding Friction: pushing an object across a surface Rolling Friction: between wheels and a surfaceFluid Friction: opposes the motion of objects traveling through a fluid (air or water), also called drag force or air resistance

Reduce the contact area by using rollers/ball-bearings/wheelsChange the surfaces of the materials that are touching by using lubrication like motor oil, wax, or greaseRoll objects instead of sliding themCreate a cushion of air Eg. Like a hovercraft or air hockey table

Reducing Friction26Friction Can Be Harmful or HelpfulHarmful frictionHoles in socks or knees of jeansWind & water cause erosionHelpful frictionFriction between pencil & paperWithout friction, you would just slip & fall when you tried to walk

Friction can be a useful force because it prevents our shoes slipping on the pavement when we walk and stops car tires skidding on the road.

Useful Friction28 Ice causes very little friction, which is why it is easy to slip over on an icy day. But this is a good thing for ice skating and sledding.

Recall the Sliding PennyWhy do things not continue to move at constant velocity?FrictionIf the sliding penny slows down, whats the force responsible?Friction

4/18/2008Lecture 830How could I keep it moving at a constant velocity?Reduce the frictionDo I need to apply a force to keep it moving? Why?No, inertia will keep it moving once the initial force was applied.

4/18/2008Lecture 831Measures of friction are based on the type of materials that are in contact.

Each material has what is known as a coefficient of friction. The coefficient is a measure of how easily one object moves in relationship to another.

When you have a high coefficient of friction, you have a lot of friction between the materials.

Concrete on concrete has a very high coefficient of friction.

A material such as Teflon (on most things) has a very low coefficient. Teflon is used on surfaces where we don't want things to stick; such as pots and pans.

How is friction measured?Standard Friction EquationFriction = coefficient of friction x normal force.Normal force comes into play any time two objects are in direct contact with one another, and always acts perpendicular to the object that applies the force. The simplest example of the Normal force can be seen in the situation of an object on a horizontal surface. If the object is on a horizontalsurface, then Normal force = Weight

2 NFriction and the Normal Force4 NThe force required to overcome static or kinetic friction is proportional to the normal force, n.fk = mknfs = msnn12 N6 Nn8 N4 Nn

= coefficient of kinetic friction= coefficient of static frictionFN= the normal forceks= Symbol for the Greek letter mu, used in the formula to represent the coefficient.Sample Coefficients of FrictionMaterial on Materials = static frictionk = kinetic frictionsteel / steel0.60.4add grease to steel0.10.05metal / ice0.0220.02brake lining / iron0.40.3tire / dry pavement0.90.8tire / wet pavement0.80.7Spring 2008364/18/2008Lecture 836The coefficient of kinetic friction between a block and the level surface it slides on is 0.45.

If the mass of the block is 10.0 kg, what is the minimum force needed to keep the block moving with uniform motion?

The applied force need only balance the kinetic frictional force in order to maintain uniform motion.Fk = kFN

Fk = kFN

Fk = (0.45)(10.0)(9.8 m/s2)

Fk = 44 N

A student pulls a 5.00 kg object and discovers that sheneeds to exert 30.0 N of force before the object moves.

What is the coefficient of static friction between theobject and the surface on which it rests?Since the applied force is equal to 30 N just before moving, the friction force must be equal in size (30 N).Therefore: = 30 N (5 kg)(9.8 m/s2) = 0.61