2 MotionWhen you are riding in a car, why don’t the people in the car look like they are moving?Why do the things outside the car look like they ARE moving?Are YOU moving right now?Are you sure?When riding in a car, some people go around you and it looks like you’re going backwards. Why?
3 Motion Motion is a change in RELATIVE position: Motion requires a reference point, usually stationary.Distance measure the path taken:To measure how far you went, you have to follow the actual pathDisplacement is the distance moved, regardless of pathStraight line from start to finish
4 Speed Speed measures how fast a motion occurs Requires distance and timeSpeed = distance/timeMiles per hour (mph = miles/hr)Feet per second (feet/second)PER means divided by (usually using time)SI unit of measure for speed is meters per second (m/s)Constant speed means same distance is covered in the same amount of time.
6 Graphing Speed Graph Time on X-axis Graph Distance on Y-axis Slope of line is speedAssumes constant speedFaster speed = more slope to lineSlower speed = less slope to line
7 Average SpeedAverage speed is calculated based on starting and ending points (distance) and total time elapsed.Used to describe objects that don’t move at constant speedExercise: You travel 100 miles in 50 minutes, then 50 miles in 50 minutes, then 50 miles in 100 minutes. What is your average speed?Answer:Distance is = 200 milesTime is = 200 minutesDistance/time = 200/150 = 1 miles/minute
8 Instantaneous SpeedInstantaneous speed describes the speed and any particular point in timeVery difficult to measure
9 Velocity VELOCITY measure how fast in a particular direction Velocity is speed with direction60 mph north4 m/s SouthVelocity is a very important concept for physicsThe DIRECTION portion makes things act differentlyIf one direction is considered ‘positive’The opposite direction is ‘negative’The velocities can cancel out based on direction
10 Velocity When you combine velocities, you get a ‘resultant velocity’: What is resultant velocity of 15 m/s east and 1 m/s east?Going same direction, so add them15 m/s + 1 m/s = 16 m/sWhat is resultant velocity of 15 m/s east and 5 m/s west?Going opposite directions, so west is negative direction15 m/s east + (– 5 m/s west) = 10 m/s eastWhat is resultant velocity of 10 m/s east and 10 m/s south?14 m/s southeast (use Pythagorean theorem)
11 AccelerationWhen you accelerate your car, what are you doing to the motion?Acceleration is a change in velocityAcceleration can be a change in speedEither up or down (positive or negative)Acceleration can be a change in directionVelocity has direction, so acceleration does alsoA uniform circular motion is constant acceleration (always changing direction)
12 Calculating Acceleration Acceleration is the change in velocity, so you have to calculate the change.Acceleration = (Velocityfinal – Velocityinitial)/timeAlso:Because final velocity can be less than initial velocity (slowing down), acceleration can be negativeNegative acceleration is called deceleration in normal speechIn science, it is still acceleration with a negative value
13 Velocity-Time GraphsVelocity changes can be graphed on velocity/time graphsVelocity on Y-axisTime on X-axisConstant Velocity
15 Forces Force – a push or a pull Science definition: an action on a body (object) in order to change the body’s motionMany forces act at the same time on any objectCombination results from many forces acting on an object is called the net force
16 Forces Balanced Forces do not cause changes in motion This includes from rest (no motion) to motion or motion to restExample: Tug of War with equal teamsUnbalanced Forces result in changes in motionThe net force is larger in one directionExample: Tug of War with one side stronger
18 Friction Friction is a Force Caused by objects rubbing together Like hands in winterActs in direction OPPOSITE the applied forceWhy your bike stops rolling!Why a ball stops rolling!Because of friction, a constant force must be applied to your car to keep it rolling (= gas)Rougher surfaces have more frictionWhy do tires have grooves & knobs?Why do court shoes have tread?
19 Types of Friction Static Friction - between stationary objects Like brakes on a car when it’s not movingKinetic Friction – between moving objectsSliding Friction – two (flat) surfaces rubbingRolling Friction – round object to flat surfaceLike tires on bike or carFluid Friction – in liquids or gasesAir Resistance is a type of fluid frictionForce of Friction:Static > Sliding > Rolling > Fluid
20 Friction and MotionFriction between 2 objects results in HEAT buildup and low efficiencyReduce Friction by:Make surfaces smootherUse of lubricants (turn it to fluid friction) (oil, grease, wax. graphiteConvert Sliding Friction to Rolling FrictionIncrease Friction by:Make surfaces rougherMake more surface area rubbingIncrease force pushing objects together
21 Assignment: Friction and Motion You are riding your bike to school. Is friction good or bad. Justify your answer using the principles of force, motion, acceleration, air resistance, sliding friction rolling friction, and fluid friction.At least 3 paragraphsAt least 3 sentences per paragraph.
23 Newton’s First Law: inertia Which is easier?Pushing a car from start to moving or keeping it moving?Starting your bicycle moving or keeping it moving?
24 Newton’s First Law: inertia An object at rest remains at rest and an object in motion remains in motion unless acted upon by an unbalanced force.Inertia is the tendency of an object to maintain the same motion.
25 Newton’s Second Law: Force The unbalanced force acting on an object equals the objects mass times its accelerationF = mass * acceleration (F=m*a)The harder you push, the faster it acceleratesIt takes more force to move a larger mass than a smaller mass.Which would you rather try to stop (-acceleration) a freight train moving slowly or a baseball moving 100mph?
26 Free-Fall and WeightFree-fall – when the only force acting on an object is gravityFree-fall acceleration near the earth’s surface is constant – 9.8 m/s2After 1st second it is moving 9.8 m/sAfter 2nd second it is moving 19.6 m/sAfter 3rd second it is moving 28.4 m/sEtc.Weight is Free-fall acceleration times massWeight is a force (mass * acceleration)
27 Weight and Mass Mass is the amount of matter in an object It doesn’t change, no matter where you go.Weight is mass times acceleration due to gravity.It changes depending upon gravityOn the moon, your mass is the same as on earth, but 1/6th of the weight
28 Terminal VelocityTerminal velocity is when air resistance (friction due to air) exactly balances the weight.A sky-diver will eventually reach terminal velocity and not fall any fasterAbout 320 km/h (200 mi/h)
29 Newton’s Third LawFor every force, there is an equal and opposite reaction forceThey are on different objectsWhen you kick a ball:Your foot applies a force to the ballThe ball also applies a force to your foot.Applications:RocketsJet-skisRam-jet engines