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MOTION, WORK , AND POWER MOTION Motion- A change in position in a certain amount of time In order to describe motion, you need a frame of reference to compare it with Motion SPEED Speed- The rate at which an object moves Velocity- Speed in a given direction Speed = Distance Time Since distance is usually measured in meters and time in seconds, the unit for speed is given in meters per second (m/sec) MEASURING SPEED If a car travels 500 meters in 20 seconds, what is its speed? 500 meters 20 seconds Speed = distance = 500 meters = 25 m/sec time 20 seconds What is the speed of a jet plane that travels 7200 km in 9 hours? Speed = 7200km / 9 hr = 800 km/hr The speed of a cruise ship is 50km/hr. How far will the ship travel in 14 hours? Distance = Speed x time = 50km/hr x 14 hrs = 700 km ACCELERATION Acceleration- The rate in change of velocity If something is accelerating, it is speeding up, slowing down, or changing direction The acceleration of an object is equal to its change in velocity divided by the time in which the change occurs Acceleration = Final velocity – Original Velocity Time DETERMINING ACCELERATION A rollercoaster has a velocity of 10 m/sec at the top of a hill. Two seconds later it reaches the bottom of the hill with a velocity of 20 m/sec. What is the acceleration of the rollercoaster? Acceleration = Final velocity – Original Velocity Time Acceleration = 20 m/ sec – 10 m/sec = 10 m/sec = 5 m/sec/sec 2 sec 2 sec A rollercoaster is moving at 25 m/sec at the bottom of a hill. Three seconds later it reaches the top of the next hill, moving at 10 m/sec. What is the deceleration of the rollercoaster? Deceleration = Final velocity – Original Velocity Time Deceleration = 10 m/ sec – 25 m/sec = -15 m/sec = -5 m/sec/sec 3 sec 3 sec LAWS OF MOTION Newton’s laws of motion describe three states of motion Rest Constant motion Accelerated motion The laws also explain how forces cause all of the states of motion NEWTON’S FIRST LAW OF MOTION The first law states that an object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by an outside force Since constant velocity means the same speed and direction, in order for an object to change velocity, or accelerate, a force must act on it The concept of inertia forms the basis for this law Inertia- the property of matter that tends to resist any change in motion The inertia of an object is related to its mass The more massive an object, the more inertia it has and the more difficult it will be to move You feel the effects of inertia when you are riding in a car that stops suddenly while you keep moving forward NEWTON’S SECOND LAW OF MOTION The second law of motion show how force, mass, and acceleration are related Force = Mass x Acceleration When mass is measured in kilograms and acceleration in meters/sec/sec, force is measured in Newtons (N) Tells us that a greater force is require to accelerate an object with a greater inertia (mass) Explains one reason why smaller cars get better gas mileage than larger ones NEWTON’S THIRD LAW OF MOTION States that for every action, there is an equal and opposite reaction According to the third law: Every force must have an equal and opposite force All forces come in pairs You demonstrate the third law when you walk Your feet push against the floor The floor pushes with an equal but opposite force against your feet You move forward The reaction engine utilizes the third law WORK Work- a force acting through a distance In order for work to be done on an object, a force must move it Work is the amount of force applied to an object times the distance the object moves in the direction of the force If an object doesn’t move, no work is done If an object doesn’t move in the direction of the force, no work is done MEASURING WORK Work = force x distance (w = f x d ) Force (f) is measured in Newtons Distance (d) is measured in meters Work (w) is measured in newton-meters (n-m) or joules (J) A force of 10,000 N is applied to a stationary wall. How much work is performed? Work = f x d = 10,000 N x 0 m = 0 N-m or 0 J A 950 N skydiver jumps from as altitude of 3000 m. What is the total work performed on the skydiver? Work = f x d = 950 N x 3000 m = 2,850,000 N-m or 2,850,000 J An ant does 1 N-m of work in dragging a 0.002 N grain of sugar. How far does the ant drag the sugar? distance = work / force = 1 N-m / 0.002 N = 500 m POWER Power- how fast work is done The rate at which work is done, or the amount of work per unit time power = work / time (p = w / t) Since power is the amount of work done per unit time it can also be written p = (f x d) / t The unit for power is the newton-meter per second (mn/sec) or the joule per second (J/Sec) Large quantities of power are measured in kilowatts (kW) one kilowatt equals 1000 watts MEASURING POWER power = work / time (p = w / t) or p = (f x d) / t A horse performs 15,000 J of work pulling a wagon for 20 seconds. What is the horse’s power? power = 15,000 J x 20 seconds = 300,000 w A 750 N pole vaulter lifts himself 5.0 m in 2.5 seconds. What is his power? power = 750 N x 5.0 m / 2.5 sec = 1500 w A tow truck pulls a car out of a ditch in 6.5 sec. If 6000 watts of power is used, how much work is performed by the truck? work = p x t = 6000 watts x 6.5 sec = 39,000 J