Motion, Work , and Power

1. Motion, Work , and Power

2. 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

3. Speed • Speed- The rate at which an object moves • Velocity-Speed in a given direction Since distance is usually measured in meters and time in seconds, the unit for speed is given in meters per second (m/sec) Speed = Distance Time

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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 (m-n/sec) or the joule per second (J/Sec) • Large quantities of power are measured in kilowatts (kW) one kilowatt equals 1000 watts

14. 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