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Motion And Forces

Motion And Forces. Describing Motion. Motion Occurs when an object changes position relative to a reference point. Distance Vs. Displacement. Distance how far an object has traveled Displacement distance and direction of an objects change in position from starting point.

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Motion And Forces

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  1. Motion And Forces

  2. Describing Motion

  3. Motion Occurs when an object changes position relative to a reference point.

  4. Distance Vs. Displacement Distance • how far an object has traveled Displacement • distance and direction of an objects change in position from starting point

  5. Change in Position • Distance • The SI unit of distance is the meter. • Longer distances are measured in kilometers • 1 km= 1000 m • Displacement • Is the distance and direction of the objects change in position Total Distance: 60 meters Total Displacement: 20 meters east

  6. Speed Speed– the distance an object travels per unit time – rate of change in position Avg Speed (v) = total distance (d)/total time (t) V = d/t

  7. A passenger elevator travels from the first floor to the 60th floor, a distance of 210m in 35s. What is the elevators speed? Speed=distance/time speed= 210m/35s speed=6.0 m/s 210m 35s

  8. A motorcycle is moving at a constant speed of 40 km/h. How long does it take the motorcycle to travel a distance of 10 km? Time=distance/speed Time= 10km 40km/h 10km Time= .25h 40 km/h

  9. How far does a car travel in .75h if it is moving at a constant speed of 88 km/h? Distance= speed x time Distance= 88km/h x .75h Distance= 66km 88km/h .75h

  10. Graphing Motion The slope of a line on a distance – time graph gives the speed of an object in motion. Y DISTANCE Motion of an object can be plotted on a distance – time graph. X TIME X axis -> TIME Y axis -> DISTANCE

  11. Graphing motion • The motion of an object over a period of time can be shown on a distance time graph. • Time is plotted along the horizontal axis • Distance is plotted on the vertical axis • Each axis must have a scale that covers the range of numbers to be plotted • Graph data points • Draw a line to connect them

  12. Speed on a Distance-Time Graph • If an object moves with constant speed, the increase in distance over equal time intervals is the same. • Causes a straight line • If the speed changes the line will not be straight • The steepness of a line on a graph is the lines slope • The slope of s line equals the object seed • A horizontal line on a distance-time graph has a zero slope and represents an object at rest

  13. Acceleration

  14. Velocity – includes speed of an object and the direction of its motion What is the difference between speed & velocity? Velocity includes direction where as speed does not.

  15. Velocity • Same units as speed, m/s • Velocity can change even if the speed of the object remains constant because the direction change.

  16. Acceleration – rate of chance of velocity. Acceleration occurs when an object changes speed, its direction, or both.

  17. Velocity and Acceleration • The rate of change of velocity is acceleration • When the velocity of an object changes the object is accelerating • Velocity includes speed and direction • Things speeding up AND slowing down have acceleration Acceleration Acceleration Velocity Velocity

  18. Calculating Acceleration Acceleration (meters/second²) = change in velocity(meters/sec) Time (seconds) * NOTE : Change in velocity = final velocity – initial velocity v= Vf - Vi Acceleration formula : a=Vf – Vi t

  19. Positive Acceleration A plane takes off… a = Vf – Vi = 80m/s – Om/s t 20s a = 4m/s² The plane is speeding up so acceleration is positive.

  20. Negative Acceleration A bike comes to a stop… a = Vf – Vi = 0m/s – 3m/s t 2s a = -1.5m/s The bike is slowing down so acceleration is negative.

  21. A skateboarder has an initial velocity of 3 m/s west and comes to a stop in 2 seconds. What is the skateboarders acceleration? • A= (Vf- Vi) t • A= (0 m/s – 3 m/s ) west 2 s • A= -1.5 m/s2 west • The acceleration has a negative sign, so the direction is reversed. • A= 1.5 m/s2 east.

  22. An airplane starts at rest and accelerates down the runway for 20s. At the end of the runway its velocity is 80 m/s north. What is its acceleration? • A= (Vf- Vi) t • A= (80 m/s – 0 m/s ) north 20 s • A= 40 m/s2 north

  23. A cyclist starts at rest and accelerates at .5 m/s2 south for 20 seconds. What is the final velocity? • Vf = at + Vi • Vf = (.5 m/s2 south)(20s) + 0 m/s south • Vf = 10 m/s south

  24. A ball is dropped and falls with an acceleration of 9.8 m/s2 downward. It hits the ground with a velocity of 49 m/s downward. How long did it take the ball to fall to the ground? • T= (Vf- Vi) a • T= (49 m/s – 0 m/s ) downward 9.8 m/ss • T= 5s

  25. Motion And Forces

  26. Force — a push or pull applied to an object.

  27. Net Force – When two or more forces act on an object at the same time FORCES ARE = BALANCED FORCES NET FORCE = 0 FORCES UNBALANCED NET FORCE = + COMBINED FORCES NET FORCE = TWO FORCES ADDED TOGETHER

  28. Friction – the force that opposes the sliding motion of two touching surfaces. Friction is caused by microscopic bumps on surfaces called microwelds.

  29. Air Resistance – friction – like force that opposes motion of objects that move through air – depends on speed, size & shape of object

  30. Friction

  31. Sliding Friction • Opposes the motion of two surfaces sliding past one another

  32. Rolling Friction • As a wheel rolls static friction acts over the area where the wheel and surface are in contact • Referred to as traction on vehicles

  33. Gravity • An attractive force between any two objects that depends on the masses of the objects and the distance between them. • One of the four basic forces • Gravity between two objects never disappears • Because of this gravity is sometimes called a field • Earths Gravitational field is stronger near Earth and becomes weaker as the distance increases.

  34. Law of Universal gravitation • States that the gravitational force increases as the mass of either object increases and as the objects move closer. • Explains why you feel the Earths gravity

  35. Weight • The gravitational force exerted on an object • Weight (N) = mass (kg) x gravitational strength (N/kg) • Fg=mg • Weight and mass are not the same • Mass is a measure of matter an object contains • Weight on Earth • g=9.8 N/kg

  36. Newton’s Laws Of Motion

  37. Newton’s First Law of Motion Also known as Law of Inertia! An object in motion stays in motion, or an object at rest stays at rest until an unbalanced net force acts upon it.

  38. Inertia – tendency of an object to resist any change in its motion. WILL STAY AT REST UNTIL FORCE ACTS IN IT WILL CONTINUE IN MOTION UNTIL THESE FORCES ACT ON IT GRAVITY FRICTION WALL

  39. Newton’s Second Law of Motion A net force acting on an object causes the object to accelerate in the direction of the force. Acceleration is determined by size of force and the mass of an object. (Kg * m/s²) (kg) (m/s²) Force = mass x acceleration F = m x a

  40. Calculating Net Force • Fnet= mass (acceleration) • This is the formula to remember 

  41. You push a wagon that has a mass of 12 kg. If the net force on the wagon is 6N south what is the wagons acceleration? • F=ma • a= f/m • m=12 kg • Fnet= 6 N south • a= 6 N south 12kg • a= .5 m/s2 south

  42. If a helicopters mass is 4,500 kg and the net force on it is 18,000 N upward, what is its acceleration? • F=ma • a= f/m • m=4,500 kg • Fnet= 18,000 N upward • a= 18,000 N upward 4,500 kg • a= 4.0 m/s2 upward

  43. What is the net force on a dragster with a mass of 900 kg if its acceleration is 32.0 m/s2 • F=ma • m= 900 kg • a= 32.0 m/s2 • F= (900 kg) ( 32.0 m/s2) • F= 28,800 N west

  44. A car pulled by a tow truck has an acceleration of 2.0 m/s2 east. What is the mass of the car if the net force on the car is 3,000 N east. • m=F/a • a=2.0 m/s2 • Fnet= 3,000 N East • m= 3,000 N east 2.0 m/s2 • m= 1,500 kg

  45. Newton’s Third Law of Motion For every action (or force), there is an equal and opposite reaction (or force). Momentum – property of moving object resulting from its mass and velocity. ACTION REACTION Momentum (p) = mass x velocity

  46. Force and Momentum • Law of Conservation of momentum • If no external forces act on a group of objects, their total momentum does not change. • A Game of Pool • Ball A hits ball B, the momentum lost by ball A is gained by ball B and the total momentum before and after the collision is equal

  47. Gravity

  48. Gravity – any two masses that exert an attractive force on each other. Gravity depends on mass & distance between objects Weight – gravitational force exerted on an object; measured in units called Newtons The greater the object mass, the stronger the gravitational force on it

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