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Motion & Forces

Motion & Forces. Describing Motion Motion Speed & Velocity. Newton’s First Law. Newton’s First Law of Motion An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force. constant velocity. net force. motion.

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Motion & Forces

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  1. Motion & Forces Describing Motion Motion Speed & Velocity

  2. Newton’s First Law • Newton’s First Law of Motion • An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force. constant velocity net force motion

  3. Motion • Problem: • Is your desk moving? • We need a reference point... • nonmoving point from which motion is measured

  4. Reference point Motion Motion • Motion • Change in position in relation to a reference point.

  5. Relative Motion If you are sitting in a chair reading this sentence, you are moving. You are not moving relative to your desk or your school building, but you are moving relative to the other planets in the solar system and the Sun.

  6. Motion • Distance • Distance describes how far an object has moved. • The SI unit of length or distance is the meter (m). Longer distances are measured in kilometers (km). • Shorter distances are measured in centimeters (cm).

  7. Motion • Displacement is the distance and direction of an object's change in position from a reference point. • Suppose a runner jogs to the 50-m mark and then turns around and runs back to the 20-m mark. • The runner travels 50 m in the original direction (north) plus 30 m in the opposite direction (south), so the total distance she ran is 80 m.

  8. Motion Problem: • You are a passenger in a car stopped at a stop sign. Out of the corner of your eye, you notice a tree on the side of the road begin to move forward. • You have mistakenly set yourself as the reference point.

  9. d s t Speed & Velocity • Speed • rate of motion • distance traveled per unit time

  10. Calculating Speed The SI unit for distance is the meter and the SI unit of time is the second (s), so in SI, units of speed Sometimes it is more convenient to express speed in other units, such as kilometers per hour (km/h).

  11. Speed & Velocity • Instantaneous Speed • speed at a given instant • Speed odometer reading • Average Speed • the total distance traveled divided by the total time of travel

  12. Changing Instantaneous Speed When something is speeding up or slowing down, its instantaneous speed is changing. If an object is moving with constant speed, the instantaneous speed doesn't change. To determine direction you need to know the velocity

  13. Speed & Velocity • Problem: • A storm is 10 km away and is moving at a speed of 60 km/h. Should you be worried? • It depends on the storm’s direction!

  14. Speed & Velocity • Velocity • speed in a given direction • can change even when the speed is constant!

  15. d t s Calculations • Your neighbor skates at a speed of 4 m/s. You can skate 100 m in 20 s. Who skates faster? GIVEN: d = 100 m t = 20 s s = ? WORK: s = d ÷ t s = (100 m) ÷ (20 s) s = 5 m/s You skate faster!

  16. d t s Calculations • Sound travels 330 m/s. If a lightning bolt strikes the ground 1 km away from you, how long will it take for you to hear it? WORK: t = d ÷ s t = (1000 m) ÷ (330 m/s) t = 3.03 s GIVEN: s = 330 m/s d = 1km = 1000m t = ?

  17. Distance-Time Graph A B Graphing Motion speed • slope = • steeper slope = • Straight line = • flat line = • Single point = instantaneous speed faster speed no motion constant speed

  18. Distance-Time Graph A B Graphing Motion • Who started out faster? • A (steeper slope) • Who had a constant speed? • A • Describe B from 10-20 min. • B stopped moving • Find their average speeds. • A = (2400m) ÷ (30min) A = 80 m/min • B = (1200m) ÷ (30min) B = 40 m/min

  19. Distance-Time Graph Graphing Motion • Acceleration is indicated by a curve on a Distance-Time graph. • Changing slope = changing velocity

  20. Speed-Time Graph Graphing Motion Specify the time period when the object was... • slowing down • 5 to 10 seconds • speeding up • 0 to 3 seconds • moving at a constant speed • 3 to 5 seconds • not moving • 0 & 10 seconds

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