# Motion & Forces Describing Motion  Motion  Speed & Velocity.

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Motion & Forces Describing Motion  Motion  Speed & Velocity

Newton’s First Law Newton’s First Law of Motion 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. 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. motion constant velocity net force

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

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

Relative Motion If you are sitting in a chair reading this sentence, you are moving. 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. 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.

Motion Motion DistanceDistance Distance describes how far an object has moved.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).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).Shorter distances are measured in centimeters (cm).

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.

Motion 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 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. You have mistakenly set yourself as the reference point.

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

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).

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

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

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

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

Calculations Your neighbor skates at a speed of 4 m/s. You can skate 100 m in 20 s. Who skates faster? 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! s d t

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? 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? GIVEN: s = 330 m/s d = 1km = 1000m t = ? WORK : t = d ÷ s t = (1000 m) ÷ (330 m/s) t = 3.03 s s d t

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

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

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

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

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