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Physical Science Ch. 2: Motion Motion Motion is the change in position of an object. A reference point is usually needed in order to determine motion or rate of motion. SR-71 Blackbird Examples: • • • • • SR-71 Blackbird Tossing an object in a moving bus Earth’s rotation Virtual reality rides Car sickness • Displacement is a change in location of an object (distance and direction from starting point). • Is displacement the same as motion? Hmmm… No, not necessarily. A spinning basketball for example, is in motion. But it’s location does not have to be changing, so it may not be displaced. Speed While motion refers to the change of position of an object, speed describes the rate of change of position, and velocity describes both speed and direction. The SI unit for velocity is m/s. Instantaneous speed is the speed at 1 given moment in time. Constant speed is speed which neither increases or decreases. • Average speed is calculated by dividing distance by time. V = D/T Try the following problem....... Cletus drives from Little Rock to Texarkana, a distance of 150 miles. If his average speed was 60 mi/hr, how long did the trip take? time = distance / velocity 150 miles / 60 mi/hr = 2.5 hrs. or 2 hours and 30 minutes. • Let’s say you go to watch the fireworks on the 4th. From where you’re sitting, you are 501 m away when a skyrocket explodes. If you hear the explosion 1.5 s later, what is the speed of sound (m/s)? • When a plane breaks the sound barrier (faster than the speed of sound, 334 m/s) it produces a loud shock wave called a sonic boom. • Suppose an F-18 flying overhead breaks the barrier, and you hear the sonic boom 12 sec. after it was produced. How high in the air was the jet flying? Acceleration Acceleration is the rate of change of velocity (how fast something speeds up or slows down) and is measured in meters/second/second (m/s/s or m/s2) For example, if a car speeds up by 20 meters per second every second, than the rate of acceleration is 20 m/s/s. If the same car continued to accelerate at 20 m/s/s for 5 sec., it will increase it's velocity by 100 m/s. Start – 0 m/s 1 sec. – 20 m/s 2 sec. – 40 m/s 3 sec. – 60 m/s 4 sec. – 80 m/s 5 sec. – 100 m/s the formula for acceleration is: A = (Vf - Vi) / t Vf - final velocity (speed it ends up at) Vi - initial velocity (speed it started at) T - time Now try this practice problem. The Thrust SSC is the only vehicle to ever break the sound barrier on land, going 760 mi/hr. If the vehicle went from 0 to 760 mi/hr in 40 sec., what was it’s rate of acceleration in mi/hr/sec? A = (Vf - Vi) / t Vf = 760 mi/hr Vi = 0 mi/hr t = 40 sec. A = (760 – 0) / 40 A = 19 mi/hr/sec. The rate of acceleration of an object can also be a negative value. Negative acceleration simply means that the object is slowing down. Forces A force is any push or pull exerted on an object. As you sit here, there are at least 3 forces currently acting on your body. What are they? Hmmm. 1. Gravity 2. Supporting force 3. Air pressure Balance Forces • Forces which are equal in size and opposite in direction are said to be balanced forces. • Ex.: -chair and gravity -tug-o-war (tied -2 people leaning back-to-back Unbalanced Forces Unbalanced or net forces are not equal in size. And since this means that the force in 1 direction must be greater, it will result in a change in velocity. Ex.: - kicking a ball - ears popping - pushing the desk Newton's First Law of Motion • Newton's 1st LOM states that an object will maintain a constant velocity unless acted on by a net force. This includes both stationary and moving objects. • Newton's 1st law is also called the Law of Inertia. Inertia is the tendency of an object to resist any change in motion. Inertia is related to the mass of an object. What is this relationship? Motion and Forces What happens in a crash? • The law of inertia can explain what happens in a car crash. • When a car traveling about 50 km/h collides head-on with something solid, the car crumples, slows down, and stops within approximately 0.1 s. Motion and Forces 2.3 What happens in a crash? • Any passenger not wearing a safety belt continues to move forward at the same speed the car was traveling. • Within about 0.02 s (1/50 of a second) after the car stops, unbelted passengers slam into the dashboard, steering wheel, windshield, or the backs of the front seats. Gravity • Gravity is a force of mutual attraction which every object in the universe exerts on every other object. The amount of gravity depends on the distance between the objects and the mass of the objects. • What is the relationship between the following: –Gravity and distance? –Gravity and mass? • A black hole is a collapsed star with so much gravitational pull that even light can not escape. Does a cat always land on it’s feet? Weight • Weight is a measure of the gravitational pull exerted on an object. • The SI unit for weight is the newton (N). 1 newton is about the weight of a medium sized apple. On earth, 1 N is just about .25 pounds. • Weight is measured using a scale. “Don't step on it ... it makes you cry.” Topics for Discussion • Special effects • Falling elevator • Tides Friction • Friction is a force which opposes motion between two surfaces which are in direct contact. • There are 2 main variables which help determine the amount of friction present between 2 surfaces: 1. The force pushing the surfaces together 2. The texture of the surfaces Topics for Discussion • Hydroplaning • Swamp buggies • Starting fires • Most meteors burn up as they are passing through the earths atmosphere, yet the space shuttle which re-enters the atmosphere at a high speed is relatively unharmed. Why do you think this is? Calculate the velocity of the car shown above. • Viewing the cars above, one is travelling at a constant velocity, one is experiencing positive acceleration, and one is experiencing both positive and negative acceleration. Which is which? • The illustrations show an elephant and a feather both falling from the top of a building at the exact same time. Explain what could cause the difference between the two illustrations. • How are each of the 3 objects in the above illustration accelerating? • A robotic explorer was sent to planet BR-549. The rate of gravitational acceleration on this planet is 29.4 m/s/s. If the explorer weighed 2,500 pounds on the earth, what was it’s weight on this other planet? • Is BR-549 larger or smaller than the earth? • For the skydiver in the illustration, tell what is happening to his acceleration and velocity during the following stages of his trip: A. Initial falling stage B. Time when the parachute is being deployed C. Falling stage with the open parachute • In the above scenario, a small cannon shoots an iron ball straight up in the air, from the back of a truck traveling at a constant velocity • Is the above situation possible in real life? Explain. • Some people may put a tube of sand in their trunk or in the bed of their truck. In terms of something that we’ve discussed in this chapter, explain why they would do that. • Before a drag race, the dragster will often “burn out” at the starting line before the race occurs, and then immediately back up to the line and get set for the start of the race. Why do you think they do this? • Some good pitchers can throw a fastball which actually rises from the time it leaves their hand until the time it reaches the catcher. How do you think this is possible when gravity is pulling it downward the whole way? • Roy has to slam on his brakes when a deer runs in front of his car. If he was going 72 mi/hr and it took him 6 seconds to stop, what was his rate of acceleration? • Rob drives from Kansas City to St. Louis (210 mi.) traveling at 70 mi/hr. From St. Louis he turns north and drives to Chicago (270 mi.) averaging 60 mi/hr. How long did it take him to make the entire trip? • A 400 m runner accelerates at a rate of .5 m/s/s over the last 6 seconds of the race, in an attempt catch an opponent at the finish line. If she ends up running at 12 m/s, how fast was she running before she started her kick. Jim is driving from Kansas City to Denver. It takes him 7.5 hrs. to drive through Kansas at an average speed of 70 mi/hr. He then drives for 2 hrs. and 30 min. in Colorado at an average velocity of 60 mi/hr., before arriving in Denver. How far is it from K.C. to Denver? At or between which point(s) on the above speed/time graph is the object experiencing: A. B. C. D. Constant speed The greatest negative acceleration Top speed The most gradual positive acceleration