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Transcript
1 Isaac Newton (1642 – 1727) Newton’s Laws The Father of Force 2 What is a Force? Force can be defined as a push or a pull… or anything else that has the ability to change motion. 3 Formula Force = mass X acceleration F = ma 4 Force: kilogram x m/sec2 kg x 2 m/s Easier than saying kilogram m/sec2 N Is even easier! 5 To Understand Force… You must understand the difference between AND 2 Kg Mass 6 What is Mass? The amount of “stuff” in an object 1 Kg Mass 1.5 Kg Mass Mass is measured in kilograms. 2 Kg Mass 7 How is Mass Different Than Weight? Weight is measurement of FORCE. 2.2 pounds 3.3 pounds 9.8 N 14.8 N (1 kg x 9.86 m/s2) (1.5 kg x 9.8 Acceleration due to gravity (9.8 m/s2) 1.0 Kg Mass 4.4 pounds 19.7 N (2 kg x 9.86 m/s2) 6m/s2) Acceleration due to gravity (9.8 m/s2) Acceleration due to gravity (9.8 m/s2) 1.5 Kg Mass 2.0 Kg Mass DO NOT USE kilograms (kg) as a measurement for weight (force). USE pounds (lb) or N for force. 8 WEIGHT WEIGHT = mass X acceleration due to gravity W = mg W = ma 9 Weight = Force W = mg kilogram x m/sec2 kg x m/s2 = N Easier than saying kilogram m/sec2 N Is even easier! 10 7 1 Kg Mass Different Gravity? Mass is the Same:1 kg But the WEIGHT… 2.2 Pounds On the moon 1.0 kg would weigh 0.3 lbs. Acceleration due to gravity 1.6 m/s2 On the sun 1.0 kg would weigh 59.5 lbs. On Mars 1.0 kg would weigh 0.8 lbs. What’s “Your Weight On Other Worlds?” http://www.exploratorium.edu/ronh/weight/ 11 The Story of Newton’s Apple Newton sat under an apple tree, and an apple fell on his head. That falling apple gave him a revelation and led him to describe a force called GRAVITY. But DON’T think of gravity as falling… It is the pull (force) that an object has on another object. 12 Newton’s apple was falling because the massive gravitational force of the Earth was pulling the apple towards Earth... 13 Gravity is the force that ALL objects in the UNIVERSE exert on each other... Newton said that gravitational force depends on 2 things: The DISTANCE of the objects The MASS of the objects LESS gravitational force compared to... MORE gravitational force here. The blue object has more gravitational force than the green object because the blue object has more MASS. 14 Newton’s apple fell because the Earth’s pull of gravity.... The itty-bitty pull of apple is too small to pull the Earth, so it is pulled to the Earth... was more massive than the apple’s pull of gravity on the Earth 15 The moon is a satellite too.. The pull of gravity keeps satellites orbiting around the Earth Satellites are sent away and placed in the 16 exact spot where the Earth’s gravity pulls them into a “continuous free fall” or orbit... The Earth is round and the satellite continues to “just miss” entering into the Earth’s atmosphere. 29 The moon does the same thing. 17 Does the moon’s gravitational pull affect the Earth? Yes, the moon’s gravity is large enough to actually pull water out of the ocean from space… Well, not quite that much...just enough to create motion of the tides. 18 Newton is even credited with inventing CALCULUS. Needless to say, Newton was very important in shaping our understanding of science. 19 These laws explain why objects move (or don't move). Let’s look at Newton's three laws of motion... 20 An object will remain at rest unless acted upon by an “unbalanced” force. An object in motion will continue with constant speed and direction, unless acted on by an unbalanced force. This law shows how force, mass and acceleration are related as shown in the equation below: Force = mass x acceleration For every action there is an equal and opposite reaction. 21 Newton’s First Law: Balanced Forces An object will maintain a constant state of motion (balanced). This means an object at rest tends to stay at rest and an object in motion tends to stay in motion. Forces that are balanced can be: Not in motion Stay at rest: a book on a table IP&C This is known as Inertia Table pushes up on book Gravity pulls down on book In Motion Stay in motion: a bowling ball tossed in space It will go on and on forever. 22 Newton’s First Law: Unbalanced Forces An unbalanced force is a force that changes the motion. The book below slides and then stops because of a resistant force called friction. Friction is force that opposes motion. In space there is no resistance to cause friction, so a bowling ball would stay in motion ...unless another It kind of object got in reminds me of the way… a few movies I’ve seen lately... Force of friction stops the book IP&C 23 Of course “frictional” force happens on earth... but, if you weren’t wearing a safety belt you would continue your motion… Do not let this happen to you; buckle up! Newton’s Second Law: Force = Mass X Acceleration 24 Force and acceleration are related. You are going down the road on your inline skates. Suddenly you look to your right and see a car accelerating fast coming right for you! You look for an escape to your left… Oh no! There’s a rock at the edge of neatly manicured lawn. Which will you choose... A massive car accelerating fast? ...or a medium size rock not in motion? Good Choice Force (car) = mass x acceleration is greater than Force (rock) = mass x acceleration You still might get hurt, but would be much worse off choosing the force of the car over the force of the rock. 25 Solving Force Problems Step 1 Read the problem. Draw a picture. Step 2 Write down what you know, What are you trying to find? Step 3 Set up the formula. Step 4 Plug-in the numbers. Solve. 26 Consider the problem… “How much force is needed to accelerate a 1400-kg car 2 m/s2? ” Step 1 1400-kg car Read the problem. Draw a picture. How much force? 2 m/s2 F= m= a= Formula Plug-in Units, units, units! Answer 27 “How much force is needed to accelerate a 1400-kg car 2 m/s2? ” Step 2 Write down what you know, What are you trying to find? 1400-kg car 2 m/s2 F= m =1400 kg a = 2 m/s2 Formula Plug-in Units, units, units! Answer 28 “How much force is needed to accelerate a 1400-kg car 2 m/s2? ” Step 3 Set up the formula. Solve. F=mxa F= m =1400 kg a = 2 m/s2 Formula Plug-in mxa Units, units, units! Answer 29 “How much force is needed to accelerate a 1400-kg car 2 m/s2? ” Step 4 Plug-in the numbers. Solve. 2800kg x m/s2 F= m =1400 kg a = 2 m/s2 Formula mxa Plug-in 1400 kg x 2 m/s2 Units, units, units! Answer 2800 kg x m/s2 30 Be sure to do the problems. • Helpful Hints: a=F/m F=mxa m=F/a 31 The unit for force: 1 kg x 2 m/s = 1 newton or 1N Which one would you rather write for your answer? 2800 N …I thought so. 32 Newton’s Third Law: Action / Reaction For every action there is an equal and opposite reaction. Look at the picture below: What is the action? As the gases push downward out of the rocket, What is the reaction? The rocket is pushed upward by gases. 33 An object will remain at rest unless acted upon by an “unbalanced” force. An object in motion will continue with constant speed and direction, unless acted on by an unbalanced force. This law shows how force, mass and acceleration are related as shown in the equation below: Force = mass x acceleration For every action there is an equal and opposite reaction. 34 Newton’s laws explain how earthquakes create such devastation. Most of the time the Earth’s massive plates push on each to move very slowly. But, sometimes a massive plate exerts a greater amount of force that accelerates another plate. This results in an earthquake. To learn more go to: http://vcourseware5.calstatela.edu/VirtualEarthquake/VQuakeIntro.html