Newton's Laws of Motion

Transcription

1 Newton's Laws of Motion Ch.5 p. 152 Ch 5 Title Newton's Laws of Motion 1st Law: Inertia 2nd Law: F = ma 3rd Law: Action Reaction Pairs Newton's Laws 1

2 Newton's 1st Law: Inertia an object at rest will remain at rest, and an object in uniform motion will remain in uniform motion... unless acted on by UNBALANCED forces (F net ) F net is the sum (addition) of all forces in a particular direction. Ex: If F g is bigger than F N, you'll fall through the floor. a F N F g N1: Inertia & Fnet Newton's 1st Law: Inertia You can think of the forces playing tug of war with the object. The larger force "wins" by the amount F net. Examples: AT REST v = 0 m/s a = 0 m/s 2 UNIFORM MOTION v = # m/s a = 0 m/s 2 ACCELERATING v = # m/s a = # m/s 2 N1: Inertia & Fnet 2

3 Newton's 2nd Law: F net = ma net F net is the sum of all forces in one dimension; a net is the resulting (ACTUAL) acceleration For instance... x direction: y direction: F net(x) = F net(y) = a net(x) = a net(y) = FA a FN Ff F net causes objects with mass to accelerate in the direction of the bigger force(s). FG N2: F=ma Newton's 2nd Law: Draw a Free Body Diagram including all forces. Explain what will happen to the stone and why. Depends on direction! Analyze x & y components separately Example #1: While pushing a curling stone. Example #2: After letting go. Ex: What & Why? 3

4 Model Problem Model Problem 4

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7 Model Problem p. 168 #4 7 Remember "a" is the key to move you between Forces and The Big 4. Use what they give you to find "a", then use it to get what you're asked to find. p. 168 #4 7 7

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10 Newton's 3rd Law: For every action force on object B due to object A, there is an equal and opposite reaction force due to object B on object A. A B Forces come in equal and opposite actionreaction pairs. N3: Action Reaction Newton's 3rd Law:...if this law is true, how can they accelerate? Don't worry! Each force acts on a different body. The forces don't cancel each other because they would be drawn on separate Free Body Diagrams. N3 10

11 Newton's 3rd Law Identify the Action Reaction Pairs: Action Reaction Pairs Applying Newton's 3rd Law Collisions Example 1: An inattentive driver accidentally rear ends a parked 1500 kg car. The moving car has a mass of 1250 kg and was originally driving 30 km/h. The collision brings it to a stop in 0.75 seconds. a) Find the force the parked car puts on the moving car during the collision. b) Find the force the moving car puts on the parked car during the collision. c) How much does the collision cause the parked car to accelerate? Ex: Collisions 11

12 Ex: Collisions Applying Newton's 3rd Law Coupled Objects Example 2: A 10,000 kg train engine is hauling 2 boxcars, each of which have a mass of 6,000 kg. The force pulling the last boxcar forward is N. The coefficient of friction between the tracks and the train is a) Find the acceleration of the train and the boxcars. b) Calculate the forward force on each hitch, between the objects. c) Calculate the force the engine must apply to move the entire train forward at this acceleration. Ex: Coupled Objects 12

13 Applying Newton's 3rd Law Coupled Objects Example 2: A 10,000 kg train engine is hauling 2 boxcars, each of which have a mass of 6,000 kg. The force pulling the last boxcar forward is N. The coefficient of friction between the tracks and the train is a) Find the acceleration of the train and the boxcars. b) Calculate the forward force on each hitch, between the objects. c) Calculate the force the engine must apply to move the entire train forward at this acceleration. Ex: Coupled Objects Applying Newton's 3rd Law Coupled Objects Example 2: A 10,000 kg train engine is hauling 2 boxcars, each of which have a mass of 6,000 kg. The force pulling the last boxcar forward is N. The coefficient of friction between the tracks and the train is a) Find the acceleration of the train and the boxcars. b) Calculate the forward force on each hitch, between the objects. c) Calculate the force the engine must apply to move the entire train forward at this acceleration. Ex: Coupled Objects 13