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Thursday Honors PS
Homework
• Pg 369
• Problems 22-27
Leading Ideas
• Newton’s 1st law
• Newton’s second law
• Weight = mass x 10 (9.8)
Difference between mass and weight
To find the weight of an object
• Weight = mass x acceleration of gravity
• On Earth: acceleration due to gravity is equal to about 10 (9.8)
Mass and Weight
• Mass:– Amount of stuff– Unchanging– Measure of Inertia– kg
• Weight:– Force of gravity– Changes because of location– N
Answers to homework
• 22) 10 m/s2
• 23) 23 N• 24) 2.1 kg• 25) .04 m/s2
• 26) 15 N• 27) 43.75 N• 28) 3.7 N• 29) 58.5 N
Additional Problems
• A 2.5 kg hula hoop is pushed forward with a force of 45.9 N. What is it’s acceleration?
Answer
• F = ma
• F = 45.9 N m = 2.5 kg
• A = 45.9 / 2.5 = 18.4 m/s2
Make up a problem
• Object : _______________
• Amount of net force: __________
• Mass of object: ____________
• Solve for acceleration
Example problem
• A 1540 kg car starts from rest and accelerates to 50.7 m/s in 13.8 seconds. What is the net force on the car that causes this acceleration?
Answer
• 1540 kg = mass• Rest = 0 m/s = initial velocity• 50.7 m/s = final velocity• 13.8 sec = time
• Want net force, but need to find acceleration
a = (vf- vi) / t then Fnet = ma
a= 3.7 m/s2 Fnet = 5700 N
New homework Handout #3
• Working with Newton’s 2nd Law• Review conversions, acceleration
• All questions are in book• Pg 852 3,4• Pg 857 89-95
• To be handed in on Monday
Newton’s third Law
3rd Law
• For every _____________
There is an __________ but opposite
__________.
• For every action there is an equal and opposite reaction
• Forces come in pairs– Equal in size– Opposite in direction– Applied to different objects
Examples of 3rd law
Actions that can be related to 3rd law in the classroom
If all forces are equal and opposite, how does anything
move?
Answer
• Though equal in size, the forces act on different objects
Force of ground on box
Force of box on ground
Trebuchet
To change the motion of something
• Need to have a net force (unbalanced forces) on the object in order to change its motion
Force of rope on box
Friction force
Free Body Diagrams
• Represent the object by a simple geometric shape or a dot
• Represent all forces on that object by arrows that originate from shape/dot
• Only represent the forces that are applied to the object, NOT forces the object applies to other things
What would be the free body diagram of …..
• What are the forces on the dog?
Answer
Dog
Applied force of ground pushing paws forward
Reactive force of ground
Force of leash
Weight of dog
Create a free-body diagram of one of the dancers
Create a free-body diagram of the ball
• Draw a free-body diagram of the rock climber
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