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