weight and gravity

8/4/2019 Weight and Gravity

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Weight and GravityWeight and Gravity


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WeightWeighty

A 75 kg person jumps off the high dive atthe pool. How much force is pulling theperson down? F = ma = 75 kg (-9.81 m/s2) = 736 N down

y What is your mass in kilograms? Divide your weight by 2.2

y If you jumped off the high dive what is themagnitude of the force pulling you down? F = ma = (100 kg) (-9.81 m/s2) = 981 N down


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WeightWeight

y

Why do you fall into the pool when youjump off the high dive? Gravity!!!

What does gravity do?

x

Pulls you down.

y When you stand next to the pool to watchyour friends dive, is gravity pulling you

down more, less, or the same amountas when you were in the air? The same.


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WeightWeight

y

The amount of force that gravity pulls youdown with when you are in the air is thesame amount of force that gravity pullsyou down with then you are on the

ground.

y Weight = mass X acceleration due togravity

y Fg = mg

y g = Fg/ m


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WeightWeight

y What is the weight of a 15 kg bag of dogfood? 147 N

y What is the mass of an 80 N block of wood? 8.2 kg

y What is the weight of a 60 kg student? 589 N

y What is the mass of a 10000 N car? 1019 kg


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WeightWeighty A 65 kg person falls from an

airplane at a constantvelocity.

y What is the magnitude ofthe force of air resistance?


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WeightWeight y An 80 kg person falls from

an airplane and theirparachute produces anupward force of 775 N.

y What is the acceleration ofthe person?


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Normal ForceNormal Force

y Remember!y With how much force does gravity pull you

down?

y 981 N for me.

yDoes it matter whether you are falling orstanding on the ground?

y So, when you are standing gravity is still

pulling you down with a strong force.

y WHY ARENT YOU ACCELERATING!?!?!?


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y

You arent accelerating because thefloor/ground is supporting your weight.

y If your acceleration equals zero, then how

much net force could be acting on you?

y NONE!

y How must the upward force of the floor

compare to the downward force ofgravity?

y They are the same!


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y A 25 kg sack of flour sits on the flour of abakery. What is the acceleration of thesack of flour? With how much force is the

floor pushing up on the sack?


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y A 60 kg person rides in an elevator whichis moving upward at a constant speed.For some reason, they are standing on a

scale. How many newtons does the scaleread?


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y The same 60 kg person is riding in anelevator again, still standing on a scale.However, this time the elevator isaccelerating upward at a rate of 2.5 m/s2.

How many newtons does the scale read?


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y Again, that 60 kg person is riding in anelevator standing on a scale, but this timethe elevator is accelerating downward at arate of 3.2 m/s2. How many newtons

does the scale read?


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Newtons Law ofGravitationNewtons Law ofGravitation

y What is gravity?

yThe fundamental

force of attractionthat all objects withmass have for eachother.


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y What properties of two objects mightaffect the gravitational force betweenthem?


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yFg = Gm1m2

y G = Universal Gravitational Constant G = 6.67 X 10-11 Nm2/kg2

y

m1,m2 = mass of objectsy r = distance between objects

Not necessarily radius.

r2


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Calculating GravityCalculating Gravity

y Calculate the gravitational force betweenyou and the person next to you. Estimate the distance between your centers.


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y Calculate the gravitational force betweenyou and the Earth.

y Calculate your weight.


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y Calculate the gravitational force betweenthe Earth and the moon.


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y Calculate the gravitational force betweenthe Earth and the sun.

weight and gravity

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