Newton s Law of Universal Gravitation What is mass?
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Newtons Law of Universal Gravitation Turn to a partner and discuss • What is mass? Mass is … • the amount of matter in an object. • a measure of the inertia of an object. • measured in units of kilograms . • constant everywhere. ! No matter where I take you, your mass doesnt change. Turn to a new partner and discuss… • What is inertia?
Transcript of Newton s Law of Universal Gravitation What is mass?
Newton�s Law of Universal Gravitation
Turn to a partner and discuss
• What is mass?
Mass is …
• the amount of matter in an object.!• a measure of the inertia of an object.!• measured in units of kilograms.!• constant everywhere.!
! No matter where I take you, your mass doesn�t change.!
Turn to a new partner and discuss…
• What is inertia?
Inertia is…
• a property of matter. • a measure of an object�s resistance to
changes in its motion. ! resistance to changes in speed and/or
direction. • directly proportional to an object�s mass.
! more massive objects have more inertia.
One more partner question…
• What is weight?
Weight is …
• another name for the force of gravity which the planet exerts on an object near its surface. ! We typically speak of our weight on Earth
so we are talking about the force of gravity that the Earth exerts on us.
• a force. • dependant on BOTH your mass and the planet
which you are on or near.
Weight Calculations
• Weight = Mass * (Gravitational Field Strength) • Gravitational Field Strength depends on the planet
and the exact location. • On Earth, gravitational field strength =
g= 9.8N/kg. ! In other words, the earth exerts 9.8N of force on
you for every 1 kg of mass. • On Earth,
! Weight (in Newtons) = Mass (in kilograms) * 9.8N/kg
English versus Metric Units
Metric Units English Units
Mass Kilograms Slugs
Weight Newtons Pounds (lbs)
Note: 9.8 Newtons = 2.2 lbs!
WMG!
W = m ⋅ gOmg! This one equation is not included on your MCAS formula sheet!
W: weight (N)m: mass (kg)g: gravitational field strength(N/kg or m/s/s) g = 10 m/s/s on earth
Question #1
• Determine the weight (in Newtons) of a 64 kg object on planet Earth.!
m = 64 kgg = 10 N/kg Weight = ?
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Weight = m ⋅ g
Weight = 64 kg ⋅ 10 N/kg
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Weight = 640 N
Question #2
• Determine the mass (in kilograms) of an object which weighs 200 N on planet earth.!
True and False
1. More massive objects have more inertia than less massive objects.!
2. If the forces acting upon an object are balanced, then it must be at rest.!
3. On Earth, the force of gravity acting upon an object is 9.8 times the weight of the object.!
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More True and False…
4. Inertia is a force which acts upon objects at rest.!
5. A feather and an elephant will have the same weight within a vacuum.!
6. If you stand on a bathroom scale and bounce, the scale reading changes. This means that you weigh differently ( more or less) than if you were to standing at rest.!
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More True and False…
7. More massive objects have the same inertia as less massive objects; yet more massive objects will weigh more.!
8. The weight of an object can be expressed in units of kilograms or grams.!
9. Weight and mass are the same thing; it's just that they have different values on different planets.!
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Turn to a partner and discuss:
• What is �the force of gravity�?
The Force of Gravity is …
• the force of attraction which any two objects that have mass exert on each other.!
• only noticeable when at least one of the objects has a very, very large mass.!! When dealing with situations on Earth, we are only
interested in the gravitational force that Earth exerts on objects.!
Newton�s Law of Universal Gravitation
• ANY two objects which have mass exert an attractive force on each other which is directly proportional to the product of the masses and inversely proportional to the square of the distance between them.
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Fg =Gm1m2
d2
where G = gravitational constantm1 is the mass of one object.m2 is the mass of the second object.d is the center to center distance between the objects.Note : g ≠ G
Newton�s Law of Universal Gravitation
• Three Things for you to know: ! bigger if the objects have more mass. ! smaller if the objects are farther
apart. ! universal. It applies to ANY two
objects which have mass.
Why is gravity so special?
• It is universal. • It does not require contact. • It is poorly understood.
• Note: When people say �gravity� they typically mean �the force of gravity.� Remember that we are talking about a force.
True and False…
1. You and your lab partner will experience a force of gravitational attraction when standing next to each other.!
2. There is no gravity on the moon.!
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Practice Problems
• Situation A) 2 objects, each with mass of �m� are distance �d� apart. What would happen to Fg if the mass of one object doubles?
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Fg =Gm1m2
d22m m
d
Practice Problems
• Situation B) 2 objects, each with mass of �m� are distance �d� apart. What would happen to Fg if the mass of both objects doubles?
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Fg =Gm1m2
d22m 2m
d
Practice Problems
• Situation C) 2 objects, each with mass of �m� are distance �d� apart. What would happen to Fg if the distance between both objects doubles?
m m
2d
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Fg =Gm1m2
d2
Practice Problems
• Situation D) 2 objects, each with mass of �m� are distance �d� apart. What would happen to Fg if the mass of one object triples and the distance between both objects doubles?
3m m
2d
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Fg =Gm1m2
d2
