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Objectives Force and • Explain the difference between weight and mass. weight • Calculate weight from mass or mass from weight.
Assessment Assessment
1. If you take a trip from Earth to the Moon, which statement 2. What is the weight of this below is correct? dumbbell, in newtons?
A. Your mass and weight both decrease. B. Your mass stays the same and your weight decreases. 3. A person weighs 150 lbs. What is this weight in newtons? C. Your mass decreases and your weight stays the same. What is this person’s mass in D. Your mass increases and your weight decreases. kilograms? A 10 kg dumbbell resting on a table is partly supported by a spring that pulls upward with a force of 50 N.
Physics terms Equations
• mass
• force Weight is mass multiplied by the strength of gravity. • weight
• newtons At Earth’ s surface the strength of • pounds gravity is 9.8 newtons per kilogram.
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Explaining motion What is mass?
Isaac Newton was the first All matter has mass and takes person to explain WHY objects up space. move the way they do. A solid rock has mass. So do He based his laws of motion gases and liquids. on two key concepts: With your hand out the window • mass of a moving car, you feel the mass in the air pushing against • force you.
Measuring mass What is force?
The concept of force:
The SI unit for mass is the kilogram. A force is a push or a pull.
This object has a mass of 2 kilograms.
What is force? Types of forces
The concept of force: The concept of force:
A force is a push or a pull. A force is a push or a pull.
Forces can cause an object Forces can cause an object to change its motion. to change its motion. Examples of forces: Can you give some Can you give some • weight examples of forces? examples of forces? • friction • tension from rope • force from a spring • electric force
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Units of force Units of force
Quantitative: Force is measured in newtons.
Weight is a type of force Weight vs. mass
Mass is an intrinsic property that measures Weight is the force of gravity the quantity of matter in an object. acting on objects with mass. • Your mass does NOT change if you go into space.
How is weight different from mass?
Weight vs. mass Weight vs. mass
Mass is an intrinsic property that measures Mass is a scalar quantity. the quantity of matter in an object. It has a magnitude (such as 60 kg), but no direction. • Your mass does NOT change if you
go into space. Weight is a vector.
It has magnitude AND direction. Weight is an extrinsic property that For objects on Earth, it always depends on the gravity force. points straight down toward the F w center of the Earth. • Your weight changes if you go into
space. Your weight depends on your location.
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Weight Weight
Weight is the force of gravity acting One kilogram of mass weighs 9.8 N. 1 kg on objects with mass. This is roughly the mass and weight of a one liter water bottle. Weight is mass multiplied by the strength of gravity.
At Earth’s surface the strength of gravity is 9.8 newtons per kilogram.
Weight Weight
One kilogram of mass weighs 9.8 N. 1 kg One kilogram of mass weighs 9.8 N. 1 kg
This is roughly the mass and weight This is roughly the mass and weight of a one liter water bottle. of a one liter water bottle.
Force is a vector, so it has a direction. What changes if you bring this bottle to the Moon? Fw = 9.8 N Fw = 9.8 N
The force of weight always points straight down.
Weight What is a pound?
One kilogram of mass weighs 9.8 N. 1 kg The pound is the English unit of force.
This is roughly the mass and weight of a one liter water bottle. The person standing on this scale is pushing down on it with a force of 125 pounds.
What changes if you bring this bottle to the Moon? Fw = 1.6 N
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What is a pound? Example problem
The pound is the English unit of force. If a person has a weight of 125 pounds, what is her mass in kilograms? Pounds are a unit of force, not mass.
One pound = 4.448 N
The newton is a smaller unit of force than the pound.
One newton = 0.225 lbs (~3.6 ounces)
Convert to SI units Calculate the mass
If a person has a weight of 125 pounds, Mass is weight divided by g. what is her mass in kilograms?
Weight and mass Exploring the ideas
Whenever you know the mass of an object, you Click this can calculate its weight. interactive calculator on page 135 Whenever you know the weight of an object, you can calculate its mass.
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Engaging with the concepts Assessment
Use the weight calculator to check 1. If you take a trip from Earth to the Moon, which statement your work when solving problems on below is correct? the student assignment. A. Your mass and weight both decrease.
B. Your mass stays the same and your weight decreases.
C. Your mass decreases and your weight stays the same.
D. Your mass increases and your weight decreases.
Assessment Assessment
1. If you take a trip from Earth to the Moon, which statement 2. What is the weight of this below is correct? dumbbell, in newtons?
A. Your mass and weight both decrease. B. Your mass stays the same and your weight decreases.
C. Your mass decreases and your weight stays the same.
D. Your mass increases and your weight decreases.
A 10 kg dumbbell is resting on a table partly supported by a spring that pulls upward with a force of 50 N.
Assessment Assessment
2. What is the weight of this 3. A person weighs 150 lbs. dumbbell, in newtons? What is this weight in newtons?
Fw = mg = (10 kg)(9.8 N/kg) = 98 N
What is this person’s mass, in kilograms?
98 N
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Assessment Assessment
3. A person weighs 150 lbs. 3. A person weighs 150 lbs.
What is this weight in newtons? What is this weight in newtons?
What is this person’s mass, in kilograms? What is this person’s mass, in kilograms?
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