1. Force and weight
The concept of force is simply a push or pull. This idea is made more quantitative with units of newtons and pounds. Students sometimes consider the force due to gravity and weight to be two different forces. This lesson reinforces the fact that weight IS the force due to gravity. The force of gravity is related to but distinct from the mass of an object.

2. Objectives Explain the difference between weight and mass.
Calculate weight from mass or mass from weight.
The lesson objectives describe what the student should know or be able to do upon completion of the lesson.

3. Assessment
If you take a trip from Earth to the Moon, which statement below is correct?
Your mass and weight both decrease.
Your mass stays the same and your weight decreases.
Your mass decreases and your weight stays the same.
Your mass increases and your weight decreases.
This first assessment is keyed to the first objective: Explain the difference between weight and mass.
It will be repeated at the end of the lesson, followed by the answer.

4. Assessment What is the weight of this dumbbell, in newtons?
A person weighs 150 lbs.
What is this weight in newtons? What is this person’s mass in kilograms?
A 10 kg dumbbell resting on a table is partly supported by a spring that pulls upward with a force of 50 N.
These assessments are keyed to the second objective: calculate weight from mass or mass from weight. They will be repeated at the end of the lesson, followed by the answer.

5. Physics terms mass force weight newtons pounds
This slide lists key vocabulary for the lesson.

6. Equations Weight is mass multiplied by the strength of gravity.
At Earth’ s surface the strength of gravity is 9.8 newtons per kilogram.
The value for g here is given to two significant digits. The value of g varies over the surface of the Earth, from 9.78 N/kg to 9.83 N/kg, depending on latitude.

7. Explaining motion mass force
Isaac Newton was the first person to explain WHY objects move the way they do.
He based his laws of motion on two key concepts:
mass
force
Point out to students that in chapters 2 and 3 they learned how to describe motion (using physics vocabulary like displacement, velocity, and acceleration). In chapter 4 they will begin the work of learning how to explain WHY objects move they way they do. This can be likened to the difference in biology between the study of taxonomy and science of genetics.

8. What is mass? All matter has mass and takes up space.
A solid rock has mass. So do gases and liquids.
With your hand out the window of a moving car, you feel the mass in the air pushing against you.
These next two slides review the concept of mass, which was introduced in Chapter 2.

9. Measuring mass The SI unit for mass is the kilogram.
This object has a mass of 2 kilograms.
if possible, pass around a few 1 kg masses so that students can develop a feel for this unit.

10. What is force?
The concept of force:
A force is a push or a pull.

11. What is force? The concept of force: A force is a push or a pull.
Forces can cause an object to change its motion.
Can you give some examples of forces?
Give the students a couple minutes to name some forces they have heard about.

12. Types of forces The concept of force: A force is a push or a pull.
Forces can cause an object to change its motion.
Can you give some examples of forces?
Examples of forces:
weight
friction
tension from rope
force from a spring
electric force
Students may also mention atomic forces, magnetic forces, gravity, air resistance, buoyancy, etc. People can also apply forces by pushing or pulling on objects.

13. Units of force Quantitative: Force is measured in newtons.
A useful reference: one newton of force is about equal to the weight of a stick of butter pressing down on your hand as you hold it.

14. Units of force
When students learn Newton’s second law, F = ma, it will provide a useful tool for remembering the definition of a Newton: 1 N = (1 kg) (1 m/s2).

15. Weight is a type of force
Weight is the force of gravity acting on objects with mass.
How is weight different from mass?
Reinforce this idea: weight IS the force of gravity. These are two names for the same force. Give students a moment to answer the question. They may have prior knowledge.

16. Weight vs. mass
Mass is an intrinsic property that measures the quantity of matter in an object.
Your mass does NOT change if you go into space.

17. Weight vs. mass
Mass is an intrinsic property that measures the quantity of matter in an object.
Your mass does NOT change if you go into space.
Weight is an extrinsic property that depends on the gravity force.
Your weight changes if you go into space. Your weight depends on your location.
Ask the students: “if you go to Mars, does your weight change? Does your mass change?”

18. Weight vs. mass Fw Mass is a scalar quantity.
It has a magnitude (such as 60 kg), but no direction.
Weight is a vector.
It has magnitude AND direction. For objects on Earth, it always points straight down toward the center of the Earth. Fw
This ALWAYS is important. Emphasize to students that the force of gravity comes from the pull of the planet itself. The direction of that pull does not change even if the object is on a ramp, upside down, or flying through the air. Gravity ALWAYS points straight down.

19. Weight Weight is the force of gravity acting on objects with mass.
Weight is mass multiplied by the strength of gravity.
At Earth’s surface the strength of gravity is 9.8 newtons per kilogram.
Point out that N/kg and m/s2 are identical units. One newton equals one kg m/s2 so N/kg =m/s2.
Ask the students to explain WHY it is helpful to use N/kg in this context.

20. Weight One kilogram of mass weighs 9.8 N.
This is roughly the mass and weight of a one liter water bottle.
1 kg

21. Weight One kilogram of mass weighs 9.8 N.
This is roughly the mass and weight of a one liter water bottle.
1 kg
Force is a vector, so it has a direction.
The force of weight always points straight down.
Fw = 9.8 N

22. Weight One kilogram of mass weighs 9.8 N.
This is roughly the mass and weight of a one liter water bottle.
1 kg
What changes if you bring this bottle to the Moon?
Fw = 9.8 N

23. Weight One kilogram of mass weighs 9.8 N.
This is roughly the mass and weight of a one liter water bottle.
1 kg
What changes if you bring this bottle to the Moon?
Fw = 1.6 N
Reinforcement: The mass doesn’t change, but the weight changes because g, the acceleration due to gravity changes. Point out to the students that they would also weigh less on the Moon, but their mass would remain the same.
Ask “What is the direction of this weight vector if the bottle is on the Moon?” It would point toward the center of the Moon.

24. What is a pound? The pound is the English unit of force.
The person standing on this scale is pushing down on it with a force of 125 pounds.
If the pound is the English unit of force, what is the English unit of mass? Students may be amused to learn that it is the slug.
Since F = mg, and g = 32.2 ft/s/s, a mass of one slug has a weight of 32.2 pounds.

25. What is a pound? The pound is the English unit of force.
Pounds are a unit of force, not mass.
One pound = N
The newton is a smaller unit of force than the pound.
One newton = lbs (~3.6 ounces)
One newton is roughly equal to the weight of a stick of butter.

26. Example problem
If a person has a weight of 125 pounds, what is her mass in kilograms?
Point out that this question ASSUMES that the person is on Earth, where g = 9.8 N/kg.

27. Convert to SI units
If a person has a weight of 125 pounds, what is her mass in kilograms?
First you must convert the weight from English units into SI units.

28. Calculate the mass Mass is weight divided by g.
THEN you can calculate the mass from the weight equation.

29. Weight and mass
Whenever you know the mass of an object, you can calculate its weight.
Whenever you know the weight of an object, you can calculate its mass.

30. Exploring the ideas
Click this interactive calculator on page 135

31. Engaging with the concepts
Use the weight calculator to check your work when solving problems on the student assignment.

32. Assessment
If you take a trip from Earth to the Moon, which statement below is correct?
Your mass and weight both decrease.
Your mass stays the same and your weight decreases.
Your mass decreases and your weight stays the same.
Your mass increases and your weight decreases.
This first assessment is keyed to the first objective: Explain the difference between weight and mass.
The answer, B, appears on the next slide.

33. Assessment
If you take a trip from Earth to the Moon, which statement below is correct?
Your mass and weight both decrease.
Your mass stays the same and your weight decreases.
Your mass decreases and your weight stays the same.
Your mass increases and your weight decreases.
This first assessment is keyed to the first objective: Explain the difference between weight and mass.
The answer, B, appears on the next slide.

34. Assessment What is the weight of this dumbbell, in newtons?
A 10 kg dumbbell is resting on a table partly supported by a spring that pulls upward with a force of 50 N.
This assessment is keyed to the second objective: calculate weight from mass or mass from weight.
The answer appears on the next slide.

35. Assessment Fw = mg What is the weight of this dumbbell, in newtons?
= (10 kg)(9.8 N/kg) = 98 N
98 N

36. Assessment A person weighs 150 lbs. What is this weight in newtons?
What is this person’s mass, in kilograms?
This assessment is keyed to the second objective: calculate weight from mass or mass from weight.
The answers appear on the next slide.

37. Assessment A person weighs 150 lbs. What is this weight in newtons?
What is this person’s mass, in kilograms?

38. Assessment A person weighs 150 lbs. What is this weight in newtons?
What is this person’s mass, in kilograms?