Lesson Plan - Mass and Weight

Suggestions for Instructors

This exercise is designed to look at the difference between mass and weight, and to move students from a misconception of objects in space being weightless to objects in space being in free fall, and experiencing microgravity.

There are two programs in this exercise. The first is a visualization of what a scale would read in different environments, showing that in all cases the mass is the same, it is the same object, but the scale reads differently. due to gravity between two objects, as well as the acceleration of each object.

The misconceptions that this activity is targeting are that mass and weight are the same thing, and that weightlessness means there is no gravity.

Teachers might also consider looking for other information on this in NASA's microgravity lesson plans

The applet meets the following National Standards:  Science Content Standards: 9-12

• CONTENT STANDARD A:
• Use technology and mathematics to improve investigations and communications.
• Formulate and revise scientific explanations and models using logic and evidence.
• Recognize and analyze alternative explanations and models.
• Communicate and defend a scientific argument.
• CONTENT STANDARD B:
• Motions and forces

Answers to Questions

1. The object in orbit around the Moon had zero mass.

   FALSE. Mass is a fundamental property of matter, and not measured by a standard scale, which measures the force of gravity acting on a mass. A true statement would be: The object in orbit around the Moon had zero measurable weight.

2. The object in orbit around the Earth did not feel any gravity.

   FALSE. If there was no gravity, there would be no orbit. While an object in orbit may not be undergoing a change in speed, it is undergoing a change in direction. It takes a force to do that. However, if you were to try to measure gravity using a standard spring scale, since both the spring scale and the object that was being measured were falling together, the object would appear to be weightless.

3. The scale in orbit around the Earth did not measure any weight.

   TRUE.

4. What is the force due to gravity of a 1 kg object 1000 km above the surface of the Earth?

   7.3 N. Be sure that the distance between the objects includes both the distance above the Earth and the radius of the Earth.

5. By how much does the force of gravity decrease between the surface of the Earth and an orbit 1000 km above the surface of the Earth?

   At the surface of the Earth, a 1 kg object feels a 9.8 N force due to gravity, so 1 km above the surface of the Earth, gravity is 26% weaker.

6. What is the force due to gravity of a 1 kg object on the Moon?

   0.41 N

7. What is the force due to gravity of a 1 kg object in orbit 1000 km above the Moon?

   0.24 N

8. By how much different is the force of gravity between an orbit 1000km above the Earth and an orbit 1000 km above the Moon?

   The force of gravity acting on an object in orbit around the Moon is 3% of the force of gravity acting on an object in orbit around the Earth.

9. What is the force on the Moon due to the Earth?

   2.0E20 N

10. What is the force on the Earth due to the Moon?

    2.0E20 N

11. What is the acceleration of the Moon due to the Earth?

    0.0027 m/s/s

12. What is the acceleration of the Earth due to the Moon?

    0.000033 m/s/s

13. Why are the forces the same, but the accelerations different?

    Any force between two object pushing or pulling each other is equal and opposite: equal in magnitude and opposite and direction. You cannot push something without feeling an equal push back. However, the two objects have different mass. A smaller mass feeling the same force as a larger mass will be accelerated more by that force. Gravity accelerates a child jumping rope more than the Earth because the Earth is more massive. A cannon recoils at a slower speed than the fired cannon ball because it is more massive (and because if the cannon is on the Earth, the Earth pushes back on the cannon too.)