SUBJECT: Science

GRADE: 7,8,9

GROUP SIZE: Small

TIME: 45 minutes

TYPE OF ACTIVITY: Student Investigation

TEACHING STRATEGY: Guided Discovery Demonstration

CONCEPTS: Mass Weight Gravity

SKILLS: Observation Inference Modeling

Objectives: To distinguish between weight and mass; to learn to compute specific gravity; to consider the effect on the weight of an object as it moves further away from the Earth.

Materials: Balance; spring scale; graduate; masses; string; object to weigh (make sure it will fit into the graduate).

Teacher Background Information:

Students sometimes have a difficult time distinguishing between mass and weight of objects. Simply put, mass is the amount of matter in an object. Weight is a measure of the force of gravity on an object. We use a scale to measure weight and a balance to measure mass. All objects in the universe have a gravitational attraction for one another. The larger the mass of the object, the more gravitational attraction it exerts. This is why Mars, with a diameter of 6794 km, has a gravitational pull of about 1/2 of Earth with a diameter of 12756 km. Jupiter on the other hand, has a diameter of 143,200 km and a gravity nearly 6 times that of Earth.

As the distance between two objects increases however, the gravitational pull decreases but the mass of the objects remains the same.

Teacher Demo:

Have the students hold two magnets close together and feel the pull. Move the magnets further apart and feel the pull again. Attach the magnets to two toy cars and place spring scales on the other end of each car. Measure the force exerted by the magnets at different distances from each other. This experiment is a model for gravitational pull, also. The popular notion is that when you get into orbit around the Earth that you are weightless, that is, out of the effect of Earth’s gravity. Actually, you would have to get very far from the Earth not to be effected by its gravity. The Earth’s atmosphere extends out some 32,000 km and is still held in place by gravity. Theoretically, you would always be under some effect of the gravity of Earth because every mass in the universe exerts an attraction for every other mass.

If you had a mass of 100 kilograms on the surface of the Earth, the Earth would exert a force of about 980 newtons on the mass, that is, it would weigh 980 newtons. You would be about 6440 km from the center of the Earth. The law of inverse squares states that, if you move to a distance twice that (12880 km) from the center of the earth, your weight would be 1/4, that is, (1/2)^{2} = 1/4. A distance three times the distance to the center would yield a gravity of 1/9 and so on. Notice these distances are measured from the center of the Earth not from the surface. Subtract the distance to the center from the total to get the distance from the surface.

What is important here is that an object moving away from the Earth loses weight but not mass. The object above, perhaps an astronaut, would still be the same astronaut floating in space. The astronaut would not be weightless (at 6440 km above the Earth, he or she would still weigh about 245 newtons) and would still have a mass of 100 kg.

Procedure

- Discuss with the students what we might mean by weightlessness in space. (By the way, the term micro-gravity is the proper term for the environment an object is in when in orbit)
- Do the Teacher Demo shown above with the students and discuss the information on computation of the weight of an object as it moves away from the Earth.
- Fill a 25 or 50 ml graduate half full with water. Mark the level.
- Attach a thread or string to a mineral sample or a lead sinker and suspend it from a spring scale. Record the weight.
- Lower the sample into the graduate and weigh again.
- Subtract the wet weight from the dry weight.
- Divide the difference into the dry weight. The result is the specific gravity.

The specific gravity is useful in the identification of rocks and minerals. This information will be useful in a future lesson.

Extensions:

Have the students consider the relationship between the buoyancy of an object in water and the equalization of forces of gravity by orbiting (centrifugal force) around a planet. Review Newton’s laws and Kepler’s laws and their application here. Have the students determine at what distance from the Earth the pull of gravity would be less than 1 newton.