|
Mass produces the effects of weight and inertia. Weight is the force of gravity acting on mass. At the Earth’s surface, 1 kg of mass weighs 2.2 lb. If your mass is 50 kg, then your weight is 50 × 2.2 = 110 lb at the Earth’s surface. Mass is measured in kilograms. In the United States, weight is commonly measured in pounds. In physics, however, it will be more useful to express weight in newtons, the SI unit of force. 
|
|
Weight and mass are not the same. Mass (in kilograms) is the same on Earth, on the Moon, on Mars, or anywhere else. Weight, however, depends on the local strength of gravity. Your weight on the Moon is only 1/6 as much as it is on Earth even though your mass is the same. The Moon’s gravity is weaker than Earth’s gravity because the Moon has only 1/10 Earth’s mass. Mass is an intrinsic property of matter because it does not depend on environmental conditions. Weight is an extrinsic property because it depends on the local strength of gravity, which is different in different places.
|
Inertia is the property of matter that resists changes in motion. Throwing a beach ball is easier than throwing a bowling ball even though both are the same size. The bowling ball, however, has more inertia because of its larger mass. Inertia plays an important role in physics.
|
Think about skating across an ice rink. Once you get going your inertia keeps you going in the same direction, at the same speed, until something applies a force to stop you. Inertia explains why massive objects are hard to start moving and hard to stop once they get moving. For most situations, an object’s inertia is equal to its mass. A 10 kg bowling ball has 50 times the inertia of a 0.2 kg beach ball.
|
|
|
Astronauts orbiting the Earth can float above the floor of a spacecraft in apparent weightlessness. In reality, orbiting spacecraft are only a few hundred miles above Earth’s surface and therefore still subject to 99% of Earth’s surface gravity. The astronauts float because the floor is falling away beneath them! You experience the same state of free fall in the moments between jumping off a diving board and landing in the water. To truly become weightless, an astronaut would have to travel far from Earth. The first humans to experience this were the Apollo 8 astronauts who flew to the Moon in 1968.
|
| |
|
