Figure 14 Weight is a measure of the force of gravity acting on an object. A An astronaut with a mass of 88 kg weighs 863 N on Earth. B An astronaut with a mass of 88 kg weighs 141 N on the moon. Calculating If the same astronaut stood on Mars where the acceleration due to gravity is about 3.7 m/s2, how much would the astronaut weigh?
If you study the weight formula, you'll see that mass and weight are proportional. Doubling the mass of an object also doubles the object's weight. 
Mass is a measure of the inertia of an object; weight is a measure of the force of gravity acting on an object. Consider the same astronaut shown on Earth and on the moon in Figure 14. On the moon, the acceleration due to gravity is only about one sixth that on Earth. Thus, the astronaut weighs only about one sixth as much on the moon as on Earth. In both locations, the mass of the astronaut is the same.
Section 12.2 Assessment
Reviewing Concepts
- State Newton's first law of motion in your own words.
- What equation states Newton's second law of motion?
- How is mass different from weight?
Critical Thinking
Applying Concepts Describe several examples of Newton's first and second laws that you observe during a normal day.
Making Judgments A steel ball is the same size as a wooden ball, but weighs twice as much. If both balls are dropped from an airplane, which of them will reach terminal velocity more quickly? Explain.
Math Practice
6. During a test crash, an air bag inflates to stop a dummy's forward motion. The dummy's mass is 75 kg. If the net force on the dummy is 825 N toward the rear of the car, what is the dummy's deceleration?
A bicycle takes 8.0 seconds to accelerate at a constant rate from rest to a speed of 4.0 m/s. If the mass of the bicycle and rider together is 85 kg, what is the net force acting on the bicycle? (Hint: First calculate the acceleration.)
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