CRITICAL THINKING: The term ”weight” in physics has the following very specific meaning:
“The weight of an object is the name given to a particular force: the gravitational force exerted by the earth on the object, giving it an acceleration of 9.8 m/s2 near the surface of Earth.”
In ordinary speech the use of “weight” is nowhere nearly so precise. What, if anything, is wrong with the following statements? Explain, and correct the statements to become scientifically accurate if they are incorrect.
a) A 3-kg object has a weight of about 30 N at the surface of Earth.
b) A 120-lb person weighs about 55 kg.
c) An astronaut orbiting Earth experiences weightlessness
d) If you eat too much you may gain weight.
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a) A 3-kg object has a weight of about 30 N at the surface of Earth.
The statement is incorrect because it confuses mass and weight. The weight of an object is the force exerted on it due to gravity, whereas mass is the measure of the amount of matter in an object. The relationship between weight and mass is given by the formula: weight = mass × acceleration due to gravity. The correct statement would be: A 3-kg object has a mass of 3 kg, and its weight on the surface of Earth is approximately 29.4 N (calculated as 3 kg × 9.8 m/s^2).
b) A 120-lb person weighs about 55 kg.
The statement is incorrect because it assumes a direct conversion between pounds and kilograms for weight. However, pounds and kilograms are units of mass, not weight. To convert between pounds and kilograms, we need to consider the acceleration due to gravity. The correct statement would be: A 120-lb person has a mass of about 54.4 kg on the surface of Earth.
c) An astronaut orbiting Earth experiences weightlessness.
The statement is correct. When an astronaut is in orbit around Earth, they are essentially falling freely due to the gravitational pull of Earth. This state of freefall gives the perception of weightlessness because there is no solid surface providing a normal force opposing the force of gravity. However, it is important to note that the astronaut still has mass and remains affected by gravity, but they experience a sensation of weightlessness due to the absence of contact forces.
d) If you eat too much, you may gain weight.
The statement is correct in the context of everyday usage. In ordinary speech, weight is often used to refer to changes in body mass or body composition. If a person consumes more calories than they burn, they are likely to gain mass and may be considered to have gained weight colloquially. However, it is important to distinguish between weight gain in the context of mass increase and weight gain in the context of the gravitational force exerted on an object.