Numerical Problems
A book has a mass of 2 kg. What is its weight on Earth (g=9.8 m/s^2)
W=mg=2×9.8=19.6N
What is the difference between mass and weight?
Mass: amount of matter; constant everywhere; measured in kilograms (kg).
Weight: gravitational force on that mass; depends on local g; measured in newtons (N).
A box has a mass of 20 kg. What is its weight on Earth (g=9.8 m/s^2)
mg=20×9.8=196N.
A rock has a mass of 10 kg. If we take it to Mars, why does its weight change but not its mass?
Mass is independent of location. Weight changes because Mars has weaker gravity.
A suitcase weighs 150 N on Earth. What is its mass?
W = 150/9.8 = 15.3 Kg
Why do engineers prefer to use mass instead of weight when describing materials?
Mass is constant and universal. Weight changes depending on gravity, so using mass avoids confusion and ensures consistency.
A 1,000 kg car is parked on Earth. If it were transported to the Moon, what would its weight be? What would its mass be? (g=1.6 m/s^2 on the moon)
W Moon =1000×1.6=1600N.
Mass stays 1000 kg (mass doesn’t change with location).
If someone says “I lost 5 kg after going to the Moon,” are they correct? Explain.
Incorrect. They lose weight, not mass. Their kg of mass stays the same, but their weight in newtons decreases.