Science Starter! John weighs 735 N on Earth. (a) What is John’s mass in kilograms? (b) What would John’s mass be on Mars where g = 3.8 m/s 2 ? (c) What would John’s weight be on Mars where g = 3.8 m/s 2 ?

1.F g = 735 N F g = mg g = 9.8 m/s = (m) (9.8 m/s 2 ) m = ? m = 75 kg 2.m = 75 kg 3. m = 75 kg F g = mg g = 3.8 m/s 2 F g = (75) (3.8 m/s 2 ) F g = ? F g = 285 N

Equilibrium

The “Normal” Force Contact force between surfaces (support)

Scales Measure the Normal force …. Not WEIGHT! Normal force – “Apparent” weight “The scale reads 540 N”  F N = 540 N Ex: – Scales on an incline – Scales on an accelerating surface (stepping on a scale in an elevator)

Example 1 Joey has a mass of 60 kg. He stands on a scale in an elevator. As the elevator begins to move upward, the scale reads 658 N. (a)Draw a free-body diagram of Joey. (b)Write an “F Net Equation” in the horizontal vertical directions. (c)Determine the acceleration of the elevator.

Example 2 Joey has a mass of 60 kg. He stands on a scale in an elevator. What would the scale read if the cable suddenly snapped and the elevator was in free-fall? A)Draw a free-body diagram of Joey. B)Write an “F Net Equation” in the horizontal vertical directions. C)Determine the scale reading.

“Weightlessness” Weightlessness: Absence of a normal force (free-fall) F N = 0 N, but F g and “g” still exist! Vomit Comet Space Station The Universe - Gravity