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/s2? (c) What would John’s weight be on Mars where

g = 3.8 m/s2?

1. Fg = 735 N Fg = mg

g = 9.8 m/s2 735 = (m) (9.8 m/s2) m = ? m = 75 kg

2. m = 75 kg

3. m = 75 kg Fg = mg

g = 3.8 m/s2 Fg = (75) (3.8 m/s2)

Fg = ? Fg = 285 N

Equilibrium

Definition: A condition of motion in which the net force on an object is zero.

According to Newton’s Second Law:

The “Normal” Force

Contact force between surfaces (support)

ScalesMeasure the Normal force …. Not WEIGHT!

Normal force – “Apparent” weight“The scale reads 540 N” FN = 540 N

Ex: – Scales on an incline – Scales on an accelerating surface

(stepping on a scale in an elevator)

Example 1Joey 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 “FNet Equation” in the horizontal vertical

directions.(c) Determine the acceleration of the elevator.

Example 2Joey 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 “FNet Equation” in the horizontal vertical

directions.C) Determine the scale reading.

“Weightlessness”Weightlessness: Absence of a normal force

(free-fall) FN = 0 N, but Fg and “g” still exist!

Vomit Comet

Space Station

The Universe - Gravity