Peoples Physics Concepts

4.7. Types of Forces http://www.ck12.org

a) If she wants to weigh less, should she weigh herself when accelerating upward or downward?

b) When the elevator is not accelerating, what does the scale read (i.e., what is the normal force that the scale exerts

on the woman)?

c) When the elevator is accelerating upward at 2.00 m/s^2 , what does the scale read?

Answera) If she wants to weigh less, she has to decrease her force (her weight is the force) on the scale. We will
use the equation

F=ma

to determine in which situation she exerts less force on the scale.

If the elevator is accelerating upward then the acceleration would be greater. She would be pushed toward the floor
of the elevator making her weight increase. Therefore, she should weigh herself when the elevator is going down.

b) When the elevator is not accelerating, the scale would read 70.0kg.

c) If the elevator was accelerating upward at a speed of 2.00m/s^2 , then the scale would read

F=ma=70kg×( 9 .8m/s^2 +2m/s^2 ) =826N

which is 82.6kg.

Example 2

Question: A spring with a spring constant ofk=400N/m has an uncompressed length of.23m and a fully
compressed length of.15m. What is the force required to fully compress the spring?

Solution: We will use the equation

F=kx

to solve this. We simply have to plug in the known value for the spring and the distance to solve for the force.

F=kx= (400N/m)(.23m−.15m) =32N