1. Aim: How can we apply Newton’s Second Law?
Do Now: Quiz
Homework: Problem Set

2. Forces on Earth Weight is force due to gravity for an object.
On Earth g = 9.8 m/s2
How would the force an object exerts on earth’s surface compare with that of the moon?
How would the force an object exerts on earth’s surface compare with that of the moon?

3. Free Body Diagram Review
Objects at at rest all have a net force of zero because they are at equilibrium.
An object on a table has a force pushed up on it by the table because the table is elastic.
This is usually called the normal force.

4. 3 Situations Station Activity
Students will answer the following questions at each station regarding a mass.
Use the 4-Step Method!
Use a 1kg mass (1000g)

5. 4 Steps for Equilibrium Problem
Draw a Free Body Diagram
Write that the “Up Force = Down Force” & “Left Force = Right Force”
Write Newton’s Second Law in all directions
Solve for Unknown
Draw free body diagram from last demo
F1 is force of person on block, F2 is force of person on block

6. Station 1
A 500 g mass is sitting on the table. If you push down on the mass with a force of 5 N, what is the normal force exerted on the mass by the table?

7. Station 2
The 500 g mass sitting on the table has a string attached to it. Pull on the string with 4 N force upwards. What is the resulting normal force?

8. Station 3
The mass sitting on the table has a string attached to it. Pull on the string with 7 N force upwards. What is the resulting normal force? Is there a net force? What is it?
FN = 0, but there is a net force.

9. Elevator Physics
A 65 kg woman descends in an elevator that briefly accelerates at 0.2g downward when leaving a floor.
What is her apparent weight during acceleration?
What does the scale read when traveling at a constant speed of 2.0 m/s?
Lets hop in an elevator and find out!
a. Sum of F = ma
FN - mg = ma
FN = mg – m0.2g (negative acceleration for right term since accelerating downwards)
b. FN - mg = ma = 0 for constant speed
c. FN – mg = m(0.2g) (Positive acceleration for right term since upwards)

10. Tension is the force between strings.
If we assume the cord has no mass then the tension is equal on both ends. FT + (-FT) = 0

11. What is the acceleration of box B and Box A? (Assume Frictionless)
Draw a FB diagram for both boxes
Set up horizontal and vertical forces separately.
3. Apply Newton’s second Law
4. Solve Unknown!
Box 1: Fx = FP – FT = mAax
Box 2: Fx = FT = mBax
aA = aB

12. Summary
1. Justify the forces between two objects connected by a taut rope. (Just explain the forces involving the rope). 2. Why can a rope only pull? 3. Explain the forces you feel when in an elevator going down and then stopping.