1. GIG
Read the passage and mark your answers on your whiteboard. NOT ON THE PAPER.
Question 34.
Don’t forget to turn in 2-D Motion homework.

2. Applying Newton’s Laws

5. Action-Reaction Pairs
How do you jump? What are your muscles actually doing?

6. Action-Reaction Pairs
Newton’s first and second laws involve the force(s) on a single object. Newton’s third law involves two objects.
An action-reaction pair is two objects exerting forces on each other, like the canoes in the last example or your feet and the floor pushing against each other.

7. Action-Reaction Pairs
A common error in physics is not recognizing whether the problem involves an action-reaction pair or forces on a single object.
For example, a bowling ball rests on a table. If the problem asks for the net force on the bowling ball, then the force of the bowling ball pushing on the table is irrelevant. It is a force on the table, not the bowling ball.

8. Make a Heading in Your Notebook:
Newton’s Laws Practice Problems
Use the textbook an solve page 159, problems 6-8.
Finished? See if you know how to solve the LessonCheck problems on page 160.

9. Free-Body Diagrams
A free-body diagram is a sketch showing all the forces acting on an object.
These diagrams are helpful when the forces are more complex than a single +- vector.

11. When Making a Free-Body Diagram
Make a dot representing the center of mass in the object.
Draw all vectors out of this dot.

12. Two Astronauts Push a Satellite
Both exert a 46N force. They push at a 90-degree angle to each other. The satellite’s mass is 940 kg. What is the resultant vector of acceleration (magnitude and angle)?

13. Two Astronauts Push a Satellite
Both exert a 46N force. They push at a 90-degree angle to each other. The satellite’s mass is 940 kg. What is the resultant vector of acceleration (magnitude and angle)?
Start with the x and y components of the vector
Fx = max Fy = may
46N = 940kg(ax) N = 940kg(ay)
Ax = 0.049m/s Ay = m/s2

14. Two Astronauts Push a Satellite
Both exert a 46N force. They push at a 90-degree angle to each other. The satellite’s mass is 940 kg. What is the resultant vector of acceleration (magnitude and angle)?
Pythagorean theorem will give you the magnitude of the resultant vector.
(0.049m/s2)2 + (0.049m/s2)2 = a2
A = 0.069m/s2

15. Two Astronauts Push a Satellite
Both exert a 46N force. They push at a 90-degree angle to each other. The satellite’s mass is 940 kg. What is the resultant vector of acceleration (magnitude and angle)?
Trigonometry will give you the angle of the vector.
Tan-1θ = Tan-1(0.49/0.49) = 45°

16. Try This on Your Whiteboard

17. Try This on Your Whiteboard

18. Normal Force
Whenever two objects are in contact, a normal force exists between them.
A normal force is the force exerted perpendicular to the surface when two objects are in contact.

19. Normal Force on a Can Resting on a Table

20. Normal Force
Normal force does not have to equal weight.
Think about a ping-pong ball...

21. Weight Equation
We’ve mentioned weight a few times recently. What is weight in physics terms?
Weight is a force equal to the mass of an object times acceleration due to gravity. It is measured in newtons (N).
W = mg g = 9.81m/s2

22. Weight
Weight and mass are related. If we stay on planet Earth, it is easy to convert between weight and mass.
81.0kg(9.81m/s2) = 795N
795N/9.81m/s2 = 81.0kg

23. Weight
Mass, however, is a scalar. It has no direction and does not exert a force. Weight is how gravity causes a vector force proportional to mass.
On the Moon, an astronaut’s mass is no different from Earth, but weight changes because the Moon’s gravity only causes acceleration of 1.62 m/s2, so the astronaut weighs much less.

24. Weight
Earth weight
81.0kg(9.81m/s2) = 795N
Moon weight
81.0kg(1.62m/s2) = 131N

25. Firefighter Down a Pole
A firefighter with a mass of 97kg needs to get downstairs quickly. If he jumped, he would accelerate at 9.81m/s2, but that would injure him. Instead he slides down a pole, accelerating at 4.2m/s2. This slower rate was caused by the pole exerting a force in the opposite direction of gravity. What is the magnitude of that force?
You can follow along on page 164.

28. Practice
Use your whiteboard to solve questions at the bottom of page 164.

29. Homework
None. Have a good weekend.

30. Closure Answer on your whiteboard:
If a person jumps out of an airplane, they accelerate downward at 9.81m/s2 due to gravity. How does a parachute alter the net force on a person?
The planet Mercury has no atmosphere. Would a parachute work on Mercury?