Standard 8.2

1. Standard 8.2 Forces Unbalanced forces cause changes In velocity.

2. Anticipatory Set How do you win at tug-o-war? What does it take?

3. Standard 8.2 (a-c) • Students know a force has both direction and magnitude. • Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces. • Students know when the forces on an object are balanced, the motion of the object does not change.

4. LANGUAGE OF THE DISCIPLINE • Force: is a push or pull. Every force has a magnitude and a direction. • Newton: (N) is the unit of measurement for the magnitude of a force • Balanced Forces: will not change the motion of the object • Unbalanced Forces: causes a change in the motion of an object. • Net Force: the sum of all of the forces acting on the object

5. Forces • What causes MOTION • Forces have a magnitude and direction. (Just like velocity) • Magnitude for Force= NEWTONS (N) • Use ARROWS to indicate direction of forces and length of arrow to indicate magnitude • Example:

6. Net Forces • NET means total • You combine all forces on an object to get NET • It determines if an object moves, which direction it moves, and how far.

7. Unbalanced Forces • If you have a net force (not = to 0) then it is unbalanced forces. • It will cause a change in velocity (direction or speed)

8. Balanced Forces • There will be NO change in velocity of object (won’t move)

9. Check for Understanding • If two girls push a box the same direction, both with 5 N of force. What is the net force? 10 N • If two boys push a wagon with 2 N each and in opposite directions what is the net force? 0 N • In a tug of war game if team A pulls the rope with 7 N and team B moves the rope with 3 N. What is the net force? 4 N (towards team A)

10. Practice & HW • Guided Practice: • Read through Guided instruction on page 41-42 and highlight important information • Complete the Guided questions in Measuring up on page 41-42 • Raise hand for a stamp • Independent Practice: • Complete questions 1-4 on page 42 • HW: • complete questions 1-4 on page 43.

11. Anticipatory Set How would these surfaces affect an obejct’s motion?

12. Standard 8.2 (d) • Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction.

13. LANGUAGE OF THE DISCIPLINE • Gravity: force that pulls objects together and pulls toward earth. • Compression: is a force that develops when the molecules of an object are squeezed together. • Tension: force that develops when molecules are stretched apart. • Friction: force between objects that works against their movement past each other

14. Gravity • Mass: amount of matter in an object • Weight: amount of gravitational force on an object • So mass never changes, but weight depends on where you are in space This is why when you step on a scale it tells your weight, because it is how much gravity is pulling down on you on to the scale

15. Gravity and Mass • Gravity= Mass x 10 m/s2 • Gravity amount doesn’t change if you put another force on it. • the more mass the move gravity pulls • Gravity pulls down and the surface an object is on pushes up

16. Elastic Forces • Matter is elastic if it returns to its original shape after being squeezed or stretches. • Compression- SQUEEZING • Tension- PULLING • These are the forces that work against gravity to keep forces BALANCED • Gravity, Compression, and Tension are forces that act “up and down”

17. Friction • Friction depends on the types of surfaces and how hard they push together. • Example: lightly put your hands together and rub them. Now push your hands together firmly and try to rub them. Notice the Difference? • Friction is a SIDE to SIDE force • So if you place a sideways force on an object it is and equal force of friction that will keep it stationary

18. Friction Problem • Ex: a boy is pulling a box along the floor with 40 N. The force of friction is 10 N between the floor and the box. What is the net force? • 40 N (from boy) MINUS (-) 10 N (of friction) • Net force: 30 N • ALWAYS SUBTRACT THE FRICTION FORCE

19. Check for Understanding • What forces are up and down forces? • Compression, tension, gravity • What is a force that deals with surfaces? • friction • What is the net force of a 6 N weight hanging from a spring and then is pulled down 4 N? • 6+4= 10

20. Practice & HW • Guided Practice: • Read through Guided instruction on page 45-47 and highlight important information • Complete the Guided questions in Measuring up on page 45-47 • Raise hand for a stamp • Independent Practice: • Complete questions 1-6 on page 47 • HW: • complete questions 1-4 on page 48.

21. Standard 8.2 (e-g) • Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction). • Students know the greater the mass of an object, the more force is needed to achieve the same rate of change in motion. • Students know the role of gravity in forming and maintaining the shapes of planets, stars, and the solar system.

22. LANGUAGE OF THE DISCIPLINE • Newton’s 2nd law of motion: motion of an object will change when unbalanced forces work upon it • Inversely proportion: as one quantity increases, the other quantity will decrease. • Universal force: force that works on all objects • Unbalanced forces: (see slide)

23. Newton’s Second Law • Acceleration depends on the net force acting on the object and the object’s mass Acceleration= Net force Mass • So to speed up acceleration you need to either increase force or decrease mass • To slow down acceleration you need to decrease force or increase mass • SO MASS IS INVERSERLY PROPORTIONAL To ACCELERATION

24. Newton’s 2nd Law

25. Calculating Force Acceleration= Net force Mass So Force= Mass x Acceleration (1 N is = to 1kg x m/s2)

26. Calculating Acceleration • ex: a wagon has a mass of 10 kg. Sarah pushes it with 3 N and Brandon pushes it with 8 N in the same direction. There is a force of 9 N of friction acting on it. What is the acceleration? • ADD UP FORCES SUBTRACT FRICTION (3+8-9) • Divide it by mass (10) • Acceleration = .2 m/s2

27. Calculating Force • Example: What force is needed to accelerate a box has a mass of 40 kg at 5 m/s2? • F= M x A • 40 x 5 = 200 N

28. UNBALANCED FORCES • Unbalanced forces will cause an object to either • Accelerate • Slow down • Or stop • Other forces that will change acceleration • Gravity- will pull objects down (especially if in the air) • Friction- will slow objects down if touching surface

29. Reading Informational Text • Read pages 55-56- ANSWER QUESTIONS GUIDED QUESTIONS AS YOU READ • Why is gravity a universal force? • It exists between all objects in the universe • Why don’t you float away when you jump or skip? • Force of gravity pulls you to earth • What influences gravitational attraction between objects • Mass and distance • What is weight • Measure of gravity on an object

30. Gravity between OBJECTS • The force of gravity between two objects • increases with greater mass • decreases with greater distance. • Gravitational force= Mass of object A X Mass of object b Distance Between objects • Example: Which pair of objects has the greatest gravitational force on each other? • The 2nd and 3rd 10 kg 6 kg 7 kg 4 m 2m

31. Calculating weight • On the moon acceleration due to gravity is 1.6 m/s2. On the earth it is 9.8 m/s2. If something weighs 784 kg on earth, how much will it weigh on the moon? • USE CROSS MULTIPLICATION! 1.6 = 9.8 X 784 1.6 (784)= 9.8 x Solve for X X= 128

32. Check for Understanding • What is the equation for force • F= M x A • What is the equation for acceleration • A= F/M • Three tennis balls are sitting on a table, ball A and B are 3 feet apart and ball B and C are 10 feet part. Which has a greater gravitational force between them? • Ball A and B

33. Practice & HW • Guided Practice: • Read through Guided instruction on page 50-52 and highlight important information • Complete the Guided questions in Measuring up on page 50-52 • Raise hand for a stamp • Independent Practice: • Complete questions 1-6 on page 52 • HW: • complete questions on page 53,56,57.