Science 8 Quarter 1 Lesson 1 Balanced and Unbalanced Forces

Science 8 Quarter 1 Lesson 1 Balanced and Unbalanced Forces and Types of Forces

1. May the Force Be With You!

2. Laws of Motion

3. Learning Objectives Identify the forces acting on an object at rest. Examine the conditions when two forces are balanced and, Explain the effect of balanced forces on the state of motion of an object.

4. What is a force? A force is a push or pull that causes any change in the motion of an object ( move, stop, or change direction). In physics, a force is anything that makes an object accelerate. If an object is stationary, it won’t start moving without a force.

5. Force Forces come in pairs Force is a vector quantity Forces have a magnitude and a direction 5N, north (up) Magnitude: 5N Direction: north (up)

6. Force Measurement and Units SI unit for force is the Newton, N A Newton is a force that causes a 1kg mass to accelerate at a rate of 1m/s/s or 1 . 1 N ( Newton ) = 1 kg * m/s/s or  

7. Does force always make objects move? NO!

8. Combining Forces Net Force – The overall force acting on an object after all forces are combined. - Forces in the same direction we add them. - Forces in opposite directions you subtract them. 10N 10N = + Net Force = 10N 10N 7N - = Net Force = 3N

9. Forces in the Same Direction When forces are applied in the same direction, they are added to determine the size of the net force.

10. When two forces act in opposite directions, you subtract the smaller force from the larger force to determine the net force. The net force will be in the same direction as the larger force. Forces in Opposite Directions

11. Activity 1 Form 6 groups. Do the 2 scenarios, observe what happens Answer the questions on a ½ sheet of paper. Do this activity in 10 minutes.

12. Scene 1 Tie a string of yarn to a pen and let it hang for 5 seconds. Question 1: Is the pen at rest or in motion? Question 2: Are there forces acting on the pen? If yes, draw the forces. You may use arrows to represent these forces. After 5 seconds, cut the string using a pair of scissors. Question 3: What happens to the pen? What could have caused the pen’s motion?

13. Scene 2 Place a book on top of a table, and observe for 5 seconds. Question 1: Is the book at rest or in motion? Question 2: Are there forces acting on the book? If yes, draw the forces acting on the book. Let one member of your group push the book in one direction and another member push it in the opposite direction at the same time with the same amount of push (force). Question 3: Did the book move? How will you make the book move?

14. Balanced and Unbalanced Forces Forces occur in pairs and they can be either balanced or unbalanced

15. Balanced Forces: The forces in each direction are “equal”. If more than one force is present, it does not have to cause an acceleration on an object. If another force “balances” the first out, there will be no acceleration at all. Think: If both guys (who weight the same) pull on a rope in opposite directions, with an equal amount of force, how much will they move?

16. Balanced Forces Balanced forces do not cause a change in motion They are equal in size and opposite in direction The net force is 0

17. Example: Gravity pulls down on you… The ground pushes back up… THIS KEEPS YOU WHERE YOU ARE! If these football players push on each other equally as hard, will either one move? Gravity pulls down Ground pushes up

18. “ Unbalanced ” Forces If the multiple forces acting at one time are not balanced out (unequal), then acceleration can/will occur on the object! Kicking the ball causes it to move quickly in a different direction If one side of the scale has more mass, then gravity will accelerate it down!

19. Unbalanced Forces An unbalanced force always causes a change in motion The net force is greater than 0 When unbalanced forces act in opposite directions you can find the net force Net force Magnitude The difference between the two forces Direction Direction of the largest force

20. Unbalanced Forces 3 N, right – 6 N, left = 3N, left

21. Unbalanced Forces 4 N, left – 10 N, right = 6N, right

22. Unbalanced Forces 5 N, right + 10 N, right = 15N, right

23. Review Balanced Forces Balanced Push i.e. Pushing a Car No Acceleration or change in motion Net Force = 0 Balanced Pull i.e. Tug-o-war No Acceleration or change in motion Net Force = 0 10N 10N 10N 10N

24. Review Un-Balanced Forces Add Together Un-Balanced Same Direction Faster Acceleration Net Force > 0 Subtract from each other Un-Balanced Opposite Direction Slower Acceleration Net Force > 0 10N 10N 10N 10N 10N

25. A. Contact Force B. Non-contact Force forces that require physical contact also called action-at-a-distance force or field forces Types of Forces

26. Contact Forces

27. 1. Applied Force An applied force is a force which is applied to an object by another object or by a person. The direction of applied force depends on how the force is applied.

28. 2. Normal Force The normal force is the support force exerted upon an object which is in contact with another stable object. It is always directed towards the object and acts perpendicularly.

29. 3. Frictional Force The frictional force is the force exerted by the surface as an object moves across it or makes an effort to move across it. The friction force opposes the motion of the object.

30. 4. Air Resistance Air resistance is a force that acts upon objects as they travel through the air. The force of air resistance always opposes the motion of the object.

31. 5. Tension Force Tension force is the force which is transmitted through a string, rope or wire when it is pulled tight by forces acting at each end. The tensional force is directed along the wire and pulls equally on the objects on either end of the wire.

32. 6. Spring Force The spring force is the force exerted by a compressed or stretched spring upon any object which is attached to it. This force acts to restore the object which compressed or stretches a spring, to its rest or equilibrium position.

33. Non-Contact Forces

34. 1. Gravitational Force Force that attracts any two objects with mass. We call it attractive because it always tries to pull masses together, never pushes them apart. Every object is pulling on every other object in the entire universe! This is called Newton's Universal Law of Gravitation.

35. 2. Electric/ Electrical Force An electric force is the force that exist between charges. It can either be attractive or repulsive force.

36. 3. Magnetic Force Magnetic Force can be defined as the attractive or repulsive force that is exerted between the poles of a magnet and electrically charged moving particles.

37. Quiz Time!

38. 9. What is it called when two or more forces acting on an object are equal in all direction and results in no movement? 10. What is it called when two or more unequal forces act on an object and it moves in the direction with more force?