1. How Forces Interact

2. I) Forces Act Together

3. A) Only a force can change the motion of an object

4. 1) It must be stronger than the inertia of the object

5. B) Whenever the motion of an object changes, more than one force is acting

6. 1) Example: a runner’s leg and foot pushes on the ground and the ground reaction pushes the runner forward

7. C) All forces are measured in newtons

8. 1) At Earth’s surface, gravity pulls on a 0
1) At Earth’s surface, gravity pulls on a 0.1-kg with the force of 1 newton

9. 2) Fun Fact – a quarter-pound burger weighs about 1 newton

10. II) Friction

11. A) Friction is a force that opposes motion whenever two surface rub against each other

12. 1) Friction happens only when one force, such as gravity or muscle power acts to move an object against something else

13. B) Overcoming Friction to put an object into motion requires energy

14. 1) It takes less energy to keep a object moving then it does to get it started.

15. C) It is possible to reduce friction

16. 1) one way to reduce friction is to use rollers or wheels

17. 2) another way is by using lubricants, such as oil and grease

18. D) Friction can be reduced, but can not be eliminated.

19. E) Friction is needed

20. 1) Walking - friction between your foot and the ground offers the resistance necessary to push your body forward.

21. 2) Tire treads, shoe soles, and cleats are all designed to increase friction and prevent slipping

22. III) Balanced Forces

23. A) Balanced forces acting on an object are equal in size but opposite in direction

24. 1) the result is the two forces seem to cancel each other out

25. 2) When forces are balanced the object does not move, or it continues it motion at a constant velocity

26. B) Every object that is at rest acted on by balanced forces.

27. 1) the force of air pressure weighing down on your body is balanced by an equal force with you pressing out (also air pressure)

28. C) Objects that move at a constant velocity are also acted on by balanced Forces

29. 1) a parachuter comes down under balanced forces at a constant velocity, because the parachute cancels out someof the force of gravity

30. IV) Unbalanced Forces

31. A) Objects often have more than one force acting on them

32. B) Net force is the combination of all of the forces acting on an object

33. C) Unbalanced forces can cause an object to speed up, slow down, or change direction

34. 1) When two unequal forces push in opposite directions on an object, the object changes its speed in the direction of the stronger force

35. 2) example: game of tug or war

36. D) A car stopped on a hill, the force of friction from the tires is balanced with the force of gravity

37. 1) speeding up: break is released, which reduces friction on tires, and gravity is stronger

38. 2) slowing down: the force of friction increases

39. 3) changing direction: driver turns the wheel