Forces

1. Forces

2. Force: An action exerted on an object (a push or a pull) in order to change the state of rest or motion of an object. Measured in Newtons (N)

3. 2 Categories of Forces • Contact Forces: two interacting objects are physically in contact with each other. • At-a-distance Forces: two interacting objects are not in physical contact but are able to exert a push or pull

4. Force of Gravity • The Earth pulls you downward • Any object near Earth is under it’s pull

5. Force of Tension (FT) • Contact force acting through a string, chain, etc…

6. Friction • Friction • When surfaces slide over one another friction acts • Direction of friction always opposing motion • Without friction • Tires of a car would not be able to push against the ground and move car forward • You would not be able to grip objects • You would slip and fall when you tried to walk

7. Force of Friction • From sliding on a surface • Always goes in the opposite direction of motion (or the opposite direction an object would move)

8. Normal Force • Why doesn’t the book fall to the floor?

9. Normal Force • Surface Force that acts perpendicular to object • Consider the following: 1.0 kg mass on a table • We know from Newton’s 3rd law the table has to…. • Push upward with the same force • How does the block know? • What if it didn’t push hard enough? • What if it pushed too hard?

10. Normal Force (FN) • Reactive Force

11. Types of Forces • WRITE THESE DOWN!!! • Fg = Force of Gravity • FA = Force Applied • FT= Force of Tension • Ff= Force of Friction • FN = Normal Force (“Support”)

12. Free Body Diagrams Identifies all of the Forces acting on an object Each Diagram has a Vector Arrow: Size of arrow: Shows the magnitude of the force Direction of arrow: Shows direction of force

13. Example: Book on Table FN Draw a circle around your object Fg force on book by the Earth Support force on book by table Fg

14. Example: Continued 2) Block Moving 1) Push Block 3) Book stops

15. A baseball is in free fall traveling toward a person. Neglect air resistance.

16. Free Body Diagrams Diagrams used to show the: magnitude (how much) and direction of all forces acting upon an object.

17. A cat is at rest on a table top. The free-body diagram for this situation is shown below:

18. A rightward force is applied to a Piano in order to move it across a desk at constant velocity. Consider frictional forces.

19. Force: An action exerted on an object (a push or a pull) in order to change the state of rest or motion of an object. Measured in Newtons (N) Net Force: • Combination of all the forces acting on an object. (add them up)

20. Balanced Forces • If all of the pushes and pulls are even on an object, then the object “keeps on doing what it was doing” • Tug-o-war • Two teams that are evenly matched play tug-o-war • NOTHING HAPPENS

21. Balanced Forces • Mechanical Equilibrium: • when net force is zero • Does not cause a change in object’s motion

22. Unbalanced Forces • Unbalanced forces cause objects to change velocities • Tug-o-war • Two teams that are unevenly matched play tug-o-war • One team will win

23. Unbalanced Force • What happens when an object has an unbalanced force acted upon it? • Unbalance forces cause acceleration!!

24. If the forces are… • Equal – • Nothing happens • Ex: A book sitting on a shelf • Fg = 25 N • FN = 25 N • Unequal – • The object changes what it is doing • Ex: • FA = 25 N right • FA = 100 N left

25. A rightward force is applied to a plate in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance.

26. A rightward force is applied to a box in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance

27. A force is applied to the right to drag a sled across loosely-packed snow with a rightward acceleration.