Unit 3 Forces

1. Unit 3 Forces

2. Forces • A force is a push or a pull • A force affects how an object moves • Can make objects start or stop moving • Can make objects move faster or slower • Can make objects change direction

3. Push/Pull Activity • Move around the classroom and find at least 10 things that you push, pull, or push and pull (3 minutes) • Complete the Venn diagram by placing the items in the appropriate categories (2 minutes)

4. Forces • Remember acceleration? • change in speed or direction • Forces cause a change in the speed or direction of an object • F=ma • Forces are measured in Newtons (N) 1N=1kg·m/s2

5. Combining Forces • Forces usually act in pairs • Forces act in a particular direction • Forces usually cannot be seen, but their effects can

6. Forces • More than one force can act on an object at a time • Forces can push or pull in any direction • What happens when more than one force acts on an object depends on: • How strong the forces are • The direction of the forces

7. Combining Forces • When more than one force acts on an object, the forces combine to form a net force • The net force is the combination of all forces acting on an object • Forces may work together or they may be opposite forces

8. Balanced Forces The total or net force is the difference between the individual forces • Total/Net force= 5N - 5N=0N • When the forces are balanced, the object is at equilibrium and does not move

9. Unbalanced Forces • When 2 or more forces act in the opposite direction and are not equal in magnitude (number), the forces will be combined by subtracting to produce a net force • The total or net force is the difference between the individual forces • The object will move in the direction of the greater force

10. Unbalanced Forces • The total or net force is the difference between the individual forces • Total/Net force= 10N - 5N=5N right

11. Practice • You are pulling a wagon with your niece in it up a hill with a force of 18N. She is holding your dog’s leash, and the dog is pulling in the opposite direction with a force of 12N. What is the force on the wagon? • Total/net Force = 18N – 12N = 6N

12. Unbalanced Forces • When 2 or more forces act in the same direction, the forces are added to produce the total force • Unbalanced forces result in motion

13. Unbalanced Forces • The total force is the sum of the individual forces • Total/Net force= 25N + 30N = 55N • Will the box move?

14. Practice • Example: You are pushing your twin brothers up a hill in a stroller with a force of 18N. Your best friend sees you and comes to help. He or she pushes with a force of 12N. What is the force on the stroller? Total/Net Force = 18N + 12N = 30N

15. Friction • Dragon Racers Activity

16. Friction • Friction exists when two surfaces are touching and comes from the fact that no two surfaces are perfectly smooth • Friction acts in the opposite direction of motion and will cause a moving object to slow down and eventually stop

17. Friction • To make an object move, the force you exert on an object must be greater than the force opposing it • What did you observe about the friction of the surfaces that were rougher or softer in the Dragon Racer Activity?

18. Friction • Four types • Static friction – occurs between stationary objects • Books on table, box on floor • Sliding friction – occurs when solid objects slide over each other • Books or box being pushed

19. Friction • Four types • Rolling friction – occurs when a solid object rolls over another solid object • Tires on a road • Fluid friction – occurs when an object moves through a fluid (liquid or gas) • Fish swimming, airplane flying

20. Friction • Identify the type(s) of friction involved in walking or running.

21. Friction • Identify the type(s) of friction involved in stopping a car with brakes. • What would be an example of fluid friction? • Rolling friction?

22. Friction • How can we reduce friction? • Use oil in an engine between moving parts • Grease the bearings in a car wheel • Have you ever used candle wax on a sled? • Use a lubricant (oil, grease, etc.)

23. Friction Practice • Consider a book on a table • If I push the book with a force of 32N and the table applies 4N of friction, what is the net force on the book?

24. Friction Practice • Consider a book on a table 32N 4N The net force on the book is F= 32N - 4N = 28N

25. Force Calculations • Force = mass x acceleration • F=ma • The triangle: Force Mass Acceleration

26. Friction Practice • Your mass is 70kg. Your friend pushes you down a hill on a sled with a force of 66N. The snow has a friction of 2N. What is your acceleration?

27. Friction Practice 66N 2N

28. Friction Practice • Calculate net force: F = 66N – 2N = 64N Then find acceleration: a = F/m a = 64N/70kg = 0.91m/s2

29. Practice • A 50kg skater pushed by a friend accelerates at 5m/s2. How much force did the friend apply? • How much force is required to push a 16kg child in a 22kg stroller with an acceleration of 2.5m/s2?

30. Practice • If a 90kg water skier is being pulled behind a boat with a force of 360N, what is the skier’s acceleration? • If a 60kg person on a 15kg sled is pushed with a force of 300N, what will be the person’s acceleration?

31. Practice • A bowling ball rolled with a force of 15N accelerates at a rate of 3m/s2. A second ball rolled with the same force accelerates at 4m/s2. What are the masses of the two bowling balls?