Section 1: The Nature of Force What is a force?

1. Section 1: The Nature of ForceWhat is a force? • Force • A push or pull on an object • Has both Size & Direction • Size: • Measured in SI units called newtons (N) • Spring Scale

2. How do you combine forces? • Direction: • Same direction: Add (+) • Diff. direction: Subtract (-) • Net force = combination of all forces acting on object • Unbalanced Forces • Result in motion

3. What does balanced forces mean? • Balanced Forces • Net force = 0 • No motion

4. Lab: The Nail Challenge! • Objective: • Balance nails on single nail head • Work in pairs

5. Section 2: Friction and GravityWhat is friction? • Friction • Resistance to motion • Opposite direction of travel • Caused when 2 surfaces rub together • resistive force (slows down objects)

6. What does Friction depend on? • Friction depends on… • Types of surfaces • How hard surfaces push together

7. What are sliding and rolling friction? • Types of Friction • SlidingFriction: solid surfaces slide over each other • RollingFriction: object rolls over surface

8. What are fluid and static friction? • Types of Friction • FluidFriction: object moves through fluid (or air) • StaticFriction: objects not moving

9. What are some uses for friction? • Is Friction harmful or helpful? • Ways to reduce friction • Ways to increase friction

10. What is a gravitational force? • Gravitational Force • Force of attraction between 2 objects • Pulls things toward each other • Depends on: • Mass • Distance

11. What is the difference between mass and weight? • Mass • Amount of matter • Same no matter where you are • SI units = kilograms (kg) • 1 kg = 1000 grams (g) • Weight • Force of gravity • SI units = newtons (N) • Depends on where you are

12. What is Free fall? • Free Fall • Only force acting on an object is gravity • Objects in free fall accelerate as they fall • All objects free fall at the same rate (9.8 m/s2)

13. Gravity & Freefall

14. What is Air resistance? • Air resistance • Type of fluid friction • Opposes motion of objects through air • Depends on: • Size, Shape, Speed

15. What is Terminal Velocity? • Terminal Velocity • As an object falls it picks up speed • Increased speed  increased air resistance • Eventually force of air resistance = force of gravity  TERMINAL VELOCITY • Object stops accelerating!

16. Terminal Velocity

17. Section 3: Newton’s First and Second LawsWhat is Inertia? • Inertia • Tendency of object to resist a change in it’s motion

18. What does inertia depend on? • Inertia depends on Mass • “Amount” of inertia depends on objects’ mass

19. What is Newton’s 1st Law? • Newton’s 1st Law of Motion • Object at rest will remain at rest • Object in motion will remain in motion unless acted on by an unbalanced force.

20. Practice Problem 1 Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose an astronaut in that place throws a rock. The rock will: a) gradually stop. b) continue in motion in the same direction at constant speed. 

21. Practice Problem 2 An 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving with the same speed and in the same direction? • 0 N (no force)

22. Practice Problem 3 Ben Tooclose is being chased through the woods by a bull moose which he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion.

23. Newton’s 1st Law Review • Unbalanced force from another car changes your CAR’s motion • You continue as before until your seatbelt changes YOUR motion

24. What is Newton’s 2nd law of motion? • Newton’s 2nd law of Motion • Force, Mass & Acceleration are related • Force = Mass X AccelerationOR Acceleration = Force ÷Mass • FYI, 1 N = 1kg X 1 m/s2

25. Force, Mass & Acceleration

26. Force, Mass & Acceleration • A 52 kg water skier is being pulled by a speedboat. The force causes her to accelerate @ 2 m/s2. Calculate the FORCE that causes this acceleration. • F = 52 kg x 2 m/s2 • = 104 kg x m/s2 = 104 kg*m/s2 • = 104 N

27. Force, Mass & Acceleration • What is the force on a 1000 kg elevator accelerating at 2 m/s2? • 1000 kg X 2 m/s2 = 2000 N • How much force is needed to accelerate a 55 kg cart at 15 m/s2? • 55 kg X 15m/s2 = 825 N

28. Section 4: Newton’s Third LawWhat is Newton’s 3rd Law? • Newton’s 3rd law of Motion • For every action, there is an equal and oppositereaction • “Action” & “Reaction” are names of forces

29. How do forces always occur? • Forces ALWAYS occur in pairs. • Single forces NEVER happen • 2 objects are involved in every force • Action force: “A pushes B” • Reaction force: “B pushes A”

30. What do equal and opposite mean? • In Newton’s Third Law, “equal” means: • Equal in size • Equal in time • “Opposite” Means: • Opposite in direction

31. Don’t Action & Reaction forces cancel each other? • Action & Reaction forces act on DIFFERENT objects • In “Net force” problems, we are talking about opposing forces acting on the SAME object

32. Try These!! • If forces are equal and opposite why don't they cancel each other out? • They occur on two different objects.  Forces can only cancel out when the forces are acting on the same object.

33. If the forces are equal and opposite how do two different objects obtain different accelerations in the same interaction? (Remember F=ma) • Different accelerations are obtained when the objects have different masses.