Forces & Motion

1. Forces & Motion Chapter 12

2. TUG-O-WAR TIME!!!

3. What is a force • A push or pull that acts on an object • Forces can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction

4. Measuring Force • Spring Scales • The stretch of a spring scale depends on the weight (type of force) acting on it

5. Units of Force • Measured in newtons (N) • 1 N = the force that causes a 1 kilogram mass to accelerate at a rate of 1 meter per second squared • 1 N = 1 kg•m/s2 • This unit is named after Sir Isaac Newton (1642-1727) • Scientist who explained how force, mass and acceleration are related

6. Force Diagrams • Use arrows to represent the direction and strength of a force (like a vector!)

7. Spring Scale Activity Choose five objects on your table. Attach a string to your objects if necessary. Use the spring scale to determine the weight (in newtons) of your objects. Draw a force arrow for each object that is to scale relative to each other force arrow.

8. Combining Forces • Back to tug-o-war… • You can combine force arrows to show the result of how forces combine • Forces in the same direction add together • Forces in the opposite direction subtract from one another

9. Net Force • The overall force acting on an object after all the forces are combined

10. Balanced Forces • Sometimes the net force acting on an object is zero. • Balanced Forces • Forces that combine to produce a net force of zero • Results in NO CHANGE in an object’s motion

11. Common Example • Two people locked in an arm wrestling match • Tug-o-War match with two evenly matched teams! • Two football players pushing against one another at the line of scrimmage

12. Unbalanced Forces • Results when the net force acting on an object is NOT equal to zero • When an unbalanced force acts on an object, the object accelerates

13. Combining Forces = Adding Forces = Subtracting Forces = 0 Equal and opposite forces

14. Friction • A force that opposes motion of objects that touch as they move past each other. • Acts at the surface where objects are in contact (includes all solids, liquids, and gases) • Friction is important! • Without friction every surface would be impossibly slippery • Food would slide right off your fork • Walking would be impossible • Cars would slide around with their wheels spinning

15. Four main types of friction • Static Friction • Sliding Friction • Rolling Friction • Fluid Friction

16. Static Friction • The friction force that acts on objects that are not moving • Always acts in a direction opposite to that of the applied force

18. Sliding Friction • A force that opposes the direction of motion of an object as it slides over a surface • LESS than static friction • This means that once an object is moving, less force is needed to keep the object moving than to start it moving

20. Rolling Friction • The friction force that acts on rolling objects • When a round object rolls across a flat floor, both the object and the floor are bent slightly out of shape at the point of contact • 100 – 1000 times less than static or sliding friction • This is why movers use wheeled dollies to move heavy objects!

22. Fluid Friction • Liquids and mixtures of air are known as fluids • Fluid friction results when fluids (like liquids and air) oppose motion of an object • Example, when you stir cake batter you can feel fluid friction • Fluid friction increases as the speed of the object moving through the object increases

23. Air Resistance • Fluid friction acting on an object moving through the air • At higher speeds air resistance is a significant force • For example, swimmers, cyclists and even runners wear slick racing suits to reduce air resistance

24. Wing Suits... (Start @ 1:00 min

25. Types of Friction Foldable Activity!

26. Gravity • A force that acts between any two masses • An attractive force (it pulls objects together) • Unlike friction, gravity can act over large distances (think skydiving!)

27. Gravity (continued) • Earth’s gravity acts downward toward the center of the Earth

28. Falling Objects • Both gravity and air resistance affect the motion of a falling object • Gravity causes objects to accelerate downward • Air resistance acts in the direction opposite to the motion, reducing acceleration

30. Flying Squirrels… • As objects fall they accelerate (gain speed) • As speed increases, air resistance increases • If an object falls long enough, the upward force of air resistance eventually will equal the downward force of gravity • Forces are balanced, acceleration is zero and the object continues falling at a constant velocity

31. Terminal Velocity • Constant velocity of a falling object when the force of air resistance equals the force of gravity

32. Projectile Motion • The motion of a falling object (projectile) after it is given an initial forward velocity • The only forces acting on a projectile are gravity and air resistance

34. Projectile Motion (Continued) • The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path

35. 12.2 Newton’s First & Second Laws of Motion

36. Newton’s First Law of Motion • The state of motion of an object does not change as long as the net force acting on the object is zero • An object at rest remains at rest • An object in motion remains in motion with the same speed and direction (i.e. no acceleration) • An unbalanced force must be acting on an object if the object is accelerating

37. Brainstorm What are some everyday events that give evidence of Newton’s first law?

38. Inertia • Newton’s first law is called the “Law of Inertia” • Inertia is the tendency of an object to resist change in motion

39. Beach Ball Activity

40. Crash Test… • Seatbelts... To wear or not to wear?

41. Unbalanced Forces • Unbalanced forces cause an object’s velocity to change • If velocity is change, the object is accelerating (a change in speed or direction) • The acceleration of an object depends on both the force acting on it and the mass of the object • Mass • A measure of the inertia of an object; depends on the amount of matter an object contains

42. 2nd Law of Motion • The acceleration of an object is equal to the net force acting on it divided by the object’s mass Acceleration = Force Mass F a = m

43. Acceleration • Always in the same direction as the net force • REMEMBER • N = 1kg•m/s2

44. Math Practice (guided) • An automobile with a mass of 1000 kilograms accelerates when the traffic light turns green. If the net force on the car is 4000 newtons, what is the car’s acceleration?

45. More 2nd Law Examples #1 A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N? #2 An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2in the forward direction. What is the net force acting on the automobile?

46. Weight vs. Mass • Weight is the force of gravity acting on an object • Weight = Mass x Acceleration due to gravity • Weight is a measure of the force of gravity acting on an object

47. 12.3 Newton’s Third Law of Motion and Momentum

48. Newton’s Third Law • Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object • For every action there is an equal and opposite reaction • Action and reaction forces are equal in size and opposite in direction

49. Action-Reaction Forces DO NOT CANCEL • Action forces and reaction forces act on different objects so they do not cancel each other out • Net force only equals zero if opposite forces are acting on the same object

50. Example 1 • A swimmer uses her arms to push against the water (ACTION FORCE) • The swimmer is propelled forward because the water exerts a force on the swimmer (REACTION FORCE)