Sections 1.Describing and Measuring Motion 2.Acceleration 3.The Nature of Force 4.Force, Mass, and Acceleration 5.Friction and Gravity 6.Action and Reaction

What is Motion? An object is in motion when its distance from another object is changing. Motion depends on your reference point (an object used for comparison in determining if something is in motion).

Describing Distance and Speed The SI unit for distance is the meter. Speed is calculated as: Distance/time

Average Speed Since most objects in motion do not travel at a constant speed, we calculate an average speed which is calculated as: Total distance/Total time

Average Speed (cont.) If Lance Armstrong cycles a total of 889 Km in the Tour De France in 49 hours, What is his average speed?

Velocity When you know the speed and the direction of an object, you know its velocity.

Graphing Motion Slope refers to the steepness of a line on a graph. When graphing motion (distance vs. time) the steeper the slope, the faster the motion. Slope = rise/run

Graphing Motion

Time Boy #1 Distance Boy #2 Distance 1 min.2 Km 2 min.4 Km 3 min.6 Km 4 min.8 Km 5 min.8 Km 1.0 Km 6 min.8 Km 1.2 Km 7 min.9 Km 1.4 Km 8 min 1.1 Km 1.6 Km 9 min 1.3 Km 1.8 Km 10 min 1.6 Km 2.0 Km

Acceleration Acceleration refers to the rate at which velocity changes … a change in velocity over time.

Acceleration (cont.) Speeding up Slowing down Changing direction

Acceleration (cont.)

If a roller coaster is traveling at 7 m/s at its highest point, and 3 seconds later its speed has increased to 28 m/s, what is its average acceleration?

Graphing Acceleration

The Nature of Force A force is a push or a pull. Forces are described by magnitude and direction.

Forces Forces can add together (when acting in the same direction), or cancel each other out (when acting in opposite directions). ++= 0=

Forces The overall force acting on an object after all of the forces have been added together is called the net force. Unbalanced forces always result in motion.

Forces Equal forces acting on an object are called balanced forcesBalanced forced do not result in motion … Equal forces acting on an object are called balanced forces. Balanced forced do not result in motion …

Newton ’ s 1 st Law of Motion Newton ’ s 1 st Law of Motion Inertia is the tendency of an object to resist change in its motion. “ An object at rest will stay at rest, and an object in motion will stay in motion, unless acted upon by an unbalanced force. ”

Inertia: An everyday experience

Inertia and Mass Mass refers to the amount of matter in an object. The more mass an object has, the more inertia it has …

Newton ’ s Second Law “ The net force on an object is equal to the product of its acceleration and its mass. ” Force (N) = Mass (Kg) X Acceleration (m/s 2 ) OR Acceleration Acceleration (m/s 2 ) = Force (N) /mass (Kg) Acceleration

Newton ’ s Second Law If the wagon has a mass of 62 Kg, and the girl is pulling it with a force of 170 N, what is the rate at which the wagon is accelerating?

Newton ’ s Second Law What force is required for this 275 Kg motorcycle to accelerate at a rate of 18 (m/s 2 )?

Friction The force that one surface exerts on another is called Friction. Even objects that appear smooth have many irregularities … which get caught on the irregularities of the opposing surface, and resist motion..

Friction Friction always acts in the opposite direction of the force/motion.

Friction: An Everyday Experience

Friction: Three types Sliding friction Rolling Friction Fluid Friction

Gravity Gravity is the force that pulls objects toward each other. The more massive the object, the more gravitational pull it has.

Freefall When the only force acting on an object is gravity (ignoring air resistance) it is said to be in “ freefall. ”

Acceleration Due to Gravity Objects in freefall will accelerate at 9.8m/s 2. All objects accelerate at the same rate regardless of mass (again, ignoring air resistance).

Acceleration Due to Gravity If the person is in freefall for 3.8 seconds, what is his approximate speed just before the bungee cord slows his fall?

Projectile motion An object that is thrown horizontally is called a “ projectile ”.

Projectile Motion When an object is simply dropped from the same height as another object that is thrown, they will hit the ground at the same time!

Air Resistance Objects falling through the air experience a type of fluid friction called “ air resistance. ” Air Resistance

Terminal Velocity When air resistance is so great that an object no longer accelerates in freefall, the object has reached “ terminal velocity. ”

Weight or Mass? ….. –Mass refers to the amount of matter an object contains, while weight refers to the gravitational pull on an object. –Gravitational pull on an object can be measured in newtons. Since F=M x A, one calculates weight (N) by multiplying mass (Kg) times the acceleration due to gravity (9.8 m/s 2 ).

What is the approximate gravitational force acting on this motorcycle if it has a mass of 225 Kg? Think …..

How much would I weigh on the moon? Because the moon has one sixth the gravity of Earth, you would weigh six times less than what you weigh on Earth. This is why the astronauts were able to move easily in their heavy space suits. In the photo to the right, John Young, commander of the Apollo 16 lunar landing mission, jumps up from the lunar surface as he salutes.

Universal Gravitation The law of universal gravitation states that the force of gravity acts between all objects in the universe.

Newton ’ s 3 rd Law of Motion “ If one object exerts force on another object, then the second object exerts a force of equal magnitude in the opposite direction on the first object. ”

Newton ’ s 3 rd Law affects everything!

Momentum In assessing what Newton referred to as “ quantity of motion ” we now call “ momentum ”. Momentum equals mass X velocity

Momentum What has more momentum, a 12,000 Kg jet traveling at 700 m/s, or a 295,000 Kg cruise ship traveling at 6 m/s?

Conservation of Momentum The Law of Conservation of Momentum states the total momentum of any group of interacting objects remains the same. (in the absence of outside forces)

Conservation of Momentum