Section 1 Describing Motion

Motion Motion is a change in position Position depends on a reference frame Frames of reference can be stationary or moving.

Frame of Reference If you are sitting in a moving car, you are not moving if the car is the reference frame But you are moving relative to someone standing on the curb

Measuring Motion Motion - A change of position Speed -The distance an object travels per unit of time Equation - speed = distance/ time (s = d / t) SI units – meters per second (m/s) Distance is usually measured in meters or kilometers Time is usually measured in seconds or hours

Speed

Instantaneous Speed Instantaneous speed of an object is the speed at a single instant in time Example: At the moment this picture was taken the car was traveling 250 mph, that was it’s instantaneous speed at that exact moment

Constant and Average Speed Constant speed Speed that does not change Example: The Earth rotates at a constant speed on it’s axis. (Once every 23 hours 56 minutes and 3seconds) Average speed Speed that does change Average speed = total distance (meters) total time (seconds)

Velocity Velocity is speed with direction Velocity is a vector that has size (speed) and direction Objects have different velocities if they are moving at different speeds or if they are traveling in different directions

Objects have different velocities if they are moving in different directions

Velocity Velocity of an object can change even if the speed remains constant If the roller coasters speed stays constant its velocity will change because it is changing direction

Velocity Circular motion In circular motion, the velocity is constantly changing. This is because the direction is constantly changing

Graphing Motion The motion of an object over a period of time can be shown on a distance-time graph Time is plotted on the horizontal axis Distance traveled is plotted on the vertical axis

Acceleration The rate at which velocity changes over a period of time Equation A = (Vf – Vo)/ T A= acceleration Vf = final velocity Vo = original velocity T = time Units = m/s2 m = meters s = seconds

Calculating Acceleration I get into my car, start the engine and drive. After 5 seconds I am moving at 10 meters per second. What is my acceleration at this point in time? Original velocity (Vo) = 0 m/s Final velocity (Vf) = 10 m/s Time = 5 seconds 10 m/s – 0 m/s = 10m/s 5s 5s = 2 m/s2

Acceleration If acceleration is negative you are slowing down If acceleration is positive you are speeding up

Result of rapid negative acceleration

Acceleration Acceleration can happen in two ways Changes in velocity Changes in direction Acceleration can be an increase or a decrease in velocity .

Speed-Time Graphs The vertical axis is the objects speed The horizontal axis is the time The line is pointing up, this indicates the rate of speed is increasing (positive) The line is also curved, this indicates the rate of acceleration was uneven

Speed-Time Graphs This graph indicates that acceleration increased at a constant rate The speed then remained constant for a while Then began to decrease The speed became constant again Then continued to decrease at a faster rate until it stopped

Forces A force is a push or pull that one object exerts on another object A force is also a vector that has size (strength) and direction

Forces The size of the force is the strength of it The direction is which way the push or pull is applied The SI unit used for force is the newton (N) Force = Mass x Acceleration N = 1kg x 1m/s2

Balanced Forces When two or more forces act on an object they are combined to form the net force When two or more forces act on an object and the net force is equal they are called balanced forces

Unbalanced Forces When forces combine to produce a net force that is not zero the are unbalanced forces The net force that caused the mule to be lifted up is the difference of the two forces (mule and cargo)

Unbalanced Forces Change Velocity When forces acting on an object are balanced the velocity of the object does not change When forces on an object are unbalanced the velocity of the object will change

Friction Friction is the force that opposes the sliding motion of two surfaces that are in contact

What Causes Friction? The size of the frictional force exerted depends on the material the surfaces are made from and the roughness of the surface All surfaces have surfaces have bumps and dips, even highly polished ones

Static Friction Static friction is the frictional force That prevents two surfaces in contact with each other from sliding past each other Static friction is what is preventing this man from moving the truck

Sliding Friction Sliding friction is the force that acts in the opposite direction to the motion of a surface sliding on another surface. Sliding friction will a eventually cause the bear to come to a stop

Rolling Friction Friction produced by objects such as wheels and ball bearings. Tends to be less than sliding friction.

Air Resistance Air resistance opposes the motion of objects that move through the air Air resistance acts in the opposite direction to the object moving through it The amount of air resistance depends on the speed, size and shape of the object

Terminal Velocity After jumping from the aircraft the skydiver increases speed up to a steady maximum called terminal velocity. The downward force of gravity (the Weight) is balanced by the upward force of air resistance (the Drag)

Momentum

Momentum – Mass of an object multiplied by its velocity Equation – P = mass x velocity P = M x V Units – Kg m/s

Law of Conservation of Momentum States that the total conservation of momentum of any group of objects remains the same unless outside forces act on the object.