What is Motion Mechanics The study of motion

What is Motion? Mechanics – The study of motion of objects Kinematics – The science of describing motion of objects

What is motion? Motion is a change in position relative to a reference point.

What is a Frame of Reference? In order to describe something that is moving you have to compare it to something that is not moving. For example, a bus is in motion compared to the sidewalk

SPEED—measure the rate of motion in relation to a reference point. Speed (S)= distance time

CONSTANT SPEED There’s no change in the rate of motion; the speed stays the same throughout motion Distance (m) The slope of any distance-time graph gives the speed of the object. The slope of an object moving at constant speed is a straight line. Time (s)

The data table to the left is an object moving at a constant speed of 6 m/s. The data table on the right is an object moving with an irregular speed. How can you tell the speed is constant by just looking at data table? Time (s) 0 1 2 3 4 Position (m) 0 6 12 18 24 Time (s) Position (m) 0 0 1 1 2 4 3 9 4 16

Constant Speed Calculate the constant speed of the object using the graph below: S=d/t S= 200 km/100 min= 2 km/min S= 160 km/80 min= 2 km/min S= 120 km/60 min= 2 km/min S= 80 km/40 min 2 km/min

AVERAGE SPEED Because most objects do not move at a constant speed one way to describe the motion of an object moving at changing speeds is to use average speed. Average speed describes the ratio of the total distance an object moved to the total time it took to move that distance. Avg. Speed= total distance total time

Whenever solving problems using formulas, you must: SHOW ALL WORK – 1. Write the Formula (for every problem) – 2. Substitute Measurements (for distance, time or speed) – 3. Solve, label units, circle final answer

Must show work!!!!

While on vacation, Lisa Carr traveled a total distance of 440 miles. Her trip took 8 hours. What was her average speed? To compute her average speed, we simply divide the distance of travel by the time of travel. s = d/t s = 440 mi/8 hr s = 55 mi/hr

Average Speed of cyclist If the total distance and/or the total time is not provided, you need to first calculate them and then solve for average speed. For example: A cyclist rode 20 miles in an hour, he rested for 0. 3 hours and rode another 30 miles in the next 2 hours. 1) Calculate total distance and total speed: 2) Calculate average speed D= 20 + 30 mi= 50 mi S=td/tt t= 1 + 0. 3 + 2 hr= 3. 3 hr S= 50 mi = 15. 15 mi/hr 3. 3 hr

VELOCITY speed of an object in a given direction. For example, the velocity of a baseball may be 85 mi/hr toward home plate Or, when travelling from Florida to Maine, the Or, average speed is 60 mi/hr north. Velocity (v) = distance time include direction object moved

Velocity problem: A car is driving on 95 south to go to New York that is 250 miles way. The car makes it to New York in 5 hours. What is its velocity? Velocity = distance = 250 mi/hr = time 5 hr 50 mi/hr south

COMBINING VELOCITIES When 2 velocities are going in same direction, Add them together. When 2 velocities are going in opposite directions, Subtract.

Combining Velocities problem For example: You are riding downstream (with the current) in a canoe along the Blackstone River. You are paddling at 5 mi/hr and the river is flowing at 13 mi/hr. What is your velocity? 5 mi/hr + 13 mi/hr = 18 mi/hr downstream You are riding upstream (against the current) in a canoe along the Blackstone River. You are paddling at 5 mi/hr and the river is flowing at 13 mi/hr. What is your velocity? 13 mi/hr + 5 mi/hr = 8 mi/hr upstream

STOP!

Acceleration is… the rate of change in velocity. If an object’s speed, direction or both change it is accelerating. Acceleration = Final velocity – Original velocity (linear) Time it takes to change velocity a= vf – vo t

ACCELERATION, continued Positive acceleration—When an object increases its speed, it experiences positive acceleration. Example, peddling on a bike after stopping Negative acceleration— When an object in motion decreases its speed, it experiences negative acceleration (deceleration). Example, slowing down to avoid a bump.

Acceleration is graphed… On a velocity-time graph (or a speed-time graph)

Constant Velocity Constant velocity shown on a velocitytime graph shows that the velocity does not change as time passes

ACCELERATION, cont. Constant Acceleration —an object’s velocity increases at the same rate as time passes

MOMENTUM—determines how difficult it is to stop the object’s motion. Momentum is always conserved; it can be transferred from object to object Momentum = mass x velocity The unit used to describe momentum is kg m/sec