Motion and Speed Chapter 2

Section 1 Describing Motion

A. Motion Motion – change in position 2. Distance - how far something has moved

Motion 3. Displacement - distance and direction from the starting point If a football player runs 100yds in one direction and turns around and runs 70 yds back, the displacement is equal to the difference. 100yds – 70 yds = ?

B. Speed Speed – Distance / time Rate - any change over time Calculation for speed: speed = distance / time Constant speed - speed that doesn’t change Instantaneous speed- speed at a specific point

S (meters/second)= d (meters)/ t (seconds) A car traveling at a constant speed covers a distance of 750m in 25s. What is the car’s speed? D=750m T=25s S=“what we need to find” What do we need to do to find the speed of the car?

C. Graphing Motion The motion of an object over a period of time can be shown on a distance-time graph Distance is plotted on the vertical axis (y) Time is plotted on the horizontal axis (x) The slope of a line gives the speed of an object in motion

D. Velocity Velocity – speed and direction 5 m/s N a. How are speed and velocity different ? b. These 2 escalators have the same speed but why do they have different velocities?

Section 2 Acceleration

A. Acceleration Acceleration - change in velocity’s rate Positive acceleration - speed is increasing Negative acceleration - speed is decreasing When an object changes speed or direction, it is accelerating

Visualizing Acceleration A C E B D A to B=speeds up as gravity pulls it down B to C=slows down as gravity is pulling down and preventing it to climb back it

B. Calculating Acceleration Acceleration= final velocity - initial velocity over time a = vf – vi / t Units of acceleration – m/s2 3. A speed/time graph tells you if the acceleration is positive or negative Positive acceleration – “+” numbers with “+” slope on graph Negative acceleration –”-” numbers with “-” slope on a graph

Review Problems: Acceleration A ball is dropped from a cliff and has an acceleration of 9.8m/s2. How long will it take the ball to reach a speed of 24.5m/s2? A sprinter leaves the starting blocks with an acceleration of 4.5m/s2. What is the sprinter’s speed 2s later?

Discussion Question Most sports rely on the ability of people to make quick changes in acceleration. What is some of the equipment used in different sports to make acceleration easier?

Answer Cleats - soccer, baseball, football Rubber-soled shoes – basketball Starting Blocks – runners, swimmers Wind-resistant clothing – racers

Section 3 Motion and Forces

A. What is force? Force – a push or a pull Net force – when 2 or more forces combined act on an object at the same time If the 2 forces cancel each other out, what do you think the net force will be?

B. Balanced vs. Unbalanced Forces Ffriction W Fpush Balanced Force – forces on an object that are equal in size and opposite in direction The net force = 0 2. Unbalanced Force – forces on an object that are unequal in size and opposite in direction The net force = the larger force

C. Inertia and Mass Inertia – the tendency of an object to resist any change in its motion The larger the mass of an object, the greater its inertia Newton’s Laws of Motion – rules that describe the effects of forces on the motion of objects Newton’s 1st Law of Motion – an object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it

D. What Happens in a Crash The law of inertia (a.k.a. Newton’s 1st Law) can explain what happens in a car crash Inertia causes this unrestrained crash test dummy to continue moving at the speed of the car before the crash

Balanced or Unbalanced… Two students push on a box in the same direction, and another student pushes on the box in the opposite direction. Is this force that is being exerted balanced or unbalanced? What does the net force equal?