# Motion Chapter 2 Section 1 Describing Motion 1

Motion Chapter 2

Section 1: Describing Motion 1. 2. 3. What you will learn: Distinguish between distance and displacement. Explain the difference between speed and velocity Interpret motion graphs.

Motion p Motion occurs when an object changes it’s position relative to a reference point. n A reference point is used to create a frame of reference, which is a coordinate system where the position of the object(s) is/are measured.

Motion Continued… p Distance is how far an object has moved. n SI unit of distance is the meter (m). Displacement is the distance and direction of an object’s change in position from the starting point. p

Speed p p Speed is the distance an object travels per unit of time. Calculating Speed: n n Any change over time is called a rate. Speed = Distance (m) Time (s) p Ex: Acura NSX top speed: 168 mph

Solve This Speed Problem: p A car travels at a constant speed and covers a distance of 750 m in 25 s. n p What is the car’s speed? 1. Write what you know: n n d= 750 m t= 25 s 2. The variable you need is speed p 3. s= d/t p 4. Use numbers: s= 750/25= 30 m/s p

More Speed Problems p 1. An elevator travels from the first floor to the 60 th floor, a distance of 210 m, in 35 s. What is the elevators speed? n n p s= d/t s= 210/35 = 6 m/s 2. How far does a car travel in 0. 75 h if it is moving at a constant speed of 88 km/h? n n n s= d/t d= st d= 88 km/h)(. 75 h) = 66 km

Changing Speed p Average speed describes speed of motion when the speed is changing. n Average speed is the total distance traveled divided by the total time of travel.

Instantaneous Speed p Instantaneous speed is the speed at a given point in time. In this example, the instantaneous speed is 100 mi/h. n This person needs to slow down!!! n Two examples of motion in which the instantaneous speed changes are acceleration and deceleration. n

Graphing Motion p The motion of an object over a period of time can be shown with a distance-time graph. Time is plotted on the x-axis and distance on the y-axis. n

Velocity p Velocity is a term that describes the speed of an object AND IT’S DIRECTION. To tell if you are in danger of a hurricane you need to know how fast it is moving and in what direction. n

Velocity Continued… p If the cars in this picture maintain the same speed, does their velocity stay the same or change? Why? n Velocity changes because direction is constantly changing.

Did We Reach Our Learning Goals for Section One? 1. Distinguish between distance and displacement. p 2. Explain the difference between speed and velocity. p 3. Interpret this graph: p

Section 2: Acceleration 1. 2. 3. What you will learn: Identify how acceleration, time, and velocity are related. Explain how positive and negative acceleration affect motion Describe how to calculate the acceleration of an object.

Acceleration p Review: Velocity is the change of position. p New: Acceleration is the change of velocity. n Velocity includes both the speed AND direction of an object. p A change in velocity can be a change in: § 1. How fast something is moving OR § 2. A change in the direction it is moving. n Recap: Acceleration occurs when an object changes it’s speed, it’s direction, or both!

Speeding Up and Slowing Down Positive Acceleration is when the speed of an object is increasing. p Negative Acceleration is when the speed of an object is decreasing. p

Let’s Do Some Practice p Is a car accelerating if it is moving in a straight line at a constant speed? n p Is a car accelerating if it is moving in a straight line but slowing down? n p NO! YES! Is a car accelerating if it is moving in a circular racetrack at a constant speed? n YES!

Changing Direction p Any time a moving object changes direction, it’s velocity changes and it is accelerating.

Calculating Acceleration p Acceleration is the rate of change in velocity. p To calculate the change in velocity: n p Final velocity- initial velocity To calculate acceleration (in m/s 2)= n a = vf –vi t

Calculating Positive Acceleration p Example: An airplane starts at rest at the end of a runway and reaches a speed of 80 m/s in 20 s. Calculate acceleration. n n a = vf –vi t a = (80 m/s – 0 m/s) = 4 m/s 2 20 s

Calculating Negative Acceleration p Example: A skateboarder is moving in a straight line at a constant speed of 3 m/s and comes to a stop in 2 s. Calculate the acceleration: n n a = vf –vi t a = (0 m/s -3 m/s) = -1. 5 m/s 2 2 s

Did We Reach Our Learning Goals For Section 2? p p p 1. Identify how acceleration, time, and velocity are related. 2. Explain how positive and negative acceleration affect motion. 3. Describe how to calculate the acceleration of an object.

Section 3: Motion and Forces 1. 2. 3. What you will learn: Explain how force and motion are related. Describe what inertia is and how it is related to Newton’s first law of motion. Identify the forces and motion that are present during a car crash.

What is a Force? p A force is a push or pull. n Some forces are obvious, others aren’t as noticeable. p Example: the floor exerts a force on your feet, the atmosphere exerts a force on your body, & gravity pulls on you.

Changing Motion p When you exert a force on an object, the motion of the object may change. n When you hit a ball with a racket, the racket strikes the ball with a force that causes the ball to stop and then move in the opposite direciton.

Balanced Forces p Forces that are equal in size and opposite in direction are balanced forces. n Force does not always change velocity!

Unbalanced Forces p Unbalanced forces are those that are not equal. n In the example below, the girl is pushing the box with a greater force than the boy so the box is going to move.

Inertia and Mass p Inertia is the tendency of an object to resist any change in its motion. n Example: What would be more difficult for you to stop with your hand…a tennis ball thrown at you or a bowling ball?

Speaking of Inertia… p Which of the following vehicles would you choose to more successfully slam into and break down a concrete wall? Why?

Newton’s First Law of Motion p p “An object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it. An object at rest will remain at rest unless an unbalanced force acts on it. ” Plain terms: An object is going to keep moving unless a force stops it, and an object that is not moving will stay still unless a force causes it to move.

How Does Newton’s Law Have Anything To Do With Me? ? p The “law of inertia” or Newton’s first law explains what happens in a car crash. n n Which vehicle has the greater inertia? Which vehicle has the greater damage?

You MUST Wear Your Seatbelt!!! If a car traveling at 50 km/h collides with something solid, the car crumples, slows down, and stops within 0. 1 s. p Any passenger NOT wearing a seatbelt continues moving at 50 km/h and within 0. 02 s will collide with the dash and windshield. . p

Why Seatbelts? Seatbelts help to slow down a person as the car slows down and helps to prevent people from being thrown out of the car. p Statistic: Car safety experts say that about half the people in fatal car accidents would survive if they had worn seatbelts. p

Did we reach our learning goals for section 3? p p p 1. Explain how force and motion are related. 2. Describe what inertia is and how it is related to Newton’s first law of motion 3. Identify the forces and motion that are present during a car crash.

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