Chapter 2 Motion & Speed

Motion & Speed motion – occurs when an object changes position in order for you to know if something has changed position you need a reference point relative motion - movement relative to another object, even if you do not move yourself example: you are moving right now relative to the other planets

Motion & Speed distance – describes how far an object has moved the SI unit for distance is the meter (m) displacement – the distance and direction an object moves in relation to the starting point must include distance as well as in what direction the object moved

Motion & Speed example: if you drive 10 km east from your home to the store and then 7 km west to a friend’s house, what is his displacement from his starting point at his home?

Motion & Speed speed – the distance an object travels per unit of time equation:

Motion & Speed Suppose you ran a 4 km race in 20 min. Calculate your speed. Sound travels at a speed of 330 m/s. If a lightning bolt strikes the ground 1000 m away from you, how long will it take for the sound to reach you?

Motion & Speed speed – the distance an object travels per unit of time motion at a constant speed does not slow down or speed up, such as when a car is set on cruise control most of the time, speeds that you experience are not constant. For example, when driving in a car you speed up, slow down, or stop for a red light So, when speed is changing you use average speed

Motion & Speed average speed – the total distance traveled divided by the total time describes the speed of motion when speed is changing example: when riding a bike, you start going 0 km/hr and then speed up to about 20 km/hr. Then you slow down when going up a hill. The 5 km trip took you 0.5 hr. What is your average speed?

Motion & Speed instantaneous speed – the speed at a given point in time when something speeds up or slows down its instantaneous speed changes

Motion & Speed velocity – describes the speed of an object and the direction of the motion speed describes only how fast something is moving & DOES NOT take into account direction example: if 2 escalators both going at the same speed but in different directions their speed of the passengers on them are the same but their velocity is different

Motion & Speed velocity – describes the speed of an object and the direction of the motion

Motion & Speed Graphing Motion use a distance / time graph time is on the X axis and distance is on the Y axis this will show the objects movement over time the slope of a distance time graph = speed

Motion & Speed Graphing Motion

Motion & Speed Graphing Motion A flat horizontal line indicates that the object or person is NOT moving A diagonal line indicates that the object or person IS moving Example #1: Jake was on his way to his soccer game when he realized he left his shin guards at home. He turned around and went home to pick them up. On his way back to his game he had to stop and wait for a train. After the train passed he was able to drive the rest of the way to his soccer game. Explain what is occurring at each event.

Motion & Speed Example #2: You’re at the park playing with your remote control helicopter and trying to land it on a picnic table. You descend the helicopter toward the table. During the descent you hover above the table while your friend clears off the food. When they are done you finish the descent. Explain what is occurring during each segment of the graph.

Motion & Speed Example #3: You leave your house and walk 2 blocks in 3 minutes to the nearest gas station. You spend 5 minutes at the gas station before heading home. You walk 1 block in 1 minute before you stop at the crosswalk. You wait 1 minute before being able to cross the street and walk the remaining 1 block home. Explain what is occurring during each segment of the graph.

Motion & Speed Example 4: Draw a distance vs. time graph for the following scenario. Sandy left her house to go to work. She travels 1 mile in 2 minutes before she is stopped and has to wait 1 minute at a red light. She then travels another 5 miles in 7 minutes to get to work. Once she parks her car she realizes that she forgot her purse. It takes her 5 minutes to drive 6 miles back to her home.

Motion & Speed Example 5: Draw a distance vs. time graph for the following scenario. Joe leaves his house to walk his dog. It takes him 5 minutes to walk 5 blocks to the park. When Joe and his dog get to the park they stop at the fire hydrant for 1 minute so the dog can do his business. They then continue to walk another 3 blocks through the park in 2 minutes towards a pet shop. After staying in the pet shop for 3 minutes to buy a bone, Joe and his dog turn around and walk home in only 7 minutes.

Motion & Speed acceleration – the rate of change of velocity acceleration occurs if an object changes its speed, its direction, or both

Motion & Speed acceleration – the rate of change of velocity positive acceleration acceleration is in the same direction as the velocity (direction) car speeds up (speed)

Motion & Speed acceleration – the rate of change of velocity negative acceleration acceleration is in the opposite direction of the velocity (direction) car slows down (speed)

Motion & Speed acceleration – the rate of change of velocity changing direction any time a moving object changes direction, its velocity changes and it is accelerating example: a horse on a carousel’s speed is always constant but the horse is accelerating because it is constantly changing direction

Motion & Speed acceleration – the rate of change of velocity calculating acceleration a =acceleration vf = final velocity vi = initial velocity t = time measured in m/s2

Motion & Speed A car is driving 10 meters per second. The driver sees a police vehicle and slows down to 5 meters per second in 5 seconds. Has the car accelerated or decelerated? By how much has the car changed in acceleration?

Motion & Speed A jet airliner starts at rest at the end of a runway and reaches a speed of 80 m/s in 20 seconds. What is the acceleration for the airliner?

Motion & Speed A skateboarder is moving in a straight line at a speed of 3 m/s and comes to a stop in 2 seconds. What is the skateboarder’s acceleration?

Motion & Speed Interpreting velocity vs. time graphs Velocity goes on the y-axis  Time goes on the x-axis  The slope of the line represents the object’s acceleration

Motion & Speed Velocity vs. Time graph

Motion & Speed Velocity vs. Time graph A flat horizontal line indicates that the object or person is NOT accelerating A diagonal line indicates that the object or person IS accelerating or decelerating

Motion & Speed force – a push or a pull that one thing exerts on another examples: force of the floor on your feet force of gravity pulling you down force of someone pushing on your arm if you exert a force on an object, it causes the motion of the object to change example: a tennis racket striking a ball- causes the motion to change

Motion & Speed net force – the sum of the forces (positive & negative) acting on an object the net force includes the size of the force and the direction of the force

Motion & Speed Two Types of Net Forces 1. balanced forces – forces on an object that are equal in size but opposite in direction the forces “cancel each other out” example: if two students push on a box with the same amount of force but in opposite directions

Two Types of Forces Balanced forces: Forces on an object that are equal in size and opposite in direction + = 0 Net Force = 0

Motion & Speed Two Types of Net Forces balanced forces – forces on an object that are equal in size but opposite in direction

Motion & Speed Two Types of Net Forces 2. unbalanced forces – occurs if two objects are pushing with unequal forces in the opposite direction or if two forces are pushing in the same direction example: two students pushing a box on opposite sides but one student is pushing with more force two students pushing a box on the same side

Motion & Speed Two Types of Net Forces 2. unbalanced forces – occurs if two objects are pushing with unequal forces in the

Motion & Speed inertia – the tendency of an object to resist any change in its motion if an object is moving, it will keep moving in that direction and at that speed unless acted on by another force in other words, the velocity of an object remains the same unless another force changes it

Motion & Speed inertia – the tendency of an object to resist any change in its motion The Law of Inertia is also called Newton’s First Law example: a ball moving in a certain direction with a certain speed will continue moving until another ball hits it

Motion & Speed inertia – the tendency of an object to resist any change in its motion the greater the force – the greater the inertia inertia is related to mass – so, the greater the mass, the greater the inertia (this is because there is more resistance to change its motion.)

Motion & Speed inertia – the tendency of an object to resist any change in its motion Newton’s 1st Law, or the Law of inertia can explain what happens in a crash. Explain why it is beneficial to wear a seatbelt.