Transcript Speed
This picture indicates that you need to refer to your notes Look around you- you are likely to see something in motion. Even if you don’t see anything moving, motion is still occurring all around you. Air particles are moving, the Earth is circling the sun, and blood is traveling through your blood vessels. Reference Points A reference point is used to determine the direction of the moving object. The earth’s surface is a common reference point for determining motion. Reference points can be moving or nonmoving. Nonmoving objects: mountains Moving objects: hot air balloon and the zebra. Motion Motion is an object’s change in position relative to a reference point. You can describe this motion in terms of direction: N, E, S, W, up, down, to the left of , just beyond, in front of, etc. Speed Speed is the distance traveled by an object divided by the time taken to travel the distance. Formula Measuring speed Units used for measuring speed can differ, but will always be a distance unit divided by a time unit. Jets: km/h Cars: mi./h Snails: cm/s Falling objects: m/s An athlete swims a distance from one end of a 50 m pool to the other end in a time of 25 s. What is the athlete’s average speed? 1. Write the equation for average speed. 2. Replace the total distance and time with the values given and solve S=50m÷25s S=2m/s Average speed Speed is usually NOT CONSTANT Examples: Cars stop and go regularly. Runners go slower uphill than downhill. . Average speed = total distance traveled/total time it took. It took me 1 hour to go 40 km on the highway. Then it took me 2 more hours to go 20 km using the side streets. Total Distance: 40 km + 20 km = 60 km Total Time: 1 h + 2 h = 3 hr Ave. Speed: total d/total t = 60 km/3 h = 20 km/h Total _ Dist. Ave. _ Speed Total _ tim e I ran 1000 m in 3 minutes. Then ran another 1000 m uphill in 7 minutes. What is my average speed? Total Dist. = 1000 m + 1000 m = 2000 m Total Time = 3 min + 7 min = 10 min Ave speed = total dist/total time = 2000m/10 min = 200 m/min = D Velocity Imagine that two birds leave the same tree at the same time. They both fly at 10 km/h for 5 min, 12 km/h for 8 min, and 5 km/h for 10 min. Why don’t they end up at the same place? The birds went in different directions! Velocity Definition: the SPEED of an object in a particular direction. Examples: An airplane moving North at 500 mph A missile moving towards you at 200 m/s What is the difference between speed and velocity? Speed is just distance/time. Velocity includes direction as well. If you say that an airplane’s velocity is 600 km/h, you would be incorrect. But you could say the plane’s velocity is 600 km/h south. Although the word accelerate is commonly used to mean “speed up,” the word means something else in science. Acceleration is the rate at which velocity changes over time. An object accelerates if its speed, direction, or both change. Acceleration Acceleration Acceleration = Final velocity – Original Velocity Time Acceleration = Final velocity – Original Velocity Time Deceleration = 10 m/ sec – 25 m/sec = -15 m/sec = -5 m/sec/sec 3 sec 3 sec Deceleration= Final velocity – Original Velocity Time In this activity, you will observe one factor that affects the speed of falling dominoes. 1. 2. 3. 4. 5. 6. 7. Set up 15 dominoes in a straight line (use the meter stick as a guide). Try to keep equal spacing between the dominoes. Use the meter stick to measure the total length of your row of dominoes, and record the length. Use a stopwatch to time how long it takes for the dominoes to fall. Record this measurement. Predict what would happen to that amount of time if you changed the distance between the dominoes. Write your predictions. Repeat steps 2 & 3 several times using distances between the dominoes that are smaller and larger than the distance used in your first setup. Use the same number of dominoes in each trial. Calculate the average speed for each trial by dividing the total distance (domino row) by the time the dominoes take to fall. S=d ÷ t (round to the nearest tenth) How did the spacing between dominoes affect the average speed? Force Definition; a push or a pull All forces have size and direction. A force can change the acceleration on an object. It can be a change in speed or a change in motion. Anytime you see a change in an object’s motion, you can be sure it was caused by a force. Scientists express force using a unit called the Newton (N) Examples of everyday forces used: Typing on the computer Walking Sitting in a chair These forces don’t necessarily mean that a motion will occur. For example: Although we can’t always see the forces that are there, they are there! Can you see the force that pulls a magnet to a refrigerator? Can you see that air around you that is held near Earth’s surface by a force called gravity? You probably hear the word force used often. People say things such as “That storm had a lot of force” or “Our football team is a force to be reckoned with.” But what exactly is a force? In science, a force is a push or a pull. All forces have two properties: direction and magnitude. A newton (N) is the unit used to describe the magnitude, or size, of a force. What two properties do all forces have? Balanced condition, in a tug of war, when the rope is still is also an example of balanced force. Let’s assume you are playing tug of war with a team. Your team starts with all its strength and the team on the other side is also pulling with the same force but in an opposite direction. Due to this the flag in the middle of the rope does not move. This is because force from both the sides cancels out each other, the net force is zero and we get a condition of balanced force. Net Force = 0 Newtons (N) =5N Usually more than one force is acting on an object. Net force is the combination of all of the forces acting on an object. Usually more than one force is acting on an object. The net force is the combination of all of the forces acting on an object. If you pull a desk towards you as your friend pushes it towards you, the forces act in the same direction. You would ADD these forces to determine the net force since they are acting in the same direction! Friction Work Work- a force acting through a distance In order for work to be done on an object, a force must move it Work is the amount of force applied to an object times the distance the object moves in the direction of the force If an object doesn’t move, no work is done If an object doesn’t move in the direction of the force, no work is done Work = force x distance (w = f x d )