Moving Objects Jayne Robinson Physical Science Biloxi High School June 2006 3-1 Describing Motion When something moves, it changes position. When changing positions, an object must have a reference point. The best reference point is a stationary object. Motion also often includes speed - how fast the object moves. If motion is described as a change in position, then speed is an expression of how much time it takes for that change in position to occur. Speed - The rate of change in position; a rate of motion. Instantaneous speed - the rate of motion at any given instant. A radar gun can determine instantaneous speed. Constant speed - a speed that does not vary; “cruise control” is the closest a car has to constant speed. Average speed - the total distance traveled divided by total time of travel; the most common expression for speed. Calculating & Graphing Speed The formula to calculate speed: v=d/t speed = distance / time solve problems page 62 a distance-time graph provides a visual expression of speed: the slope of the line equals the speed. Look at figure 3-3; What total distance did each swimmer cover? (red: ; blue: What was each swimmer’s average speed for the 30 minute period? 3-2 Velocity & Acceleration What is the difference between speed and velocity? Velocity describes both speed and direction. How can velocity change if speed remains the same? Acceleration is the rate of change of velocity; how quickly speed or direction changes. If a car changes direction but remains at constant speed, velocity & acceleration is changed. Calculating acceleration a = (vf - vi) / t = Dv/ t acceleration = final velocity - initial velocity / time = change in velocity / time the Greek symbol D stands for “change in” when calculating, include all units and symbols the unit for velocity is m/s (meters per second) the unit for acceleration is m/s2 (meters per second per second or meters per second squared) work the problems on page 67 3-3 Crashing to Save Lives Crash tests with lifelike dummies are performed on all vehicles to determine what happens to people during crashes. They determine what can be done to prevent injuries and death. Seat belts and airbags are two of the devices developed from these crash tests. The seat belt allows a person to become “part of the car” and slow down when the car comes to an abrupt stop. 3-4 Force & Motion A force is a push or pull one body/object exerts on another body/object. Some effects of forces are obvious, such as a car crash, but others are not obvious (the force exerted by the floor on your feet) Some forces cause a change in velocity, either a change in speed or direction or both. Balanced forces are equal in size and opposite in direction: When you stand up the ground pushes up equal to your weight pushing down. A net force changes the velocity of an object; it either changes direction or speed or both direction and speed. Inertia is the tendency of an object to resist any change in its motion. If an object is moving, it tends to keep moving unless a force acts upon it, velocity will remain constant. If at rest, it tends to remain at rest unless a force acts upon it, velocity will remain zero. The more mass an object has, the greater its inertia Newton’s First Law Sir Isaac Newton developed several laws to describe the effects of forces. Newton’s First Law of Motion - states that an object moving at a constant velocity keeps moving at that velocity unless a net force acts on it. If an object is at rest, it stays at rest unless a net force acts upon it. This first law is also known as the law of inertia. Friction Friction is the force that opposes motion between two surfaces that are touching each other. The amount of friction depends on two factors: the kinds of surfaces and the force pressing the surfaces together. If there were no friction you would not be able to walk, to hold things between fingers, or keep objects together. 3-5 Effects of Gravity Gravity is a force that every object exerts on every other object. Many forces of gravity are too small to notice: when you hold a pencil, your hand exerts gravity on the pencil and the pencil exerts gravity on your hand. The amount of gravity depends upon two things: the masses of the objects and the distance between the objects.The masses of your hand and pencil are so small compared to the mass of Earth, gravity between them isn’t noticeable. Weight is the measure of the force of gravity on an object. Weight is dependent upon mass: the greater the mass, the greater the gravitational force on it, the the greater the weight. Mass is measured in kilograms (kg) & grams (g) Weight is measured in Newtons (N) On Earth, a kilogram weighs 9.8 N. Since the mass of the moon is about 1/6 that of Earth, a person’s weight on the moon would be about 1/6 their weight on Earth, even though their mass has not changed. Measuring Forces Scales use the principle of balanced forces to measure weight. Weight is most often measured on a spring scale. The force of gravity is taken in the calculation of weight on the scale. Answer questions 1 and 4 on page 77.
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