Exploring Motion and Forces Chapter 3 Review Physical Science Mrs. Nell

Motion and Speed Motion is a change in position. Speed is the rate of change in position (how much time it takes for the change in position to occur).

Instantaneous & Constant Speed Instantaneous speed is the rate of motion at any given instant. When you glance at your speedometer- that is an instantaneous speed. Constant speed is a speed that doesn’t vary. When you set your cruise control you go at a constant speed.

Changing Speed In cases where rate of motion varies a great deal, the best way to describe speed is to use average speed. Average speed is the total distance traveled divided by the total time of travel. Cheetahs can run more than three miles-at an average speed of 45 miles per hour. The cheetah is capable of speeds up to 72 mph!

Calculating Speed The relationship between distance, speed, and time is shown in the equation d = v x t (distance = speed x time). You can change the equation to calculate the average speed: v = d/t (speed = distance divided by time) Ex. The Wright brothers' airplane flew 120 feet in 12 seconds. Calculate its speed in feet per second. 120 ft = 10 ft/s 12 s

Graphing Speed A distance-time graph can be used to “see” the motion of an object over a period of time. The graph to the right shows how Mrs. Nell’s distance from home varied with time as she walked to her friend's house, bought a magazine in a shop on the way, and walked back home again. Now answer these questions using the graph. How far is it from Mrs. Nell's house to the shop? How far is it from Mrs. Nell's house to her friend's? How many hours did Mrs. Nell spend in the shop? How long did Mrs. Nell spend at her friend's? How long did the trip take altogether? 1 mile 2 miles 1 hours 1.5 hours 4 hours

Velocity and Acceleration Velocity describes both speed and direction of an object. Velocity can change even if the speed remains the same. If the object turns the velocity changes because the direction changes.

Acceleration Acceleration is the rate of change of velocity. Because velocity includes both speed and direction, an object is said to be accelerating if it speeds up, slows down, or turns. The drag parachute is used to change the direction of the dragster’s acceleration. When acceleration and velocity are in opposite directions, negative acceleration occurs.

Positive acceleration occurs when acceleration and velocity are in the same direction.

Calculating Acceleration To calculate average acceleration, divide the change in velocity by the time interval. To find the change in velocity subtract the initial velocity (starting velocity) from the final velocity. a = vf-vi t The unit for velocity is m/s and the unit for time is seconds… so the unit for acceleration is m/s/s or m/s2

Balanced Forces A force is a push or pull one body exerts on another. Force does not always change velocity. Forces on an object that are equal in size and opposite in direction are called balanced forces.

Unbalanced Forces An unbalanced force or net force on an object always changes the velocity of the object.

Inertia is the tendency of an object to resist any change in its motion. The more mass an object has, the greater its inertia.

The Law of Inertia Newton’s First Law of Motion (also called the Law of Inertia) states that an object in motion will stay in motion in a straight line and at a constant speed unless acted upon by a net force… and an object at rest will remain at rest unless acted upon by a net force.

Friction Friction is the force that opposes motion between two surfaces that are touching each other. The amount of friction depends on the kinds of surfaces touching and the force pressing the surfaces together.

Life Without Friction

Gravity Gravity is the force that every object in the universe exerts on every other object. The measure of the force of gravity on an object is the object’s weight. Weight is not the same thing as mass. Mass is the amount of matter in an object.

Mass and Distance The amount of gravitational force between objects depends on their masses and the distance between them. The closer a satellite is to Earth, the stronger the pull of gravity.

Weight vs. Mass Mass is measured in grams (g) and kilograms (kg). Weight, which is a force, is measured in newtons (N). Your weight varies depending on where you are. The stronger the gravitational pull- the more you will weigh. Unlike weight, mass doesn’t change with changes in gravity.

Your Weight on Other Worlds If you weigh 100 lbs… how much would you weigh at the following locations- take a guess! Mercury Venus Moon Mars Jupiter Saturn Mars = 37.7 Mercury= 37.8 Moon = 16.6 Venus = 90.7 Saturn = 106.4 Jupiter = 253.3 Mercury, Venus, Moon, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Sun Uranus = 88.9 Neptune = 112.5 Pluto = 6.7 The Sun = 2707.2 Uranus Neptune Pluto Sun

Practice: Problem #1 Are you ready for the test? Try the following problems. Goldy Goldfish, a speed swimmer, loves to race around the park’s pond, which is 0.5 miles around. If she can swim 20 laps around the track in 2 hours, what is her average speed? Answer: 20 * .5 = 10 miles/2 hours = 5mph

Practice: Problem #2 2) At exactly 2:00pm, Sammy the Snail crawls onto a meter stick at the 10 cm mark. If he reaches the 65 cm mark at exactly 2:10pm, what is his speed? Answer: v = d/t 55cm/10minutes = 5.5 cm/minute

Practice: Problem #3 3) Katie the Penguin loves to sled down her favorite hill. If she hits a speed of 50m/s after 5 seconds, what is her rate of acceleration? Hint: She starts at 0 m/s at the top of the hill. a = vf-vi 50m/s - 0m/s = 50/5 = 10 m/s/s or 10 m/s2 t 5s

Practice: Problem #4 Crazy Kaleb’s fire truck decelerates from 72m/s to 0 m/s in 6 seconds. What is his rate of deceleration? Answer: a = vf-vi 0m/s-72m/s = -72 / 6s = -12m/s2 t

Practice Problem #5 Tiny Train is traveling at the speed of 10 m/s at the top of a hill. Five seconds later it reaches the bottom of the hill and is moving at 30 m/s. What is the rate of acceleration of Tiny Train? Answer: a = vf-vi 30m/s -10m/s = 20 / 5s = 4 m/s2 t

The End Now go study!!!