# Ch8MotionandForces by peirongw

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```									Chapter 8 Motion and Forces Section 1: Motion Speed and Velocity
Motion Motion is an object's change in position over time when compared with a reference point. Frame of Reference A frame of reference is a system for specifying the precise location of objects in space and time. Speed Speed is the rate at which an object moves. Speed depends on the distance traveled and the time taken to travel that distance; the distance traveled divided by the time interval during which the motion occurred Speed can be determined from a distance-time graph. Graphical Representation of Average Velocity The slope of the line connecting two points on a graph of position versus time equals the average velocity over that time interval.

Equation for Average Speed Average speed is the distance traveled divided by the time interval of the motion. Velocity Velocity is the speed of an object in a particular direction. Average Velocity Average velocity describes how fast and in which direction an object is moving. It is calculated by dividing the total displacement by the time interval during which the displacement occurred. Distinguishing Between Speed and Velocity Speed is the distance traveled by an object divided by the time interval during which the motion occurred. Velocity is the speed of an object in a particular direction. Explore these interactive demonstrations to learn how to use equations to solve science problems. Velocity

Take a quiz that helps you find the correct answers by giving you feedback to your responses.

Section 2: Acceleration and Force Acceleration
Acceleration Acceleration is the rate at which velocity changes over time. An object accelerates if it changes speed, direction, or both. Graphical Representation of Average Velocity The slope of the line connecting two points on a graph of position versus time equals the average velocity over that time interval. Equation for Average Acceleration Average acceleration is the average rate at which an object's velocity changes over some time interval. This rate includes both magnitude and direction. Explore these interactive demonstrations to learn how to use equations to solve science problems. Acceleration

Acceleration can be determined from a velocity-time graph Graphical Representations of Acceleration Acceleration measures the rate of change in velocity. The average acceleration equals the change in velocity divided by the time required to make the change. The acceleration at any moment can be calculated from a velocity versus time graph.

Force
Force A force is an action exerted on a body in order to change the body's state of rest or motion. A force—a push or a pull—has magnitude and direction. Balanced forces do not change motion. Velocity of the object is constant!

Unbalanced forces do not cancel completely. Velocity of the object is not constant!

Friction and Air Resistance
Friction Friction is a force that opposes motion between two surfaces that are in contact.

Surface texture, electrical attraction, and other forces that push objects together affect the friction experienced by the objects. Air resistance is a form of friction. Why have the shapes of cars changed?

Gravity Gravity - a force of attraction between objects that is due to their masses and that
decreases as the distance between the objects increases Mass and distance affect gravitational force Law of Universal Gravitation Newton's law of universal gravitation states that the force of gravity depends on the product of the masses of the objects divided by the square of the distance between them.

Take a quiz that helps you find the correct answers by giving you feedback to your responses.

Section 3: Newton's Laws of Motion Newton’s First Law
Newton's First Law Newton's first law states that an object at rest remains at rest and an object in motion continues in motion with constant velocity unless the object experiences a net external force. Objects in motion tend to stay in motion. In other words, objects keep on doing what they’ve been doing. Truck and ladder There are many more applications of Newton's first law of motion. Several applications are listed below – try to provide explanations for each application.
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blood rushes from your head to your feet when riding on a descending elevator which suddenly stops. the head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface. to dislodge ketchup from the bottom of a ketchup bottle, the bottle is often turned upside down, thrust downward at a high speed and then abruptly halted.

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headrests are placed in cars to prevent whiplash injuries during rear-end collisions. while riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb, a rock or another object which abruptly halts the motion of the skateboard.

Mass and Inertia An object's mass is a measure of its inertia, or tendency to resist a change in motion. Newton's Second Law Newton's second law states that the acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the object's mass.

Force is measured in Newtons (N). Explore these interactive demonstrations to learn how to use equations to solve science problems. Newton's Second Law

Free Fall and Weight
Free Fall Free fall is the motion of a body when only the force of gravity is acting on the body. Free-fall acceleration near the Earth’s surface is constant. Weight equals mass times free-fall acceleration. Weight is different than mass. Comparing Mass and Weight A fundamental property of an object is its mass, the amount of matter in the object. An object's weight is a measure of the gravitational force acting on it. Velocity is constant when air resistance balances weight.

Newton’s Third Law
Action and Reaction Forces Each force in an action–reaction pair has the same magnitude, but the forces act simultaneously in opposite directions on different objects. According to Newton's third law, if object A exerts a force on object B, object B exerts an equal force in the opposite direction on object A. These two forces are called an action-reaction pair.

Take a quiz that helps you find the correct answers by giving you feedback to your responses. Arrange the concepts from this chapter in this interactive concept-mapping activity. Newton's Laws of Motion

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