Chapter 8 Motion Forces Lesson 1 Describing Motion

Chapter 8: Motion & Forces

Lesson 1: Describing Motion

Reference Point ► The starting point you choose to describe the location, or position, of an object. § Example: I am at the library. ► Position: describes an objects distance and direction from a reference point. § Includes direction, distance, & reference point. § Example: I’m at the library which is down the hall

Displacement ► The difference between the initial, or starting, position and the final position

Displacement

Motion ► An object’s change in position relative to a reference point

Speed ► The distance traveled divided by the time interval during which the motion occurred § Constant speed: when an object moves the same distance over a given unit of time (gradual speed increases) § Changing speed: when an object moves a greater distance each second. ► Average Speed = total distance total time

Speed

Constant Speed A uniform distance is covered for each unit of time. A constant speed graph shows a constant & positive slope

Velocity ► The speed and direction of a moving object. § Constant velocity: an object moves with constant speed and its direction doesn’t change. § Changing velocity: Velocity changes when an objects speed or direction of motion changes

Velocity

Velocity ► Velocity appears to be very similar to speed, however, when describing the velocity of an object you need to provide a magnitude and a direction § Magnitude – the speed of the object § Direction – the direction the object is moving

Difference between Speed & Velocity ► Speed: That's how fast I am moving ► Velocity: That's my speed and direction ► Motion: Keep moving ► Direction: Which way, anyway ► Acceleration: Let's go faster

Acceleration ► The measure of the rate of velocity over a period of time § An object accelerates if its speed, direction, or both change ►Speeding up: velocity increases in the same direction as its motion (positive acceleration). ►Slowing down: velocity decreases in the opposite direction as its motion (negative acceleration). ►Changing direction: velocity changes due to direction

Acceleration

Acceleration and Velocity ► As velocity increases, so does acceleration ► As velocity decreases, so does acceleration ► When direction changes, so does acceleration ► When there is a constant velocity, there is no acceleration

Lesson 2: Graphing Motion

Distance-Time Graph ► A graph that shows how distance and time are related ► Y-axis: shows distance an object travels from its reference point ► X-axis: shows how an objects position changes during each interval ► Can be used to compare the motion of two different objects

Distance-Time Graph Constant: shows a straight, diagonal line on the graph Rest is shown with a horizontal line Changing: Shows a curving line. This shows acceleration.

Describe the motion in each section of the graph. Decelerating Stopped Steady speed Accelerating

Speed-Time Graph ► Shows the speed changes during an interval of time ► y-axis: the speed of an object ► x-axis: shows time

If the graph is a horizontal line, there is no change in velocity, therefore there is no acceleration (the slope is 0). If the acceleration is positive then the slope is positive (the line moves upward to the right). If the acceleration is negative, then the slope is negative (the line moves downward to the right).

If you make a graph by hand it should always be on graph paper. The graph should fill the available space. Carefully choosing the best scale is necessary to achieve this.

The graph should always have a title.

Always label the x and y axes in 3 ways: title, numerical values, and units.

Lesson 3: Forces

Force ► A push or a pull on an object ► Unit of force is the newton (N) ► Arrows show size & direction of force ► Non contact force: a force that one object applies to another without touching it. § Example: Electrical force (static) causes socks to stick together when pulled out of the dryer. ► Contact force: a push or pull one object applies to another object that is touching it § Example: a finger pressing a button

Non-Contact Force ► Gravity: an attractive force that exists between all objects that have mass (the amount of matter in an object) ► Gravitational force depends on mass: mass increase = gravitational force increase Gravitational force depends on distance: the greater the distance the smaller the force ► ► Mass & Weight are different: § Weight is the measure of the gravitational pull on an objects mass § Weight depends on the masses of the objects and the distance between them ► the moon has less mass therefore you weigh less on the moon!

Contact Force ► Friction: a contact force that resists the sliding motion of two surfaces that are touching ► Rough surfaces produce more friction, smooth surfaces produce less friction ► Air resistance: frictional force between air and objects moving through it § Example: crumbling paper into a ball has less surface area in contact with the air. Therefore, resistance decreases making it fall faster.

Combining Force Balanced Forces: forces are equal in size and opposite in direction Unbalanced Forces: forces act in opposite directions, but they are not balanced

Combining Force

Newton’s Laws of Motion ► Law 1 : if the net force acting on an object is zero, the motion of the object does not change. § AKA inertia: the tendency of an object to resist a change in its motion

Newton’s Laws of Motion ► Law 2 : the acceleration of an object is equal to the net force exerted on the object divided by the objects mass § The greater the mass, the greater the force § Acceleration (a)= force (f) mass (m)

Newton’s Laws of Motion ► Law 3 : for every action, there is an equal and opposite reaction § When an object exerts a force on a second object, the second object exerts an equal force of equal size in the opposite direction

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