Describing and Measuring Motion Are you in motion right now? Motion: an object is in motion if the distance from another object is changing

 Describing and Measuring Motion  Reference Point: is a place or object used for comparison to determine if something is in motion. An object is in motion is it changes position relative to a reference point.  Examples of good reference points:  Tree, sign or building  Why are these good reference points????

 Measuring Distance  Measuring distance is a way of describing motion precisely  International System of Units  When you measure distance you are measuring length  SI Unit (for distance) is the meter

 Speed and Velocity  Speed: the distance an object travels in a certain amount of time.  Calculating Speed  Speed = distance/time  Unit: m/s

 Average Speed:  is the total distance divided by the total time traveled  Instantaneous Speed:  rate at which an object is moving at any given instant  Velocity:  speed in a given direction

 Graphing Motion:  Plotting distance vs time on a line graph will show you the motion of an object  The slope of the line (rise/run) tells you the rate of change of one variable in relation to another. In this case the slope equals the speed of the object’s motion

 Acceleration:  Rate at which velocity changes  Increasing speed  Decreasing speed  Change in direction

 Calculating Acceleration:  To calculate the acceleration of an object moving in a straight line, you must calculate the rate of change in the speed of the object per unit of time  Acceleration= final speed – initial speed/time

 Graphing Accelration  Two different graphs can be used to describe the acceleration of an object.  1. Speed vs. Time  2. Distance vs. Time

 On a Distance vs Time graph a curved line shows that the object is accelerating  Since the slope of the graph is changing it can be concluded that the speed is changing, there for the object is accelerating.

 On a Speed vs Time graph a straight, slanted line means that the object is accelerating at a constant rate.  The acceleration can be calculated by calculating the slope of the line  Slope = rise/run

 Force: is a push or a pull  Force is described by its strength and direction in which it acts  Unit of Force: newton (N)  One newton is the force required to cause a mass of one kilogram to accelerate at a rate of one meter per second squared in the absence of other force-producing effects.

 Forces are represented by arrows. The arrow points in the direction of the force and the length of the arrow indicates the strength of the force (the longer the arrow, the greater the force)  The combination of all the forces acting on an object is called Net Force.  The Net Force determines whether an object moves and also in which direction it moves.

 When forces act in the same direction:  the net force can be determined by adding the strengths of the individual forces.  When forces act in opposite directions:  The forces combine by subtraction  If the opposing forces are of equal strength, there is no net force and there is no resulting change in the objects motion.

 Unbalanced Forces  if the is a net force then the forces are unbalanced that results in a change in the object’s motion.  Balanced Forces  Equal forces acting on one object in opposite directions. Balanced forces acting on an object do not change the object’s motion

 Friction and Gravity  Friction: the force that 2 surfaces exert on each other when they rub against each other.  The strength of the friction depends on:  1. How hard the surfaces push together  2. The types of surfaces involved  * smooth surfaces produce less friction than rough surfaces

 Types of Friction 1. Static Friction: the friction that acts of objects that are not moving 2. Sliding Friction: occurs when 2 solid surfaces slide over each other. 3. Rolling Friction: occurs when one object rolls across a surface 4. Fluid Friction: occurs when a solid object moves through a fluid

 Gravity: is a force that pulls objects toward the center of the earth.  Law of Universal Gravitation: states that the force of gravity act between all objects in the universe.  Factors Affecting Gravity: 1. Mass of objects 2. Distance between objects

 Free Fall: occurs when the only force acting on an object is gravity.  an object in free fall is accelerating  Acceleration due to gravity near the surface of the earth is 9.8 m/s2  All objects in free fall accelerate at the same rate regardless of their masses.

 Air Resistance: - Type of fluid friction - Falling objects with greater surface area experience greater air resistance - As a falling object speeds up, the force of air resistance becomes greater and greater. - Eventually the forces become balanced and the object stops accelerating.

 Projectile Motion:  an object that is thrown is called a projectile

 Newton’s First Law of Motion states:  An object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless it is acted upon by an unbalanced force.  Inertia: is the tendency of an object to resist change in its motion. Newton’s first law is also called law of inertia.

 The greater the mass of an object, the greater its inertia.  Newton’s Second Law of Motion states:  Acceleration depends on the object’s mass and the net force acting on it.  Acceleration = Net Force/Mass  What would be 2 ways to increase the acceleration of and object?

 Newton’s Third Law states: if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction of the first object.  In other words: for every reaction there is an equal and opposite reaction.  Do Action-Reaction forces cancel?

 Momentum:  Newton called it a “quantity of motion”  Momentum is a characteristic of a moving object that is related to the mass and the velocity of the object  Momentum = mass x velocity  Momentum is described by its direction and its quantity

 What is the unit for momentum?  Law Conservation of Momentum -states that the amount of momentum objects have before a collision is the same as after the collision. (this is true only when there are no outside forces)