Motion Section 7.1

Objectives Relate speed to distance and time Distinguish between speed & velocity Recognize that all moving objects have momentum Solve problems involving time, distance, velocity, and momentum

We’re switching units! In chemistry, grams was a convenient unit of measuring mass. In physics, kilograms is used instead. Note that 1 kilogram = 1000 grams

Speed and Velocity Speed Definition: The rate of change of position with respect to time. Velocity Definition: Same as speed but also indicates direction. SI units are meters per second (m/s) You need to know two things to calculate speed: How far the object moved How much time it took to move that far.

Equation Speed = distance / time V = d/t We use “V” to indicate speed, even if we don’t know the direction. V = d/t

Example: An object traveled 110 meters north in 72 seconds. Calculate its velocity V = d / t V = 110 m / 72 s V = 1.5 m/s V = 1.5 m/s, north

Position-time graphs Position is on y-axis Time is on the x-axis Slope of the curve is change in position / change in time = speed

An example of a position-time graph

Momentum Definition: A quantity defined as the product of an object’s mass and velocity Mass: A measurement of an object’s inertia. Measured in “kilograms,” kg Useful in predicting speeds after collisions take place (Hyundai vs. Hummer)

About momentum If either mass or velocity is zero, an object has no momentum In general, higher velocities or higher masses lead to higher momenta (plural of momentum) BUT, sometimes light objects can have greater momentum than heavier moving objects.

Momentum Equation Momentum = mass * velocity p is the variable used to describe momentum p = m*v Units are kg*m/s

Sample problem A 5.0 kg rock is dropped from rest from a building and three seconds later it has a velocity of 30 m/s downward. What is its momentum at this point? p = m*v p = 5.0 kg * 30 m/s p = 150 kg*m/s

A final word about momentum Momentum, like mass and energy, is a CONSERVED quantity in nature. Momentum before = momentum after