1. Motion
Section 7.1

2. 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

3. 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

5. 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.

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

7. 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

9. 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

10. An example of a position-time graph

12. 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)

13. 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.

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

15. 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

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