Chapter 9 Sections 1 and 2

How do you know that an object is moving?

• When its distance from another object is changing.

• Motion is described relative to (compared to) an object that is not moving. This is called the reference point.

Describing Motion

• The motion of an object is described relative to a reference point.– For example: You are sitting in a seat on a

train traveling 85 Km/hr.– What is your speed relative to the train?– What is your speed relative to the ground?– What is your speed relative to your friend

sitting in his seat 6 cars behind you?

How do you measure the motion of an object?

• Measure its change in position using standard metric units for distance (meters).– Kilometer (Km), centimeter (cm), millimeter (mm)

How do you measure how fast an object is moving?

• The speed of an object is measured by how far the object moves (distance) in how much time it takes to get there (time).– Speed = distance ÷ time ( s = d / t )– Units: m/s, mi/hr, Km/min, mm/yr, au/century

d = 30 cm t = 3.8 s s = d / t s = 30 / 3.8 s = 7.9 cm/s

4 steps to get full credit !

• Write down what it is you know from the problem.force = 12 N distance = 18.5 m

• What equation would you use to solve the problem based on the information given?

work = force x distance

• Substitute what is given into the chosen equation.work = 12 N x 18.5 m

• Calculate and clearly express your answer making sure to use the correct units of measurements.

work = 222 J

Now try these…

• A rock is thrown at a bell 50 m away. If it took 4 seconds to travel that distance, what was the speed of the rock? (Ignore the effects of gravity and air resistance.)

• A student has to run the mile in P.E. every week. The final week of the quarter, she ran it in 5.6 minutes. What was her speed?– What was her speed in mi/hr ?

Average Speed

• The total distance traveled divided by the total time to get there.

Speed Graph

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d = 35 m t = 6 min s = d / t s = 35 / 6 s = 5.83 m / min

Now try these…

• Your family starts at 6 am for a trip to your Uncle Lou’s duplex on the river. At 8:30, you stop for breakfast having gone 175 miles. You drive 320 miles before lunch at 12:30. The last stretch is 450 miles, and you arrive at 6:30 pm. What was your average speed for the trip?

• At an average speed of 0.197 Km/min, how far will a bicyclist travel in 175 minutes?

What’s the difference between speed and velocity?

• Velocity is speed in a given direction.– v = d ÷ t– Units: mi/hr east, m/s up, mm/min to the right

• Net velocity problems– Use arrows to indicate motion– Arrows same direction, add velocities– Arrows opposite directions, subtract velocities

Now try these…

• A elderly gentleman is riding an elevator from the ground floor to the 65th floor of his apartment building. It takes 5 minutes to travel the 1300 ft to his floor. What is his velocity?

• You are walking up the down escalator in the mall. Your velocity is 1.7 m/s and escalator is moving 2.0 m/s. What is your net velocity?

Speed = Slope of the Line

Speed Graph

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• Slope is rise over run, or the change in Y over the change in X

• Horizontal lines have a slope of 0.

• The steeper the line, the faster the speed.