think – pair - sharemmccolgan/gp130f12/lectures/phys130_ch2_d2.pdfa frictionless hill. each is...
TRANSCRIPT
Think – Pair - Share
1. A 1-pound block and a 100-pound block are placed side by side at the top of
a frictionless hill. Each is given a very light tap to begin their race to the
bottom of the hill. In the absence of air resistance
A. the 1-pound block wins the race.
B. the 100-pound block wins the race.
C. the two blocks end in a tie.
D. there’s not enough information to determine which block wins the race.
A 1-pound block and a 100-pound block are placed side by side at the top of a frictionless hill. Each is given a very light tap to begin their race to the bottom of the hill. In the absence of air resistance
A. the 1-pound block wins the race. B. the 100-pound block wins the race. C. the two blocks end in a tie. D. there’s not enough information to
determine which block wins the race.
Position-Versus-Time
• Trucker Bob starts the day 120 miles west of Denver. He drives east for 3 hours at a steady 60 miles/hour before stopping for his coffee break.
• Draw a position-versus-time graph for Bob, including appropriate numerical scales along both axes.
• Let Denver be located at x = 0 and assume that the x-axis points to the east.
Uniform Motion
Straight-line motion in which equal displacements occur during any successive equal-time intervals is called uniform motion. For one-dimensional motion, average velocity is given by
Notice the symbol Δ. It represents a displacement and an interval in time - not position or instant in time
Problem Solving Strategy
• Sally opens her parachute at an altitude of 1500 m. She then descends slowly to earth at a steady speed of 5 m/s. How long does it take her to touch down?
• Draw pictorial representation – coordinates, symbols • Motion diagram • Mathematical Representation • Are results reasonable?
Acceleration
Average Acceleration • The time rate of change of
velocity
• Change in velocity divided by change in time
Instantaneous Acceleration • The as at a specific instant
of time t is given by the derivative of the velocity
Trajectory of a ball
Turning point – What is the acceleration as the ball rises, drops, at the top?
Tactics: Interpreting graphical representations of motion
Motion with Constant Acceleration
Finding Velocity from the Acceleration If we know the initial velocity, vis, and the instantaneous acceleration, as, as a function of time, t, then the final velocity is given by
Or, graphically,
EXAMPLE 2.21 Finding velocity from acceleration
QUESTION:
EXAMPLE 2.21 Finding velocity from acceleration
Problem-Solving Strategy: Kinematics with constant acceleration
Example 1 Bob throws a ball straight up at 20 m/s, releasing the ball 1.5 m above the ground. What is the maximum height of the ball? What is the ball’s impact speed as it hits the ground?
Example 2 A sprinter accelerates at 2.5 (m/s)/s until reaching his top speed of 15 m/s. He then continues to run at top speed. How long does it take him to run the 100-m dash?
Kinematic Curves
Upcoming Assignments
• MasteringPhysics online HW due tonight by 11 pm • Pre-class MasteringPhysics assignment due before
class on Tuesday • Pre-class MasteringPhysics assignment due before
class on Thursday • MasteringPhysics online HW due Thursday by 11 pm