Kinematics 2

What’s Most Difficult

Right now I am finding remembering stuff from old classes...its too early in the semester =). I took trigonometry last summer and i passed it with flying colors, but then looking at problems, i totally forgot how to do SIN, and COS. Once I look at the problem though it comes to me...it just takes me a while. Otherwise nothing is that difficult right now. =) Finding positive and negative velocities on a graph.

There are no problems thus far.

Im having trouble keeping my thoughts straight on what is velocity and what is acceleration .

The problems aren't really all that difficult as of yet. I'm just having trouble getting my brain back in to the swing of things.

05

Review

• Kinematics : Description of Motion – Position – Displacement – Velocity • average

v

  • Instantaneous 

x t

– Acceleration • average • instantaneous

a

 

v



t

07

Acceleration Units

• Speed/time • m/s/s = m/s 2 • mi/hr/s = mi/hr-s • km/hr/s = km/hr-s • Distance/time/time • or distance/time 2

The Ninja

• The Ninja motorcycle can accelerate from 0 to 60 mi/h in 3.8 s.

• What is its average acceleration?

• 15.8 mi/h/s • 23 ft/s/s or 7 m/s/s

Preflight 4.1

(A) (B) (C) Which x vs t plot shows positive acceleration?

86% got this correct!!!!

“The slope of the graph is getting increasingly positive; the velocity is increasing over time.” 08

Equations for Constant Acceleration (text, page 34)

• v = v 0 + at • v av = ½ (v o + v) • x = x 0 • v 2 = v 0 2 + v 0 t + ½ at 2 + 2a(x-x 0 ) x (meters) 200 150 100 50 0 0 v (m/s) 20 15 10 5 5 10 t (seconds) 15 20 0 0 a (m/s 2 ) 2 5 10 t (seconds) 15 20 1.5

1 0.5

0 0 5 10 t (seconds) 15 20 10

Acceleration ACT

An car accelerates uniformly from rest. If it travels a distance D in time t then how far will it travel in a time 2t ? A. D/4 B. D/2 C. D D. 2D E. 4D Correct x=1/2 at 2 Follow up question: If the car has speed v at time t then what is the speed at time 2t ?

A. v/4 B. v/2 C. v D. 2v E. 4v Correct v=at 24

# 43 Ch 2

• A cheetah can accelerate from rest to 25.0 m/s in 6.22 s. Assuming constant acceleration, – A. how far has the cheetah run in this time?

– B. after sprinting for just 3.11 s, is the cheetah’s speed 12.5 m/s, > 12.5 m/s, or < 12.5 m/s?

– C. what is the cheetah’s average speed for the first 3.11 s?

– D. calculate the distance covered by the cheetah in the first 3.11 s.