lecture 03: relative and circular motion - astronomybelz/phys2210/lecture03.pdf · lecture 03:...

Lecture 03: Relative and Circular Motion

Physics 2210Fall Semester 2014

Announcements

● Homeworks due 11:59 PM Tues/Thurs ● Schedule next few weeks:

● 9/08 – Unit 3● 9/10 – Unit 4● 9/15 – Unit 5● 9/17 – Unit 6● 9/22 – Unit 7, Exam Review● 9/24 – Exam 1, Units 1-6

Mechanics Lecture 2, Slide 3

Monkey Troubles

You are a vet trying to shoot a tranquilizer dart into a monkey hanging from a branch in a distant tree. You know that the monkey is very nervous, and will let go of the branch and start to fall as soon as your gun goes off. In order to hit the monkey with the dart, where should you point the gun before shooting?

A) Right at the monkeyB) Below the monkeyC) Above the monkey


Monkey

x = xo

Dart

x = vo t

2

2

1gty −=

Shooting the Monkey…

2

2

1gty −=


Shooting the Monkey…

y = voy t − 1/2 g t 2

Still works even if you shoot upwards!

y = yo − 1/2 g t 2

Dart hits themonkey


Monkey Troubles

You are a vet trying to shoot a tranquilizer dart into a monkey hanging from a branch in a distant tree. You know that the monkey is very nervous, and will let go of the branch and start to fall as soon as your gun goes off. In order to hit the monkey with the dart, where should you point the gun before shooting?

A) Right at the monkeyB) Below the monkeyC) Above the monkey


Classical Mechanics Lecture 3

Today’s Concepts:a) Relative motionb) Centripetal acceleration

“Lecture Thoughts” Responses● Numerical Examples!● Reference frames

● Sign conventions● Graphing

● Centripetal acceleration● Derivations● Angular velocity

Relative Motion

On your equation sheet...


Relative Motion

What is vMike,Ground

?

Follow-up: How would the previous problem change if Mike were running at 3 m/s with

respect to the train, to the left?


Prelecture 3, Question 1


vbelt,ground = 2 m/s

vdog,belt = 8 m/s

A) 6 m/s B) 8 m/s C) 10 m/s

CheckPoint:What is the speed of the dog relative to the ground?


vdog, ground = vdog, belt + vbelt, ground

= (−8 m/s) + (2 m/s) = −6 m/s

+x


vdog,belt = 8 m/s

What is the speed of the dog relative to the ground?

A) 6 m/s B) 8 m/s C) 10 m/s


CheckPoint

What is the speed of the dog relative to the woman?


vdog,belt = 8 m/s

A) 6 m/s B) 8 m/s C) 10 m/s


A) Because she is actually moving and the two vectors are opposite, so together they make 6 m/s

B) Because she is not moving relative to the belt, and the dog is going 8 m/s relative to the belt, the dog is also moving 8 m/s

relative to her C) The dog and woman are running towards each other so when

you add the two velocities together it would be 8+2.

What is the speed of the dog relative to the woman?


vdog,belt = 8 m/s

A) 6 m/s B) 8 m/s C) 10 m/s


Flashcard Question

Moving sidew

alk

2 m/s

1 m/s

4 m

A man starts to walk along the dotted line painted on a moving sidewalk toward a fire hydrant that is directly across from him. The width of the walkway is 4 m, and it is moving at 2 m/s relative to the fire-hydrant. If his walking speed is 1 m/s, how far away will he be from the hydrant when he reaches the other side?

A) 2 m

B) 4 m

C) 6 m

D) 8 m


Three swimmers can swim equally fast relative to the water. They have a race to see who can swim across a river in the least time. Relative to the water, Beth swims perpendicular to the flow, Ann swims upstream at 30 degrees, and Carly swims downstream at 30 degrees.

Who gets across the river first?

A) Ann B) Beth C) Carly

x

y

AnnBeth

Carly

Flashcard Question



Who gets across the river first?

A) Ann B) Beth C) Carly

x

y

AnnBeth

Carly

Flashcard Question


x

y

Flashcard Question


Who gets across the river second?

A) Ann B) Carly C) Both same

Ann Carly

Centripetal Acceleration


A girl twirls a rock on the end of a string around in a horizontal circle

above her head as shown from above in the diagram.

If the string breaks at the instant shown, which of the arrows best represents the resulting path of

the rock?

A

B

C

D

Top view looking down

CheckPoint (& Demo)


Centripetal Acceleration

Example: What is the angular speed of the Earth as it rotates about its axis? What is the linear speed of a person on the equator, due to the Earth's motion? What about in Salt Lake City?

Radius of earth is 6,371 kmSLC is at 40.75 degrees North latitude

What is magnitude of centripetal acceleration?


v(t1) = v

1

v(t2) = v

2

Note that angle between v2 and

v1 is just !

So, a = dv/dt = vd/dt = v = v2/R


We can ignore acceleration due to Earth's rotation since it's small

Example: What would be radius of a sphere, with the same linear speed on its equator as the Earth, for which an object on its surface would experience a centripetal acceleration of g = 9.81 m/s2?


Prelecture Slide: Discuss

lecture 03: relative and circular motion - astronomybelz/phys2210/lecture03.pdf · lecture 03:...

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