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Using the “Clicker”

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Using the “Clicker”

If you have a clicker now, and did not do this last time, please enter your ID in your clicker.

First, turn on your clicker by sliding the power switch, on the left, up. Next, store your student number in the clicker. You only have to do this once.

Press the * button to enter the setup menu.

Press the up arrow button to get to ID

Press the big green arrow key

Press the T button, then the up arrow to get a U

Enter the rest of your BU ID.

Press the big green arrow key.

Moving with the Earth

Remember: for uniform circular motion, the acceleration has magnitude v2/r = w2r, where w =v/r = 2p/T

So let’s calculate the acceleration:

caused by the Earth’s rotation about its axis.

caused by the Earth’s orbit around the Sun.

Note: 1 yr = 60*60*24*365 s = 3.15 x 107 s = p x 107 s (good approx)

rorb = 1.50 x 1011 m, rrot = 0.7(6.38x106 m) = 5 x 106m

worb = 2p/T = 2 x 10-7 rad/s,wrot = 365 worb = 7.30 x 10-5 rad/s

agrav = g = 9.8 m/s2

arot= w2r = (50 x 10-10)(5 x 106) = 0.025 m/s2

aorb = w2r = (4 x 10-14)(1.5 x 10-11) = 0.006 m/s2

Gravitron (or The Rotor)

In a particular carnival ride, riders are pressed against the vertical wall of a rotating ride, and then the floor is removed. Which force acting on each rider is directed toward the center of the circle?

A normal force.

A force of gravity.

A force of static friction.

A force of kinetic friction.

None of the above.

Gravitron (see the worksheet)

Gravitron simulation

Sketch a free-body diagram for the rider.

Apply Newton’s Second Law, once for each direction.

Gravitron (work together)

FS

He’s blurry because he is going so fast!

Axis of rotation

FN

a

y

mg

x

Sketch a free-body diagram for the rider.

Apply Newton’s Second Law, once for each direction.

y direction: FS- mg = may = 0 (he hopes)

x direction: FN = max = m (v2/r)

Vertical circular motion

Examples

Water buckets

Cars on hilly roads

Roller coasters

Ball on a string

When a ball with a weight of 5.0 N is whirled in a vertical circle, the string, which can withstand a tension of up to 13 N, can break.

Let’s see how to answer questions such as:

Why?

Where is the ball when the string is most likely to break?

What is the minimum speed of the ball needed to break the string?

Ball on a string – free-body diagrams

Sketch one or more free-body diagrams, and apply Newton’s Second Law to find an expression for the tension in the string.

At the topAt the bottom

Ball on a string – free-body diagrams

Do the bottom first

Assume same speed ma = mv2/r = 8 N

Breaks at T = 13 N

mg = 5 N

So, T = 3 N

So, ma = mv2/r = 8 N when it breaks

Sketch one or more free-body diagrams, and apply Newton’s Second Law to find an expression for the tension in the string.

At the topAt the bottom

(Actually, v will be smaller at the top)

Weight = mg = 5 N

T + mg = mv2/r

T = mv2/r – mg

T – mg = mv2/r

T = mv2/r + mg

A water bucket

As long as you go fast enough, you can whirl a water bucket in a vertical circle without getting wet.

What is the minimum speed of the bucket necessary to keep the water in the bucket?

The bucket has a mass m, and follows a circular path of radius r.

If you go too slow, the string will go slack, and the water and the bucket will stay together along a parabolic free fall path.

Free-body diagram for the water bucket

FN on water, or T on bucket + water

Ma = Mv2/r

Mw g or Mb+wg

Sketch a free-body diagram for the bucket (or the water), and apply Newton’s Second Law.

Mg + FN = Mv2/r

But critical speed is when FNor T= 0

So Mg = Mv2min/r or vmin= (rg)1/2

“Toward center” is down

Roller coaster

On a roller coaster, when the coaster is traveling fast at the bottom of a circular loop, you feel much heavier than usual. Why?

Draw a free-body diagram and apply Newton’s Second Law.

Roller coaster

On a roller coaster, when the coaster is traveling fast at the bottom of a circular loop, you feel much heavier than usual. Why?

Draw a free-body diagram and apply Newton’s Second Law.

FN

ma = m(v2/r)

FN – mg = mv2/r so

FN = mg + mv2/r

The faster you go, the larger the normal force has to be. The normal force is equal to your apparent weight.

mg

Driving on a hilly road

As you drive at relatively high speed v over the top of a hill curved in an arc of radius r, you feel almost weightless and your car comes close to losing contact with the road. Why?

Draw a free-body diagram and apply Newton’s Second Law.

r

Driving on a hilly road

Warning to drivers: Your braking is worst at the crest of a hill.

As you drive at relatively high speed v over the top of a hill curved in an arc of radius r, you feel almost weightless and your car comes close to losing contact with the road. Why?

Draw a free-body diagram and apply Newton’s Second Law.

Mv2/r

FN -> 0

Mg

FN – Mg = M(-v2/r) loses contact when FN = 0 at v = (rg)1/2