We just finished a section on friction

Now we’re studying circular motion

In studying circular motion:• students often don’t understand what actual

forces can fill the role of centripetal force.• this activity shows that friction can be the

cause of centripetal acceleration (as can gravity, normal force, string tension, etc).

• We play some Pink Floyd first. Gets kids thinking that record players may be useful for something.

Using our classroom record player:

we show that friction force can be the centripetal force.

• Take measurements and calculate centripetal force and friction force

• Convert rotational to linear velocity( v =𝛱 r / t²

• Determine the coefficient of friction (𝜇)

What you need

• Turntable, an LP record• Construction paper, compass meter stick,

button• Mark paper with concentric circles ( I space

circles every 2 centimeters)• Place paper over record on turntable• Set turntable speed to 45 RPM

Find the distance from center at which button starts to slide off.

• Place button a small distance from center of record.

• Turn player on.• If button doesn’t slide of, repeat, moving

button further outward until it DOES slide off.• Record this distance from center .

At this distance,fsmax = Fc

• Therefore

To calculate linear velocity of button:

• Measure distance (r) of button from center

• Determine RPM of player

To calculate coefficient of static friction:

• Since static friction force equals centripetal force at this point,

• And then,

Skills developed:• Understand that different forces can

cause centripetal acceleration• Convert rotational velocity to linear

velocity• Find coefficient of static friction