Force

Purpose Analyze force vectors in several situations of suspended

masses.

Predicting x and y components of forces (Free body diagrams: Done in the homework).

Measuring x and y components of the forces.

Encounter static and dynamic equilibrium situations.• Static: acceleration = 0• Dynamic: acceleration 0

Force

Equipment

Two force sensors (static situations) One force sensor and one photo gate (dynamic situation)

Force

Static Suspension of Mass on Strings

Force Sensors must be aligned with strings for proper measurement of the force.

Force sensors must be aligned and mass must be temporarily removed while taring the sensor.

Taring must be redone whenever force sensor is realigned.

Tare button is on the side of the force sensor.

tare buttons

Force

WRONG (not aligned)

Force sensor will notmeasure the propertension in the string.

Force

Dynamic Situation (Swinging Mass)

Photogate at low point of motion

Force sensor oriented so string is aligned at low point of motion.

Photogate triggers force measurement at the right moment and measures velocity (actually measures time and calculates v).

Force

Dynamic Situation: Free Body Diagram

At the lowest point of the swing:

y

xW=mg

T

cy mamgTF

0xF

r

ac = v2/r (direction of a is in the direction of the net force)

(Note: This is just Newton’s second law: F = ma.Only T and W are forces! “ m ac“ is not a force!)

mg cos q

mg

mg sin q

q

Q

Force

Data acquisitionstarts here. Thisis also the momentin which the massswings throughthe photo gatefor the first time.

Follow instructions

Hit the “Scale toFit” button hereif you cannot seeoscillations (F <0 if done correctly).

Force

Hints

In the static situations (no acceleration) we always have:

In the free body diagram the forces must always add up to zero.

For the dynamic situation you need only one force sensor: It must be the force sensor that is plugged into port A of the 750 interface.

Tare the force sensor before you hang the swinging mass on it.

00 yx FandF

Force

Hints

100)(

)()(%

acceptedg

measuredgacceptedgdifference

In Part I you will be asked to calculate the % difference between your measured g and the accepted value of g (use 9.8 m/s2 for accepted value):

100)(

)()(""%

calculatedT

measuredTcalculatedTyuncertaint

In Part III you will also be asked to calculate the % uncertainty in your measurement of tension. With this we actually meant again the %deviation from the expected (calculated) value of T:

Force

Hints: Measuring the Angle of the String

String

90°60°

30°

0°

small weight (acts as plumb level)

Q

angle measured -90 In this case:

3060 -90

Protractor

Force

Hints: Measuring the Angle of the String – Alternative Method

String

90°

60° 30°

0°

Q

angle measured In this case:

30