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CSULA (SOLUTION) MIDTERM PHYSICS 211 Fall 2007

I) CONCEPTUAL QUESTIONS

1. Consider the following controls in an automobile:gas pedal, brakes, andsteering wheel. Thecontrols in this list that cause an acceleration of the car are:(e)All three controls

Gas pedal causes the car to speed up. The brakes cause the car to slow down. The steering wheel

changes the direction of the velocity vector.

2. If vectorAis added to vectorB which twoconditions must be satisfied in order for the resultant

vector to be equal zero?

(a)AandBare parallel and in opposite direction(b)AandBhave the same magnitude.

3. After a ball is thrown upward and is in the air, its acceleration(e)Remains the same

For the entire time interval that the ball is in the air the acceleration is that due to gravity.

4. ou push an ob!ect, initiall" at rest, across a frictionless floor with a constant force for a time intervalt

resulting in a final speed of vfor the ob!ect. ou repeat the e#periment, but with a force that is twice as

large. $hat interval is now required to reach the same final speed v?(d)t !"

#ith twice the force the ob$ect will e%perience twice the acceleration. &ecause the force is

constant the acceleration is constant and the speed of the ob$ect (starting from the rest) is given byv = at. #ith twice acceleration the ob$ect will arrive at speed vat half the time.

II) Sol! "#! $ollo%&' *o+l!,-

1. (10 "-). A person wal%s first at constant speed of 5.00 m/salong a straight line from pointAto pointB

and then bac% along the line fromBtoAat a constant speed of 3.00 m/s. &ind the average speedover the

entire trip.

'et drepresent the distance between A and (. 'et 1tbe the time for which the wal%er has the higher

speed in1

5.00 ms d

t= .

'et 2trepresent the longer time for the return trip in2

3.00 ms d

t = .

Then the times are ( )15.00 ms

dt= and ( )

23.00 ms

dt= .

The average speed is:

( ) ( )( )

( )

( )

2 2

8.00 ms5.00 ms 3.00 ms

15.0 m s

2 2

Total distance 2

Total time

215.0 m s3.75 ms

8.00 ms

d d d

d d dv

v

+= = =

+

= =

)idterm &all*+ -

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2. (10 "-). A ball is dropped from rest from a height habove the ground. ow long does the ball ta%e toreach a height h/2?

/osition at an" time for ball:0

0-0

0- gtgttvyy iif =+=

1eplacing: hy f 0-=

=

0

0-

0-

gth gh

t =

3. (10 "-). 2iven the following displacements with rectangular coordinates:(3/ 2) m (/ 3)mand (/ 1)m

A) 3btain a unit-vectore#pression for the resultant displacementof the three vectors.

( ) ( ) mmmmd 45,064-,567,80,7 =++= .

In unit9vector notation: ( )m$id :5:0 +=

) ;etermine the magnitudeand directionof the resultant displacement.

)agnitude: mmmd -+0

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gmF AA+ = and 5!6 !P !T mAa

") sing second Bewton*s law, write the equation of motion for m. (3 "-).

gmF &&+ = and 5!6 !T ma

#) sing parts and C, find the acce$eration of each bo#. ( "-).

!P !T mAa 40 N !T %0a

!T ma !T %2a

40 N 22a a 40822 ,8-2 1.92 ,8-2

&) sing parts and C, find the tensionin the cord connecting the bo#es. ( "-).

!T ma !T %2(%.'2) = 2%.'

. (20 "-). The s"stems shown in the figure are in equilibrium. If the spring scales are calibrated in newtons6N).$hat do the spring scales read? 6Assumefrictionless, and neglectthe masses of the pulle"s4

0m= + + = F n T g Take the component along the incline

0x x xm+ + =n T g

( )

0 sin30.0 0

5.009.80sin30.0

2 2

24.5 N .

T mg

mgT mg

+ =

= = =

=

Isolate either mass0

.

T mg ma

T mg

+ = =

=

The scale reads the tensionT.

( )25.00 kg9.80 ms 49.0 NT mg= = = .

)idterm &all*+ 7