phy

1. Some physics topics question
Amitabh Kumar Singh
Velocity
Question:On a cross-country trip, a couple drives 500 milesin 10 hours on the first day, 380 miles
in 8 hours on the second day, and 600 miles in 15 hours on the third day. What was the average
speed for the whole trip?
Solution: The couple drove an overall 1480 milesina 33 hour period.Sotheiraverage speedforthe
whole trip would be
1480/33 =44.84 mph.

2. Acceleration
QUESTION:
A rifle bulletwith a muzzle speedof330 m/s is fireddirectlyinto a special dense material that
stops the bulletin25 cm. Assumingthe bullet'sdecelerationtobe constant, what is its
magnitude?
Thisgraph showshow the bulletisdecelerating.The parabolicshape canbe describedbythe bullet
travelingtothe right.The curve shape iscausedby the slowingdownof the bullet.Itgetsflatterat
the endbecause ittravelsslowerastime goeson.
Thisgraph showsthe slope of the xt andthe startingvelocityof the bulletasthe bulletisslowing
down.The bulletismovingtothe rightand isabove the time-axis.Attime zero,the velocityis330
m/s.
The accelerationisthe slope of velocity.the accelerationisnegativebecause the velocityis
decreasing.Gravityispulingthe bulletdownata constantrate.

3. Projectile motion

4. QUESTION
A quarterback passesa football ata velocityof 50 ft/sat an angle of 40 degreestowardanintended
receiver30 yd. downfield.The passisreleased5ftabove the ground.Assume thatthe receiveris
stationaryandthat he will catch the ball if it comesto him.Will the assbe completed?If not,will the
throwbe longor short?

5. xt- Demonstratesthe horizontalchange,the distance the ball coversinordertoreachthe receiver
30 yd downfield.Gravitydoesn'taffectthe footballsothe ball hasa constant velocityresultingina
straightline
Vx- Demonstratesthe constantvelocitythe ball hadthroughoutthe time itflew tothe receiver.The
velocityof the ball doesn'tchange,whichmakesthe graphlooklike aline.
yt- Showsthe vertical change of the ball.The graph isan upside downparabolabecause thisshows
howthe ball goesup and thendowndue to gravity
Vy- Demonstrateshowthe quarterbackthrew the football ata certainvelocity.How itsloweddown
at its peakpointandthencame back downwithmore velocityheadingtowardsthe ground.The
negative slopedlineiscausedbythe ball deceleratingtowardthe earth.

6. POWER QUESTION
A 60kg womanrunsup a staircase 15m highin20s.
a) How muchpowerdoesshe expend?
b) What isher horsepowerrating?
Solution:
a) P = W/Δt
The time rate of doingwork.The amountof work dividedbychange intime.Inordertofind
w = mgh
Power,we needtoknowthe numberof wattsit tookand the amountof time ittook.Watts
P = mgh/ Δt
Equalsmass timesgravitytimesheight.We were given:
m = 60 kg
p = (60kg)(9.8 m/s)(15m)/20s g = 9.8m/s
h = 15m
Δt = 20s
b) 1 hp= 746 W
440/746 = .59hp
We knowthat 1 hpequals746w, we divide 440 by 746.
CONSERVATION OF ENERGY
A .5kg ball thrownupwardhas an initial Kof 80J.
a) What are it'skineticandpotential energieswhenithas travelled3/4of the distance toits
maximumheight?
b) What isthe ball'sspeedatthis point?
c) What is itspotential energyatitsmaximumheight?
Solution:
a) U at 3/4 height= 3/4 mgh
At the top of the maximumheight,it'spotential energyis80J.At 3/4's of the way,it wouldbe
3/4(80) = 60J 3/4(80).
Since 60J out of the 80J is transferredtopotential energy,thatmeansthe other20J isfor kinetic
energybecause of it's motioninthe air.

7. b) K = .5mv²
v = sqrt(2k/m)
V at 3/4 height= sqrt(2(20)/.5) = 8.9 m/s
We needto re-write the equationforkineticenergy,.5mv²,solvingforv.Since we are
askedforthe velocityat3/4's of the way,we use the kineticenergyatthatpoint.
c) Since there isnomotionat the max point,there will onlybe potentialenergysoitequals80J.
WORK
A particularspringhasa force constantof 2500 N/m.
a) How much workisdone in stretchingthe relatedstringby6.0 cm.
b) How muchmore workisdone instretchingthe springan additional 2.0cm.
Solution:
a) .5kx²
Since workinthiscase equalsthe sprintpotential energy,we solve foritwiththe givenvalues.
.5(2500 N/m)(.06m)²
The work done bythe appliedforce incompressingastringequalshalf of the springconstant times
= 4.5J
the amountof stretchsquared.
b) .5(2500 N/m)(.8m)² - .5(2500 N/m)(.06m)²
8 - 4.5 = 3.5 J
Pullingthe spring2more cm makesit go 8cm away fromstartingpoint.To findthe workdone,we
subtract the amountof work done bypullingit6cm fromwork done bypullingit8cm.