Physics 207
Homework Set #3
1) Given that there are 500 Calories in one hamburger, 4186 Joules in 1 Calorie, and 3,600,000 Joule
in 1 kWh, how many hamburgers would it take to have the same energy content of 1 kWh?
3600000 J 1Cal 1Hamburger
= 1.72 Hamburger
1kWh 4186 J 500Cal
1kWh
2) Suppose a person drops a 5 kg bowling ball off of the top of 300 m high building. a) How long
does it take the ball to reach the ground? b) What is the speed of the ball when it reaches the
ground? c) What is the kinetic energy of the ball when it reaches the ground?
t=
2d
2(300m)
=
= 7.82 s
g
9.8m / s 2
v = gt = (9.8m / s 2 )(7.82 s) = 76.7m / s
KE =
1 2
mv = (0.5)(5kg )(76.7m / s) 2 = 14700 J
2
3) Explain how an object can have a constant speed, and yet be accelerating.
An object that is moving on a curved path (e.g. a circle) with a constant speed is accelerating
because its direction is changing.
4) A 1000 kg car runs into a brick wall at 30 m/s. a) What is the momentum of the car before it hits
the wall. b) What is the impulse that makes the car stop? c) If the car stops in 0.2 s, what is the
average force on the car?
pi = mv = (1000kg )(30m / s) = 30,000kgm / s
Δp = ( p f − pi ) = (0 − 30,000kgm / s) = −30,000kgm / s
F=
Δp − 30,000kgm / s
=
= −150,000 N
Δt
0.2s
5) A 2kg rifle files a 0.03 kg bullet with a speed of 500m/s. a)What is the recoil velocity of the rifle?
b)What is the kinetic energy of the bullet? c) What is the kinetic energy of the rifle?
To begin with, total momentum is zero, thus after it fires it must still be zero.
pbullet + prifle = 0 = (0.03kg )(500m / s) + (2kg )(vrifle ) = 0
− (0.03kg )(500m / s)
= −7.5m / s
2kg
1
2
KEbulllet = mbullet vbullet
= (0.5)(0.03kg )(500m / s) 2 = 3750 J
2
1
2
KErifle = mrifle vrifle
= (0.5)(2kg )(7.5m / s) 2 = 56.3J
2
vrifle =