Phy 121- Assignment 7
A. 1.
= current position, ω = how fast it’s turning,
= rate it picks up speed.
a. ω: Revolutions per minute is how fast the motor turns.
b. : 30° is the swing’s current position.
c. ω: This is how fast the earth turns.
2.
B. 1. a. angular speed:
b. angular acceleration: Zero. Acceleration is the rate of change, and it says the
45 rpm is constant.
2.
C. 1. Each force acts on a different body. (A exerts a force on B, B exerts an
equal & opposite force on A.) If I bump into you, the reaction that pushes back
on me has no effect on you. Only forces that push on you affect you. So, my
push on you does not get cancelled out by its reaction.
2.
D. First, consider the snowball being thrown:
Initial momentum = final momentum
(65 kg)(2.5 m/s) + (.71 kg)(2.5 m/s) = (65 kg)(v) + (.71 kg)(30 m/s)
162.5 + 1.775 = 65v + 21.3
142.975 = 65v
v = 142.975/65 = 2.20 m/s
(v of thrower.)
Then, consider a little later, when the snowball is caught:
Initial momentum = final momentum
(60 kg)(0 m/s) + (.71 kg)(30 m/s) = (60 kg)(v) + (.71 kg)(v)
21.3 = 60.71v
V = 21.3/60.71 =
.351 m/s
(v of catcher.)
E. 1. The only force on the falling ball is its weight (Earth’s gravity pulling
down on the ball).
The reaction to this is the ball’s gravity pulling up on the Earth. (Earth pulls on
ball, ball pulls on Earth.)
2.
(Objects stick together in a perfectly inelastic collision.)
a. Cons. of x momentum: (90)(5) = (185) vv
vv = 450/ 185 = 2.432 m/s
Cons. of y momentum: (95)(3) = (185)vy
vy = 285/185 = 1.541 m/s
speed (magnitude of v):
direction: arc tan (1.54/2.43) = 32.4° (ans)
ans:2.88 m/s