A 90 − kg man walks on a sawhorse, as shown below. The sawhorse is 2.0 m long and 1.0 m high, and its mass is 25.0 kg . Calculate the normal reaction force on each leg at the contact point with the floor when the man is 0.5 m from the far end of the sawforce. ( Hint : At each end, find the total reaction forces first. This reaction force is the vector sum of two reaction forces, each acting along one leg. The normal reaction force at the contact point with the floor is the normal (with respect to the floor) component of this force.)
A 90 − kg man walks on a sawhorse, as shown below. The sawhorse is 2.0 m long and 1.0 m high, and its mass is 25.0 kg . Calculate the normal reaction force on each leg at the contact point with the floor when the man is 0.5 m from the far end of the sawforce. ( Hint : At each end, find the total reaction forces first. This reaction force is the vector sum of two reaction forces, each acting along one leg. The normal reaction force at the contact point with the floor is the normal (with respect to the floor) component of this force.)
A
90
−
kg
man walks on a sawhorse, as shown below. The sawhorse is
2.0
m
long and
1.0
m
high, and its mass is
25.0
kg
. Calculate the normal reaction force on each leg at the contact point with the floor when the man is
0.5
m
from the far end of the sawforce. (Hint: At each end, find the total reaction forces first. This reaction force is the vector sum of two reaction forces, each acting along one leg. The normal reaction force at the contact point with the floor is the normal (with respect to the floor) component of this force.)
8.114 CALC A Variable-Mass Raindrop. In a rocket-propul-
sion problem the mass is variable. Another such problem is a rain-
drop falling through a cloud of small water droplets. Some of these
small droplets adhere to the raindrop, thereby increasing its mass
as it falls. The force on the raindrop is
dp
dv
dm
Fext
=
+
dt
dt
dt
=
Suppose the mass of the raindrop depends on the distance x that it
has fallen. Then m kx, where k is a constant, and dm/dt = kv.
This gives, since Fext
=
mg,
dv
mg = m
+ v(kv)
dt
Or, dividing by k,
dv
xgx
+ v²
dt
This is a differential equation that has a solution of the form
v = at, where a is the acceleration and is constant. Take the initial
velocity of the raindrop to be zero. (a) Using the proposed solution
for v, find the acceleration a. (b) Find the distance the raindrop has
fallen in t = 3.00 s. (c) Given that k = 2.00 g/m, find the mass of
the raindrop at t = 3.00 s. (For many more intriguing aspects of
this problem, see K. S. Krane, American Journal of…
8.13 A 2.00-kg stone is sliding Figure E8.13
F (kN)
to the right on a frictionless hori-
zontal surface at 5.00 m/s when
it is suddenly struck by an object
that exerts a large horizontal
force on it for a short period of 2.50
time. The graph in Fig. E8.13
shows the magnitude of this force
as a function of time. (a) What
impulse does this force exert on
t (ms)
15.0
16.0
the stone? (b) Just after the force stops acting, find the magnitude
and direction of the stone's velocity if the force acts (i) to the right
or (ii) to the left.
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