Transcript Sects. 4.5 through 4.8

Chapter 4 – part B
Newton’s Third Law
Reaction forces
Multiple bodies problems
Newton’s Third Law

If an object experiences a force then there is another object that
experiences the opposite force
Reaction forces

The reaction forces appear when an object has
some prohibited positions, The object is confined
in a particular part of the space. Examples:
 A book cannot sink beneath the table
 A door cannot translate, it rotates maintaining
the hinges fixed
 Two objects linked by a rope cannot move apart
by more than the rope length (unless it snaps…)
Solving a problem of
dynamics

Exercise 4.18
18.
A bag of cement of weight
325 N hangs in equilibrium from three
wires as suggested
in Figure P4.18.
Two of the wires
make angles
θ1 = 60.0° and
θ2 = 25.0° with the
horizontal. Find the tensions T1, T2,
and T3 in the wires.
Exercise 4.26
26.
Draw a free-body diagram of a
block that slides down a frictionless
plane having an inclination of
θ = 15.0° (Fig. P4.26). Assuming that
the block starts from rest at the top
and that the length of the incline is
2.00 m, find (a) the acceleration of the
block and (b) its speed when it
reaches the bottom of the incline.
Exercise 4.30
30.
Two objects are connected
by a light string that passes over a
frictionless pulley as shown in
Figure P4.30. Draw
free-body diagrams
of both objects. The
incline is frictionless,
and m1 = 2.00 kg,
m2 = 6.00 kg, and
θ = 55.0°.
Study the motion
Exercise 4.33
33.
In Figure P4.33, the man
and the platform together weigh 950
N. The pulley can be modeled
as frictionless. Determine
how hard the man has
to pull on the rope to
lift himself steadily
upward above the
ground.