Download Sects. 4.1 through 4.4

Chapter 4 – part A
Newton’s First Law
Newton’s Second Law
The Dynamics

Question
A ball is held in a person’s hand. a) Identify all the external forces acting on
the ball and the reaction to each. b) If the ball is dropped, what force is
exerted on it while it is falling? Identify the reaction force in this case.
Exercise 4.3

3. A 3.00-kg object undergoes
an
acceleration given by

a  ( 2.00iˆ  5.00ˆj) m/s2. Find the
resultant force acting on it and
the magnitude of the resultant
force.
Exercise 4.4

ˆ ˆ
4.
 Two forces, F1  (6i  4 j) N and
F2  ( 3iˆ  7 ˆj) N, act on a particle of
mass 2.00 kg that is initially at rest at
coordinates (–2.00 m, +4.00 m).
(a) What are the components of the
particle’s velocity at t = 10.0 s?
(b) In what direction is the particle
moving at t = 10.0 s?
(c) What displacement does the
particle undergo during the first 10.0
s?
(d) What are the coordinates of the
particle at t = 10.0 s?
Exercise 4.6
6.
A 3.00-kg object is moving in
a plane, with its x and y coordinates
given by x = 5t 2 – 1 and y = 3t 3 + 2,
where x and y are in meters and t is
in seconds.
Find the magnitude of the net force
acting on this object at t = 2.00 s.
Exercise 4.8
8.

Three forces,
 given by
F1  (2.00iˆ  2.00ˆj) N, F2  (5.00iˆ  3.00ˆj) N,

and F3  (45.0iˆ) N act on an object to
give it an acceleration of magnitude
3.75 m/s2.
(a) What is the direction of the
acceleration?
(b) What is the mass of the object?
(c) If the object is initially at rest, what
is its speed after 10.0 s?
(d) What are the velocity components
of the object after 10.0 s?
Exercise 4.20
20.
Figure P4.20 shows a worker poling a boat—a
very efficient mode of transportation—across a shallow
lake. He pushes parallel to the length of the light pole,
exerting on the bottom of the lake a force of 240 N. The
pole lies in the vertical plane containing the keel of the
boat. At one moment the pole makes an angle of 35.0°
with the vertical and the water exerts a horizontal drag
force of 47.5 N on the boat, opposite to its forward
motion at 0.857 m/s. The mass of the boat including its
cargo and the worker is 370 kg. (a) The water exerts a
buoyant force vertically upward on the boat. Find the
magnitude of this force. (b) Model the forces as
constant over a short interval of time to find the velocity
of the boat 0.450 s after the moment described.