1. 7
3rd Law – Action-Reaction
When one object exerts a force on another
object, the other object exerts an equal
and opposite force on the first.
Newton’s Laws of Motion
1st Law – Inertia
An object at rest, or moving with a
constant velocity, will remain that way
unless acted upon by an outside force.
2nd Law –
The net force on an object is equal to the
product of the object’s mass and its
acceleration.
Newtown’s laws are valid only in an inertial
(non accelerating) reference frame.
A free body diagram is a figure showing
all the forces acting on an object.
+x
+y
A force, which causes things to accelerate,
is loosely speaking, a push or pull on an
object. Forces are vector quantities
Dynamics 2
Review
Dynamics, cont.
Weight, Normal Force and Tension
Review

2. 8
Weight, Normal Force and Tension
m+y
The weight W of a body is the magnitude
of the net force required to balance the
gravitational force on the body .
m
+y
When ever two surfaces are in contact
there is a force between the surfaces
called the normal force Fn. It is
perpendicular to the surface.
The normal force is what a bathroom type
scale reads. Be careful, the normal force is
not always mg, it may be larger or smaller
depending on the situation.
m
If we neglect friction, all objects near the
surface of the earth fall with the same
acceleration, g = 9.8 m/s2. The force that
causes this is called the gravitational
force Fg and has a magnitude of mg.
When a cord is attached to a body and
pulled taut, the cord pulls the body with a
force T, called the tension, directed away
from the body and along the cord.
m
+y
m
scale reads
the tension.

3. 9
Ex: A box with mass 5 kg is pulled with a
force of F1 = 10N at an angle of q = 30o
and a retarding force F2 = 3N opposes the
motion as shown. (a) What is the
acceleration of the mass? (b) What is the
normal force? (c) If the force F1 is gradually
increased, what force would be required to
lift the box off the ground?
m
(c) FN = 0 when box is off ground(a)
(b)
+x
+y

4. 10
Ex: A box with mass 5 kg slides down a
frictionless incline at an angle of q = 30o.
(a) What is the acceleration of the mass?
(b) What is the normal force?
(c) A horizontal force F1 is then applied
such that it pushes the block up the incline
at a constant speed of 2 m/s. What is the
force F1?
(a) (b)
(c)
(d) What is the normal force now?
(d)

5. 11
Ex: For the following setup. Assume m1 >
m2. (a) What is the acceleration of the
masses in terms of m1, m2, and g? (b)
What is the tension in the string in terms of
m1, m2, and g?
+x
m1
m2
+x
+y
For m1 For m2
substitute in
substitute in
(a)
(b)