4 – Forces & the Laws of Motion
4-1 Changes in Motion
 
When we imagine a force, we usually imagine a push or
pull exerted on some object.
 
Force represents the interaction of an object with its
environment.
 
Force is the cause of acceleration or the change in the
object’s velocity.
 
The SI unit of force is the Newton.
 1 N = 1 kg·m/s2

Contact Vs. Field Force
 
Forces can act through contact or at a distance.
 
Contact forces result from physical contact between two
objects.
 
This kind of force is usually easy to identify when you
analyze a situation.
 
Another class of forces—called field forces—does not
involve physical contact between two objects.
 
The force of gravity and electrical forces are examples.
Force Diagrams
  The effect of a force on an object depends on the
magnitude and direction of the force.
  Force is a vector.
  Diagrams that show force vectors as arrows are called
force diagrams.
  A free-body diagram helps analyze a situation.
  Refer to fig. 4-3 on p. 126.
Free-body Diagrams
  Free-body diagrams isolate an object and the forces acting
on it.
  Figure 4-4 on p. 127, 128 illustrates the steps for drawing a
free-body diagram.
4-2 Newton’s First Law
 
An object at rest remains at rest, and an object in motion
continues in motion with constant velocity (that is, constant
speed in a straight line) unless the object experiences a net
external force.
 
Inertia is the tendency of an object not to accelerate.
 
In other words, when the net external force on an object is
zero, its acceleration is zero.
Net External Force
 
Acceleration is determined by net external force.
 
The net external force is the vector sum of all the forces
acting on an object.
 
When all external forces acting on an object are known,
the net external force can be found using the methods for
finding resultant vectors.
 
Tug-of-war is a good example of net forces.
 
Refer to sample and practice 4A on pp. 132, 133.
Mass & Inertia
 
Mass is a measurement of inertia.
 
The inertia of an object is proportional to its mass.
 
Seatbelts are placed in cars because of the laws of inertia
and holds the passenger firmly in place in the event of a
collision.
 
A seat belt may also lock when a car rapidly slows down

or turns a corner.
 Fig. 4-14 illustrates how one type of shoulder harness
operates.
Equilibrium
 
Objects that are either at rest or moving with constant velocity are
said to be at equilibrium.
 
The force that brings an accelerating object into
equilibrium must be equal and opposite to the force causing
the object to accelerate.
 
An object is in equilibrium when the vector sum of the
forces acting on it is equal to zero.
4-3 Newton’s 2nd and 3rd Laws
 
Force is proportional to mass and acceleration.
 
Newton’s 2nd law – the acceleration of an object is directly
proportional to the net external force acting on the object and
inversely proportional to the object’s mass.
 
Fnet = F = ma where “a” is the acceleration in m/s2, “m”
is the mass in kg, and F represents the vector sum of all
external forces acting on the object.
 
Refer to sample and practice 4B on pp. 137, 138.
Newton’s 3rd Law
  Forces always exist in pairs.
rd
  Newton’s 3 law – if two objects interact, the magnitude
of the force exerted on object 1 by object 2 is equal to the
magnitude of the force simultaneously exerted on object 2 by
object 1, and these two forces are opposite in direction.
 
In other words, for every action there is an equal and
opposite reaction.
 
An action-reaction pair is a pair of simultaneous equal


but opposite forces resulting from the interaction of two
objects.
 Action and reaction forces each act on different objects.
 Field forces also act in pairs.
4-4 Everyday Forces
 
The force of gravity exerted on the ball by Earth, Fg, is a
vector quantity, directed toward the center of Earth.
 
The magnitude of this force, Fg, is a scalar quantity called
weight.
 
When the mass and the acceleration due to gravity are
known, the weight of an object can be calculated using Fg =
mg where g = 9.81 m/s2.
The Normal Force
 
A force exerted by one object on another in a direction
perpendicular to the surface of contact is a normal force.
 
FN = mgcos where  represents the angle between the
normal force and a vertical line, and also the angle between
the contact surface and a horizontal line.
 
The normal force is always perpendicular, but is not
always opposite in direction to the force of gravity.
The Force of Friction
 
Friction opposes the applied force.
 
The resistance force that keeps an object from moving
from rest is called static friction.
 
The resistive force that opposes the relative motion of two
contacting surfaces that are moving past one another is known
as kinetic friction.
 
Refer to fig. 4-21 on p. 142.
Friction
 The force of friction is proportional to the normal force.
  Friction depends on the surfaces in contact.
  It is easier to push a desk across a tile floor than across a
floor covered with thick carpet.
  Refer to fig. 4-22 on p. 143.

Coefficient of Friction
 
The quantity that expresses the dependence of frictional
forces on the particular surfaces in contact is called the
coefficient of friction.
 
The coefficient of friction is the ratio of forces.
 
The coefficients of static friction are always greater than
kinetic friction.
 
Refer to table 4-2 on p. 144.
  Air resistance is a form of friction.
  Refer to sample and practice 4C on p. 145 and sample and
practice 4D on pp. 146, 147.