Newton

1
Newtons Second Law

• The net force on a body is equal to the product
  of the bodys mass and its acceleration.

2
Units
Problem 20   A car traveling at 53 km/h hits a
bridge abutment. A passenger in the car moves
forward a distance of 65 cm (with respect to the
road) while being brought to rest by an inflated
air bag. What magnitude of force (assumed
constant) acts on the passengers upper torso,
which has a mass of 41 kg?
3
Gravitational Force and Weight

• Gravitational force is the force that the
  Earth exerts on any object. It is directed toward
  the center of the Earth.
• The magnitude of the gravitational force is
  equal to the product of mass and acceleration due
  to gravity.

The weight W of a body is the magnitude of the
net force required to prevent the body from
falling freely, as measured by someone on the
ground.
The weight W of a body is equal to the magnitude
of the gravitational force on the body. A bodys
weight is related to the bodys mass by,
4
Contact Forces As the name implies, these
forces act between two objects that are in
contact. The contact forces have two components
one that is acting along the normal to the
contact surface (normal force) and a second
component that is acting parallel to the contact
surface (frictional force).
Normal Force When a body presses against a
surface, the surface deforms and pushes on the
body with a normal force perpendicular to the
contact surface. An example is shown in the
picture to the left. A block of mass m rests on
a table. Note In this case FN mg. This is
not always the case.
Friction If we slide or attempt to slide an
object over a surface, the motion is resisted by
a bonding between the object and the surface.
This force is known as friction. More on
friction in Chapter 6.
(5-7)
5

• Tension This is the force exerted by a rope or
  a cable attached to an object. Tension has the
  following characteristics
• It is always directed along the rope.
• It is always pulling the object.
• 3. It has the same value along the rope (for
  example, between points A and B).
• The following assumptions are made
  
• The rope has negligible mass compared to the mass
  of the object it pulls.
• The rope does not stretch.
• If a pulley is used as in fig.(b) and fig.(c), we
  assume that the pulley is massless and
  frictionless.

A
B
6
Newtons Third Law
When two bodies interact by exerting forces on
each other, the forces are equal in magnitude and
opposite in direction.

7
Recipe for the Application of Newtons Laws of
Motion
1. Choose the system to be studied. 2. Make a
simple sketch of the system. 3. Choose a
convenient coordinate system. 4. Identify all the
forces that act on the system. Label them on the
diagram. 5. Apply Newtons laws of motion to the
system.
P 17, page 109In the figure , let the mass of
the block be 8.5 kg and the angle ? be 30. Find
(a) the tension in the cord and (b) the normal
force acting on the block. (c) If the cord is
cut, find the magnitude of the resulting
acceleration of the block.
8
Sample Problem
Figure 5-12 shows a block S (the sliding block)
with mass M 3.3 kg. The block is free to move
along a horizontal frictionless surface and
connected, by a cord that wraps over a
frictionless pulley, to a second block H (the
hanging block), with mass m 2.1 kg. The cord
and pulley have negligible masses compared to the
blocks (they are massless). The hanging block H
falls as the sliding block S accelerates to the
right. Find (a) the acceleration of block S, (b)
the acceleration of block H, and (c) the tension
in the cord.