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Chapter 5: Force and Motion – I
Reading assignment: Chapter 5.5-5.8
Homework : (due Wednesday, Sept. 14, 2005)
Chapter 5: 34, 38, 39, 48, 58, 66
• In this chapter we will learn about the relationship
between the force exerted on an object and the
acceleration of the object.
• Forces
• Newton’s three laws.


II. F  m  a
Contact forces
- Involve physical contact between
_________________.
Field forces:
-No _______________________
between objects
- Forces act through __________
space
gravity
electric
magnetic
Measuring forces
- Forces are often measured by determining the ___________________ of a
calibrated spring.
- ______________ are vectors!! Remember vector addition.
- To calculate ________ force on an object you must use vector addition.
Newton’s first law:
In the absence of external forces (no net force):
• an object at rest __________ at rest
• an object in motion continues in motion with constant
_____________ (constant ____________, straight line)
(assume no _____________).
Or: When no force acts on an object, the acceleration of
the object is ____________________.
Inertia: Object resists any attempt to change its _____________
Inertial frame of reference:
-A frame (system) that is not ____________________.
- Newton’s laws hold true ____________ in non-accelerating
(inertial) frames of reference!
Are the following inertial frames of reference:
- A cruising car?
- A braking car?
- The earth?
- Accelerating car?
Mass
- Mass of an object specifies how much inertia the object
has.
- Unit of mass is ___________.
- The greater the mass of an object, the _____________
it accelerates under the action of an applied force.
- Don’t confuse mass and ______________ (more later).
Newton’s second law
(Very important)
The acceleration of an object is:
- directly ____________ to the net force acting on it
- and ___________ proportional to its mass.


Fnet  m  a

 F
a
m
Fnet , x  m  a x Fnet , y  m  a y Fnet , z  m  a z
Unit of force:
• The unit of force is the Newton (1N)
• One Newton: The force required to _______________ a 1
kg mass by 1m/s2.
• 1N = 1______/s2
Black board example 5.1
F2 = 8.0 N
q2 = 60°
Two forces act on a hockey puck
(mass 0.3 kg) as shown in the
figure.
F1 = 5.0 N
q1 = - 20°
(a) Determine the magnitude and direction of the net force acting
on the puck
(b) Determine the magnitude and the direction of the pucks
acceleration.
(c) What third force (direction and magnitude) would need to be
applied to the puck so that its acceleration is zero?
The force of gravity and weight
• Objects are attracted to the Earth.
• This attractive force is the force of gravity Fg.


Fg  m  g
• The ________ of this force is called the weight of the object.
• Thus, the weight of an object is _______ (Force required to keep
mass m from falling to the ground).
The weight of an object can very with _____________ (less weight on the
moon than on earth, since g is smaller).
The mass of an object does not vary.
Don’t confuse mass and weight