Forces: Changes in
Motion
Holt Physics
Chapter 4 section 1
Pages 123-129
What is Force?
 Force – the cause of acceleration or the
cause of a change in motion.
 a “push” or “pull”
Units of Force
 SI unit of force  Named
for Sir Isaac Newton
 Newton (N) = 1 kg x m/s2
 The amount of force that
causes a 1 kg object to
accelerate at 1 m/s2
What Forces Equal
Pounds = force  action by
gravity on mass
1 N = .225 lbs
* Force due to gravity = 14,700N
Types of Forces
1. Contact Forces - pushes and pulls
resulting from physical contact between 2
objects.
Examples – tug of war, shove in the back,
pushing a box
Contact Forces
Types of Forces
2. Field Force - force existing between objects
even with no physical contact.
Examples – Gravity, electrical charges (+ & -)
Field Forces
Diagrams
1. Force Diagrams
 A diagram of objects involved in a situation
and the forces exerted on objects.
- Sample diagram on page 126 – figure 4-3a
(next slide)
Diagrams
Diagrams
2. Free body diagram
 A diagram showing only the force acting
on a specific object (of interest)
- Sample diagram on page 126 – figure 4-3b
(next slide)
Diagrams
Homework:
1. Read p. 129 and then write:
 1 paragraph reaction (at least 5 sentences)
 1 paragraph summary (at least 5 sentences)
2. Page 128 #1-6
3. Read p. 123-129
4. Read p. 130-135