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Chapter 4, Section 1
Changes in Motion
Pg. 124 -128
What is a force?
A force causes an object to change its
It can cause:
- stationary objects to move.
- moving objects to stop.
- moving objects to change its direction.
 Force
is expressed in newtons (N).
Sir Isaac Newton (1642-1727)
“The amount of force that, when acting
on a 1kg mass, produces an acceleration
of 1m/s².”
1 N = 1 kg·m/s²
Types of Forces
1. Contact Forces – a force that results
from the physical contact of two
ex. pushing, pulling, hitting
Types of Forces
2. Field Forces – a force that can exist
between objects, even in the absence of
physical contact between the objects.
ex. gravitational force (weight), attraction or repulsion
between electrical or magnetic charges
Weight is also a force
 There
is an interaction between an
object’s mass and gravity.
 Weight
is a measure of the magnitude
of the gravitational force exerted on the
Weight (w) = (mass) (gravity)
1 lb = 4.448 N
1 N = 0.225 lb
Sample problem
Find the weight on earth of a body
whose mass is 3kg.
w = (3 kg) (9.81 m/s²)
29.4 N
Weight in N = _____
29.4 N x 0.225 lb/1N
6.62 lb
Weight in lb = _____
Pg. 128 #1 & 2
An object has a mass of 300g.
a. What is its weight on earth?
b. What is its weight on the moon if
g = 1.6 m/s² ?
Measure your weight in lbs. and determine your
mass in kilograms.