Force

1
Force

• Chapter 4, Section 1
• Changes in Motion
• Pg. 124 -128

2
What is a force?

• A force causes an object to change its
  velocity/acceleration.
• It can cause
• - stationary objects to move.
• - moving objects to stop.
• - moving objects to change its
  direction.

3
Force

• 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 kgm/s²

4
Types of Forces

• 1. Contact Forces a force that results from the
  physical contact of two objects.
• ex. pushing, pulling, hitting

5
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

6
Weight is also a force

• There is an interaction between an objects mass
  and gravity.
• Weight is a measure of the magnitude of the
  gravitational force exerted on the object.
• Weight (w) (mass) (gravity)
• 1 lb 4.448 N
• 1 N 0.225 lb

7
Sample problem

• Find the weight on earth of a body whose mass
  is 3kg.
• Weight in N _____
• Weight in lb _____

w (3 kg) (9.81 m/s²)
29.4 N
29.4 N x 0.225 lb/1N
6.62 lb
8
Assignment

• Pg. 128 1 2
• And
• 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.