Weight is a force that is defined from the gravitational attraction between two masses. The gravitational force causes the less massive object to accelerate towards the more massive object.

mgFW g Near the surface of the Earth the acceleration due to gravity is considered constant, therefore we can directly relate mass and weight using this acceleration.

This relationship assumes that the only force acting on the object is gravity. The object may actually have a different acceleration if there were other forces acting on it.

Newton’s Third Law of Motion

When an external force is applied to an object it means that there is an interaction between the two different objects.

Newton’s Third Law of Motion• For every action there is an equal but opposite reaction.• If two objects interact, the force F12 exerted by object 1 on object 2 is equal in magnitude and opposite in direction to the force F21 exerted by object 2 on object 1.

2112 FF

This law requires you to examine the interaction between two objects!

The action force is the applied force.The reaction force is the force due to the object resisting the imposed change.

Example: A book rests on a table. What are all the action reaction force pairs for this situation?

Earth

• The book pushes down on the table the table pushes up on the book.

• The table pushes down on the Earth and the Earth pushes up on the table.

• The attractive gravitational force between: Earth and book, Earth and table and book and table.

When two objects are placed in contact and one is supporting the other the force perpendicular to the supporting surface is called the Normal Force.E.g.: The contact force the table exerts on the book, to support the book.

A free body diagram is a diagram that show how all forces are applied to an object. This can include an outline of the object for forces applied at a specific location on the object, but we will be primarily considering forces applied to the object as a whole and therefore draw all forces from a point in the center of the object.

Free-Body Diagrams

Example: A 50 g hockey puck is sliding across a “frictionless” ice surface at a constant speed of 12 m/s.

a) Draw a free-body diagram showing all the forces acting on the hockey puck.b) What acceleration is the hockey puck undergoing?c) What is the magnitude of the normal Force?

N

W

v = 12 m/s

N

W

a)

b) The velocity along each direction is constant, therefore the acceleration of the hockey puck is zero.

c)amF

xx amF

yy amF

xam

0 0xa

yamWN

0

0 mgN mgN N49.0

x

y