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Main » Kinematics & Dynamics » Newton's Laws » Lesson I.6.1

### Force - Lesson I.6.1

 Key Terms: Force |Applied Force | Force of Gravity | Weight | Normal Force | Friction Force |

Newton's Laws of Motion explain the relationship
between acceleration and its cause,
force.

Force ()
• Is a push or pull

• May cause the object to change its shape
• May cause the object to accelerate
• May not change the object in any way
• Is a vector quantity

Magnitude = Size + Unit

One Newton is the amount of force required to give a 1 kg mass an acceleration of 1. Direction "Down" is used as the direction gravity pulls.

There are several types of forces. We are only going to discuss 4 basic kinds.

Applied Force ()
• A force which is applied to an object by another object.

 When Shelly is pushing the wheelbarrow across the yard, there is an applied force acting on the wheelbarrow. The applied force is the force exerted on the wheelbarrow by Shelly.

Force of Gravity ()
• The force which the earth, moon, and other massive bodies attracts an object toward itself.  The gravitational force of the Sun on the Earth holds the Earth in its orbit.
• On Earth, all objects experience a "downward" force of gravity.
• The force of gravity on an object is always equal to the weight of the object.

m = mass (kg)
g = acceleration due to gravity ()

Normal Force ()
• Can be called the support force.
• It is exerted on an object which is in contact with another stable object.
• On a level surface, the normal force is equal in magnitude but opposite in direction to the force of gravity.
• The normal force is perpendicular to the surface at which the object is on.
Friction Force ()
• The force exerted by a surface as an object moves across it ( sliding friction) or makes an effort to move across it ( static friction).
• This force is opposite to the motion of the object.
• The force of friction is equal in magnitude but opposite in direction to the applied force when the object is moving with constant velocity.
• Friction depends on the nature of the two surfaces interacting and the force pushing the two surfaces together ().

= coefficient of friction ( tables can be found in various textbooks)  Curling rock on ice. Which would have the higher coefficient of friction? Discuss with you classmates. Sandpaper on wood.

#### Free-Body Diagrams

• Used to analyze situations involving more than one force acting on an object.
• The object is represented as a square.
• The forces acting on the object are drawn from its center.
• The size and direction of the vector represents the size and direction of the force.
• Include a scale and reference coordinates.
• Vector addition can then be used to determine the net force.

 Scale: 1 cm represents 1000 N

Ex: A clock at rest on a table.

A forward force is applied to a large box in order to move it across the floor with constant velocity.