FORCE – CAUSES CHANGES IN MOTION Physical Science Contact vs. Noncontact Instructions: As you go through this Power-point, you will want to take notes in your notebook over what you have learned about FORCE (LOOK at the OBJECTIVES on the next slide to help you know what to write down). At the end of this presentation, you will make a summary of what you have learned and turn it into me by the end of class TODAY! October 2, 2018

OBJECTIVES ESSENTIAL QUESTION Define Force. Distinguish between contact and noncontact forces. Explain how to make force diagrams. ESSENTIAL QUESTION What forces may hinder the motion of a soccer ball rolling across a field?

FORCE: A PUSH OR PULL Contact Force Non-Contact Force A force that can cause or change motion of an object by touching it. Motors and gears operate through contact forces. Force that results when the two objects are not in physical contact with each other, yet are able to exert a push or pull. Non-Contact Force Examples: Gravitational Force Electrical Force Magnetic Force Contact Force Examples: Friction Normal Force Air Resistance Applied Force

https://youtu.be/Qngs_o8-FMk UNITS OF FORCE https://youtu.be/Qngs_o8-FMk

CONTACT FORCES Applied Force: A force typical push or pull, pushing a door open, pulling a rope. Friction: Happens when two objects rub against each other. Friction opposes motion. Normal Force: The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. Air Resistance: The air resistance is a type of frictional force that acts upon objects as they travel through the air.

CONTACT FORCES Normal Force= 10N

NON-CONTACT FORCES Magnetic Force: A kind of non contact force, materials such as iron, nickel, cobalt and other magnets just need to be near the magnet to be attracted or repelled. Electric Force: Noncontact force of electrical charges. Gravitational Force: The force of attraction between any two objects in the universe. All objects have gravitational force. The mass of an object and its distance determines the strength of its gravitational force.

GRAVITY Mass and distance affect the force of gravity. The greater the mass and the closer the object, the stronger its gravitational pull.

GRAVITY How does the force of gravity change as the mass of objects and distance between them changes? The force of gravity between two objects depends on two factors: Mass Distance *Longer arrows represent a greater force of gravity.

MASS VS. WEIGHT The amount of matter in an object. Unit: Grams Does not depend of gravity. Measured with a triple beam or digital balance. Measure of the Earth’s gravitational pull (a force). measured in lbs. or N Changes depending on the gravitational force. Measured using a spring scale.

FREE-BODY (FORCE) DIAGRAMS Free-body (FORCE) diagrams are pictures that show the size and direction of all forces acting on an object.

FREE-BODY (FORCE) DIAGRAMS Steps to drawing a free body diagram Pick one object to analyze Draw a box to represent the object Draw an arrow to represent each force acting on the object Make sure the arrow shows the direction and relative size of the force

Force Symbol Definition Direction Friction Ff The contact force that acts to oppose sliding motion between surfaces Parallel to surface & opposite direction of sliding Normal FN The contact force exerted by a surface on an object Perpendicular to & away from the surface Spring Fsp A restoring force, that is, the push or pull a spring exerts on an object Opposite the displacement of the object at end of spring Tension FT The pull exerted by a string, rope, or cable when attached to a body & pulled taut Away from object & parallel to spring, rope or cable at point of attachment Thrust Fthrust A general term for the forces that move objects such as rockets, planes, cars & people In same direction as acceleration of object Weight Fg A long-range force due to gravitational attraction between two objects, generally Earth & an object Straight down toward center of Earth

EXAMPLE PROBLEM 1 A book is at rest on a table top. Diagram the forces acting on the book.

Example Problem 1 In this diagram, there are normal and gravitational forces on the book.

The forces are balanced (they cancel each other out) EXAMPLE PROBLEM 1 The forces are balanced (they cancel each other out)

EXAMPLE PROBLEM 2 An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved.

Gravity is the only force acting on the egg as it falls. EXAMPLE PROBLEM 2 Gravity is the only force acting on the egg as it falls.

The forces are unbalanced, so the egg will accelerate downward. EXAMPLE PROBLEM 2 The forces are unbalanced, so the egg will accelerate downward.

EXAMPLE PROBLEM 3 A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. A free body diagram for this squirrel looks like…

EXAMPLE PROBLEM 3 Gravity pulls down on the squirrel while air resistance keeps the squirrel in the air for a while.

EXAMPLE PROBLEM 4 A rightward force is applied to a book at rest, in order to move it across a desk. Consider frictional forces. Neglect air resistance. Construct a free-body diagram for the book.

EXAMPLE PROBLEM 4 Note the applied force arrow pointing to the right. Notice how friction force points in the opposite direction. Finally, there are still gravity and normal forces involved.

EXAMPLE PROBLEM 5 A skydiver is falling with a constant velocity. Consider air resistance. Draw a free-body diagram for the skydiver.

EXAMPLE PROBLEM 5 Gravity pulls down on the skydiver, while air resistance pushes up as she falls.

EXAMPLE PROBLEM 6 A football is moving upwards toward its peak after having been booted by the punter. Neglect air resistance. Draw a free-body diagram of the football in mid-air.

EXAMPLE PROBLEM 6 The force of gravity is the only force described. It is not a windy day (no air resistance).

Make a Book! Time to tap into some creativity! You will create a 6-page book that covers what you learned today. Front Cover: “Force: A Push or Pull” Your name and the period. Page 1: Contact Force Page 2: Examples Page 3: Non Contact Force Page 4: Examples Page 5: Gravity Page 6: Force Diagrams