3.3 Motion & Force

Objectives Explain how forces and motion are related. Explain how forces and motion are related. Compare and contrast static friction and sliding friction. Compare and contrast static friction and sliding friction. Describe the effects of air resistance on falling objects. Describe the effects of air resistance on falling objects.

Force Push or pull that one object exerts on another Push or pull that one object exerts on another Vector with size (strength of force) & direction Vector with size (strength of force) & direction SI Units = Newtons (N) SI Units = Newtons (N) Can cause the motion of an object to change Can cause the motion of an object to change Example: hitting a tennis ball Example: hitting a tennis ball shooting pool/billiards shooting pool/billiards

Balanced Forces Forces equal in size but opposite in direction Forces equal in size but opposite in direction doesn ’ t change velocity doesn ’ t change velocity

Net Force When 2 or more forces act on an object at the same time When 2 or more forces act on an object at the same time Sum of all forces acting on an object Sum of all forces acting on an object

Unbalanced Forces When forces combine to produce a net force that is not zero When forces combine to produce a net force that is not zero Forces that aren ’ t equal  The object moves Forces that aren ’ t equal  The object moves Changes the velocity of an object Changes the velocity of an object

Friction The force between two objects, in contact, that opposes the motion of either object The force between two objects, in contact, that opposes the motion of either object Unbalanced force Unbalanced force Why does a ball stop rolling? Why does a ball stop rolling? Why do we have to keep applying gas to the engines in our cars to keep moving? Why do we have to keep applying gas to the engines in our cars to keep moving? Better yet, why do we put oil in a car ’ s engine Better yet, why do we put oil in a car ’ s engine Why do runners wear running shoes? Why do runners wear running shoes? Why do parachutes work? Why do parachutes work?

Friction Friction depends on the surfaces of the objects in contact Friction depends on the surfaces of the objects in contact Smooth things tend to have less frictional force  they are more slippery Smooth things tend to have less frictional force  they are more slippery Ice, oil on concrete, my old shoes, my road bike tires Ice, oil on concrete, my old shoes, my road bike tires Rough things tend to have more frictional force  less slippery Rough things tend to have more frictional force  less slippery Asphalt, my new running shoes, my mountain bike tires Asphalt, my new running shoes, my mountain bike tires

Frictional Force Increases when the force pushing surfaces together increases Increases when the force pushing surfaces together increases Static Friction = frictional force that prevents 2 surfaces from sliding past each other Static Friction = frictional force that prevents 2 surfaces from sliding past each other

Sliding friction = force that acts in opposite direction to the motion of a surface sliding past another surface Sliding friction = force that acts in opposite direction to the motion of a surface sliding past another surface Rolling friction = similar to sliding friction Rolling friction = similar to sliding friction why it is easier to move something on wheels why it is easier to move something on wheels

Friction Air Resistance is a form of friction Air Resistance is a form of friction Caused by the interaction between an object and the air molecules it comes into contact with Caused by the interaction between an object and the air molecules it comes into contact with The bigger the object the more air resistance The bigger the object the more air resistance The faster the object the more air resistance The faster the object the more air resistance

Air Resistance Causes objects to fall at different acceleration and speed Causes objects to fall at different acceleration and speed Acts in opposite direction to velocity Acts in opposite direction to velocity Air resistance not mass causes objects to fall at different rates Air resistance not mass causes objects to fall at different rates

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Terminal Velocity When an object falls with constant velocity When an object falls with constant velocity Upward air resistance becomes large enough to balance downward force of gravity Upward air resistance becomes large enough to balance downward force of gravity

In-Class Assignment/Homework Chapter Review WKT Chapter Review WKT Later this week: Later this week: Bill Nye Friction Video Bill Nye Friction Video Can a penny kill someone? Can a penny kill someone? (Mythbusters episode) (Mythbusters episode)