Chapter 3 Forces Mrs. Gerlach Foundations of Physical Science

Objectives 3-1 Define force 3-2 Compare and contrast forces 3-3 Use Newton’s 3 laws of motion to explain different situations 3-4 Complete force, weight and momentum calculations using the correct formula

A Force DEF: a push or a pull. A force can change the motion of an object.

Different types of forces –Friction –Air resistance –Weight –Gravity Net force –When 2+ forces act on an object, they combine.

Balanced force DEF: Two forces are the same size force but act in opposite directions. No change in motion occurs with a balanced force.

10 N Net force =

DEF: Two forces are not equal in size AND/OR are not acting in the same direction Now a change in motion occurs. Unbalanced force.

10 N Net force =

Newton’s Laws of Motion

First Law of Motion DEF: An object in motion remains in motion unless acted upon by an outside unbalanced force. Law also known as –Law of Inertia

Other information An object at rest remains at rest unless acted upon by an outside unbalanced force. Inertia –DEF: the tendency of an object to resist a change in motion

Examples Table cloth and dishes trick Card, Penny and Drinking Glass Wearing a seat belt

Second Law of Motion DEF: If an object is accelerating, there must be an unbalanced force acting on it. Law also known as –F = ma

Examples Pushing an empty shopping cart is easier to do than a filled shopping cart Takes more force to hit a golf ball than a bowling ball with a stick.

VS

Third Law of Motion DEF: To every action force, there is an equal and opposite reaction force. Law also known as –Action - Reaction Forces act in pairs.

Examples Rowing a kayak Walking Shooting a cannon

Formula circle – Newton’s Second Law

MeasurementVariableUnits ForceF N – newton Massmkg Accelerationam/s/s

What is the force required to accelerate a 15 kg bike to a rate of 10 m/s/s?

If the mass of a helicopter is 4,500 kg and the net force on it is 18,000 N, what is the helicopter’s acceleration?

If a football is accelerating at 10 m/s/s and a 50 N force was placed on it, what is the football’s mass?

Friction DEF: force opposes motion when 2 objects are touching.

When do you want a large amount of friction? When do you want a small amount of friction?

Types of Friction Static friction –Two objects against each other Sliding friction –two objects sliding against each other

Friction types Rolling friction –one object rolling across the other Fluid friction –resistance in a fluid, like water or air

Air Resistance DEF: a force that opposes motion of object moving through the air. Terminal Velocity is the highest speed a falling object will reach Achieved when the pull of gravity is equaled by the push air resistance. Balanced forces = no change in motion.(No acceleration)

Draw what I draw at the bottom of the page!

Gravity DEF: A pulling force between any two objects Depends on two factors

Mass of objects If we increase the mass of either object  force of gravity goes UP If we decrease the mass of either object  force of gravity goes DOWN

Distance between objects If we increase the distance between objects  force of gravity goes DOWN If we decrease the distance between objects  force of gravity goes UP

Weight DEF: Measure of the pull of gravity on an object Weight and mass are not the same!!

Weight Formula Circle

MeasurementVariableUnits How do you know? WeightWt N – newton What is the weight? Massmkg What is the mass? Acceleration due to gravity g 9.8 m/s/s

45 kg

Force of Gravity Problems What is the weight of the boy on Earth?

Will the boys mass be the same on the moon? Will the boy’s weight be the same on the moon?

What is the weight of the boy on moon where the acceleration due to gravity is 1.67 m/s/s?

Momentum DEF: Relationship between mass and velocity. Related to the force needed to change motion of an object

Example If I threw a bowling ball and a tennis ball down a bowling alley at 10 m/s, which object would knock down more pins? –How do you know?

A car, the bird or the semi truck moving at 10 m/s runs into you. Which one has the most momentum? –How do you know?

Activities that use transfer momentum:

Momentum can be conserved. –This is the Law of Conservation of Momentum. DEF: When two objects collide, their total momentum stays the same.

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