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Chapter 2 Physical Science
The Laws of Motion Chapter 2 Physical Science
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Ch2 L.1 Gravity and Friction
What are some contact forces and some noncontact forces? What is the law of universal gravitation? How does friction affect the motion of two objects sliding past each other?
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Ch2 L.2 Bellwork Force: a push or pull on an object
Contact force: a push or pull on one object by another that is touching it Noncontact force: a force that one object can apply on another without touching it Gravity: an attractive force that exists between all objects that have mass Mass: the amount of matter in an object Weight: the gravitational force exerted on an object Friction: a force that resists the motion of two surfaces that are touching
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Types of forces Contact force Noncontact force
One object applies force to another MUST be touching Can be weak or very strong Noncontact force Gravitational force Magnetic force Electric force
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Forces Have strength and direction Unit= Newtons
In arrows, longer arrow= stronger force Unit= Newtons 1 N = 1 kg⋅m/s2
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Gravity Gravity: attractive force between objects that have mass
Mass: amount of matter in an object (in kg) Earth exerts attractive force on you and you exert attractive force on Earth
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Law of universal gravitation
Sir Isaac Newton (1600’s English scientist and mathematician) developed this law All objects are attracted to each other by gravitational force How strong the force is depends on the mass of the objects
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Law of universal gravitation cont’d
Gravitational force and mass The greater the masses of the objects, the greater the gravitational force between them EVEN if the objects have DIFFERENT masses, the force between them is the SAME for each one Gravitational force and distance The force decreases as the distance between the objects increases
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Weight Weight: gravitational force exerted by an object
Biggest object near us: Earth. On Earth, an object’s weight is the gravitational force exerted on the object by the Earth Weight is proportional to mass. If mass doubles, so does the weight. Mass is constant. Weight depends on location.
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Friction Friction: force that resists the motion of two surfaces that are touching Three types Static Sliding Fluid
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Static friction Prevents surfaces from sliding past each other
Static means not moving Static friction equals the applied force up until the limit THIS MEANS THE STATIC FRICTION CAN CHANGE Applied force needs to be greater than the static friction in order to get something to move
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Sliding friction Once you overcome static friction, the object will move Opposes the motion of surfaces sliding past each other DOES NOT CHANGE (as long as the object is sliding) Increasing applied force just makes box move faster Stop pushing= box slows down and stops moving because of sliding friction
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Fluid friction Friction between a surface and any fluid that flows
water, air, oil, etc. Resistance between surface and air= air resistance Decreasing surface area decreases the resistance
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What causes friction? Microscopic dips/bumps on surfaces rub against each other Slows sliding Atoms on the opposing surfaces can form weak attractive forces and slow sliding
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Reducing friction Reduce contact between bumps on surfaces
Reduce ability for attractive forces Lubricants can do both Reducing surface area reduces air resistance
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Ch2 L.1 Homework p Outline Ch2 L.1 Study for quiz
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Ch2 L.2 Newton’s law What is Newton’s first law of motion?
How is motion related to balanced and unbalanced forces? What effect does inertia have on the motion of an object?
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Ch2 L.2 Bellwork Net force: a combination of all the forces acting on an object Balanced forces: forces acting on an object that combine and form a net force of zero Unbalanced forces: forces acting on an object that combine and form a net force that is not zero Newton’s first law of motion: if the net force on an object is zero, the motion of the object does not change Inertia: the tendency of an object to resist a change in its motion
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Identifying forces Net force- combining forces
When two or more forces act on an object, the forces combine How they combine depends on the directions of the forces Forces in the same direction: add forces to get net force direction of net forces is the same as the direction of the individual ones MUST specify a reference direction (usually to the right is positive)
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Identifying forces cont’d
Combining forces in opposite directions: Still the sum of the forces One force will have a positive value and one will have a negative value depending on your reference direction
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Balanced and unbalanced forces
When the net force on the object is zero Same effect as if there were no forces acting on it Do not change the motion of the object Unbalanced When the net force on the object is not zero
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Newton’s first law of motion
If the net force on an object is zero, the motion of the object does not change. Balanced forces Cause no change in velocity of an object Unbalanced forces First law DOES NOT apply These forces DO change an object’s velocity
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Inertia Tendency of an object to resist change
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Why do objects stop moving?
Book on table Stays still because of inertia You push it You overcome static friction Book moves You stop pushing Friction stops book IF* there were no friction on the table You push book Book continues to move forever because of inertia
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Ch2 L.2 Homework p Outline Ch2 L.2 Study for quiz
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Ch2 L.3 Newton’s Second Law
What is Newton’s second law of motion? How does centripetal force affect circular motion?
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Ch2 L.3 Bellwork Newton’s second law of motion: the acceleration of an object is equal to the net force acting on the object divided by the object’s mass Circular motion: any motion in which an object is moving along a curved path Centripetal force: in circular motion, a force that acts perpendicular to the direction of motion, toward the center of the curve
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How do forces change motion?
Forces change an object’s motion by changing its speed, direction or both speed and direction. Remember: balanced forces do not change an object’s velocity, only unbalanced forces do Cyclist’s speed will not change unless the force on the petals overcomes the friction between the wheels and the road
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Unbalanced forces on an object at REST
Object is originally not moving Balanced forces keep object still Unbalanced forces cause object to move In direction of net force
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Unbalanced forces on object in MOTION
Speeding up If net force acting on object is in the SAME direction that the object is already moving Slowing down If net force is in the opposite direction that the object is already moving Example: friction
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Unbalanced forces on object in MOTION cont’d
Change direction Object moves at constant velocity until it hits something and then it changes direction Acceleration When velocity changes Unbalanced forces make an object accelerate by changing the speed, direction, or both
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Newton’s second law of motion
The acceleration of an object is equal to the net force acting on the object divided by the object’s mass Acceleration (m/s2)= Force(N)/mass(kg) N= kg*m/s2
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Math Practice You throw a .5kg ball with a force of 10N. What is the acceleration of the ball? A 24N net force acts on an 8kg rock, what is the acceleration of the rock? A 30N net force on a skater produces an acceleration of .6m/s2, what is the mass of the skater? What net force acting on a 14kg wagon produces an acceleration of 1.5m/s2?
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Circular motion Any motion in which an object is moving along a circular path Centripetal force Velocity= tangent to circular motion. Wants straight path because of inertia Circular motion because string pulls ball inward Force that acts perpendicular to direction of motion, to center of curve= centripetal force Acceleration in direction of centripetal force
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Circular motion cont’d
Motion of satellites and planets Gravity (not a string) pulls the satellite inward towards the Earth Moon a satellite of Earth Earth a satellite of the Sun
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Ch2 L.3 Homework p Ch2 L.3 outline Study for quiz
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Ch2 L.4 Newton’s third law What is Newton’s third law of motion?
Why don’t the forces in a force pair cancel each other? What is the law of conservation of momentum?
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Ch2 L.4 Bellwork Newton’s third law of motion: when one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object. Force pair: the forces two objects apply to each other Momentum: a measure of how hard it is to stop a moving object Law of conservation of momentum: the total momentum of a group of objects stays the same unless outside forces act on the objects
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Opposite forces Objects push against each other in opposite directions
Girl wants to move away from the wall so she pushes against the wall Wall exerts equal but opposite force on the girl
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Newton’s third law of motion
Equal but opposite rule Equal magnitude of force Opposite direction Force pairs The forces the two objects apply to each other Don’t add up to be zero like balanced forces because aren’t applied to the same object
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Newton’s third law of motion cont’d
Force pair One force= action force Other force= reaction force Example: girl jumping off boat Girl applies action force on boat Boat applies reaction force on girl
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How to use the third law If reaction force results in an unbalanced force, there is a net force and the object accelerates Swimming Action force: Push arms against water Reaction force: water pushes back on arms (in forward direction) Jumping Action force: push down on ground Reaction force: ground pushes up on you plus gravity pushing down to make net force acting on you Rocket Action force: Engine pushes gas out downward Reaction force: gas pushes upward on engine (needs to overcome gravity)
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Momentum How hard is it to stop a moving object
In same direction as velocity p=m*v p= momentum (kg*m/s) m= mass in kg v= velocity (m/s)
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Momentum practice What is the momentum of a 12kg bicycle moving at 5.5 m/s? What is the momentum of a 1.5kg ball rolling at 3.0 m/s? A 55kg woman has a momentum of 220kg*m/s. What is her velocity?
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Conservation of momentum
In any collision, one object transfers momentum to another object TOTAL momentum does NOT change Law of conservation of momentum: total momentum of a group of objects stays the same unless outside forces act on the objects ex.Friction
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Types of collisions Elastic- colliding objects bounce off one another
Inelastic- colliding objects stick together after collision
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Ch2 L.4 Homework p.75 1-9 Outline Ch2 L.4 Study for quiz
Study for test
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