Ch. 12 Motion & Forces

Ch. 12Motion & Forces I. Newton’s Laws of Motion “If I have seen far, it is because I have stood on the shoulders of giants.” - Sir Isaac Newton (referring to Galileo)

Newton’s First Law • Newton’s First Law of Motion • An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force.

Newton’s Second Law • Newton’s Second Law of Motion • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. F = ma

Newton’s Third Law • Newton’s Third Law of Motion • When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

III. Defining Force Force Newton’s First Law Friction

A. Force • Force • a push or pull that one body exerts on another • What forces are being exerted on the football? Fkick Fgrav

A. Force • Balanced Forces • forces acting on an object that are opposite in direction and equal in size • no change in velocity

N N A. Force • Net Force • unbalanced forces that are not opposite and equal • velocity changes (object accelerates) Fnet Ffriction Fpull W

Calculating Net Force from Images • Net force is the sum of forces acting on an object • If forces are acting in the same direction they combine (add together) • If forces are acting in opposite directions they reduce (subtract) • If net force is zero the motion of the object will NOT change • If net force is not zero the object will accelerate (this may be positive or negative acceleration).

What is the net force on the objects below? Opposite = subtract 8N – 3N = 5 N Same = addition 5N + 2N = 7 N 2 N + 5N – 8N = 1 N 7N + 2N – 3N = 6N

B. Newton’s First Law • Newton’s First Law of Motion • “Law of Inertia” • Inertia • tendency of an object to resist any change in its motion • increases as mass increases • Newton’s First Law of Motion • An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force.

ConcepTest 1 TRUE or FALSE? The object shown in the diagram must be at rest since there is no net force acting on it. FALSE! A net force does not cause motion. A net force causes a change in motion, or acceleration.

ConcepTest 2 You are a passenger in a car and not wearing your seat belt. Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door. Which is the correct analysis of the situation? ... 1. Before and after the collision, there is a rightward force pushing you into the door. 2. Starting at the time of collision, the door exerts a leftward force on you. 3. both of the above 2. Starting at the time of collision, the door exerts a leftward force on you.

C. Newton’s Second Law • Newton’s Second Law of Motion • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. F = ma

F a m Newton’s Second Law F = ma F: force (N) m: mass (kg) a: accel (m/s2) 1 N = 1 kg ·m/s2

F a m Calculations • What force would be required to accelerate a 40 kg mass by 4 m/s2? GIVEN: F = ? m = 40 kg a = 4 m/s2 WORK: F = ma F = (40 kg)(4 m/s2) F = 160 N

F a m Calculations • A 4.0 kg shot-put is thrown with 30 N of force. What is its acceleration? GIVEN: m = 4.0 kg F = 30 N a = ? WORK: a = F ÷ m a = (30 N) ÷ (4.0 kg) a = 7.5 m/s2

D. Gravity • Gravity • force of attraction between any two objects in the universe • increases as... • mass increases • distance decreases

more mass less distance B. Gravity • Who experiences more gravity - the astronaut or the politician? • Which exerts more gravity - the Earth or the moon?

D. Gravity • Would you weigh more on Earth or Jupiter? • Jupiter because... • greater mass • greater gravity • greater weight

Free Fall • When gravity is the only force acting on an object • Represented by the letter g • Near Earth’s surface g = 9.8 m/s2

B. Gravity • Weight • the force of gravity on an object W = mg W: weight (N) m: mass (kg) g: acceleration due to gravity (m/s2) W: weight (N) m: mass (kg) g: acceleration due to gravity (m/s2) W: weight (N) m: mass (kg) g: acceleration due to gravity (m/s2) MASS always the same (kg) WEIGHT depends on gravity (N)

elephant feather D. Gravity • Accel. due to gravity (g) • In the absence of air resistance, all falling objects have the same acceleration! • On Earth: g = 9.8 m/s2

W g m Calculations • Mrs. J. weighs 557 N. What is her mass? GIVEN: W = 557 N m = ? g= 9.8 m/s2 WORK: m = W ÷ g m = (557 N) ÷ (9.8 m/s2) m = 56.8 kg

ConcepTest • Is the following statement true or false? • An astronaut has less mass on the moon since the moon exerts a weaker gravitational force. • False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.

E. Newton’s 3rd Law • For every action force, there is an equal and opposite reaction force.

3rd law: Action / Reaction • The action force and the reaction force occur to different objects so the force is not balanced. • Not all action and reaction forces produce motion. • Unbalanced forces equal changes in motion

Newton’s third law • Kicking a soccer ball • Action force = your foot hits the ball • Reaction force = the ball pushes against your foot. • Ball moves b/c action force is larger than reaction force • Leaning against a wall • Action force = you pushing against a wall • Reaction force = wall pushing against you • Nothing moves b/c the action force equals the reaction force.

F. Friction • Friction is a force that opposes the motion of objects that touch as they move past each other. • Friction acts at the surface where object are in contact • Four main types of friction: static friction, sliding friction, rolling friction and fluid friction.

C. Friction • Friction • is the force that opposes motion between 2 surfaces • depends on the: • types of surfaces • force between the surfaces

C. Friction • Friction is greater... • between rough surfaces • when there’s a greater force between the surfaces (e.g. more weight) • Pros and Cons?

Static Friction • Static friction is the friction force that acts on objects that are not moving. • Static friction always acts in the direction opposite to that of the applied force • Prevents objects from sliding. • Example: pushing a dresser that does NOT move

Sliding Friction • Sliding Friction is a force that opposes the direction of motion of an object as it slides over a surface. • Occurs when there is enough force to overcome the static friction • There will be a net force in the direction of motion • Pushing a desk that slides against the floor.

Rolling Friction • Rolling friction is the force that acts on rolling objects. • Rolling friction is about 100 to 1000 times less than the force of static or sliding friction • Skate boarding down a hill

Fluid Friction • Fluid friction opposes the motion of an object through a fluid. • Fluid examples: air, water, quick sand, and cake batter • Air Resistance is the fluid friction action on an object moving through the air. What are the forces acting on a falling leaf?

Fair Fgrav Air Resistance • Terminal Velocity • maximum velocity reached by a falling object • reached when…Fgrav = Fair • no net force  no acceleration  constant velocity  still falling

Projectile Motion • Projectile motion the motion of a falling object after it is given an initial forward velocity. • this is a curved path • Objects with different mass fall at the same rate. • The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path.

Momentum • Momentum is the product of an object’s mass and its velocity, an object with large momentum is hard to stop • momentum for any object at rest is zero • Law of conservation of momentum if not net forces acts on a system, then the total momentum of the system does not change • In a closed system the loss of momentum of one object equals the gain in momentum of another object--- momentum is conserved

Ch. 12 Motion & Forces