Chapter 12 Forces and Motion

Chapter 12Forces and Motion 12.1 Forces

What Is a Force? A forceis a push or a pull that acts on an object. It can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

Units of Force One newton(N) is the force that causes a 1-kilogram mass to accelerate at a rate of 1 meter per second each second (1 m/s2). 1 N =1 kg•m/s2

Combining Forces When the forces on an object are balanced, the net force is zero • There is no change in the object’s motion. When an unbalanced force acts on an object, the object accelerates.

The net forceis the overall force acting on an object after all the forces are combined. • Forces in the same direction add together. • Forces in opposite directions subtract from one another.

Balanced Forces Balanced forces are forces that combine to produce a net force of zero. An unlimited number of individual forces can act on an object to produce a net force of zero.

Unbalanced Forces An unbalanced force is a force that results when the net force acting on an object is not equal to zero.

Activity Calculating • Tugboat A exerts a force of 4000 N on a barge. Tugboat B exerts a force of 8000 N on the barge in the same direction. What is the combined force? Using arrows draw the combined forces. Then draw the forces involved if the tugboats were pulling in opposite directions.

ANSWER: In the same direction: 4000N + 8000N = 12000N In opposite directions: 4000N – 8000N = 4000N toward the direction of tugboat B

Friction Friction is a force that opposes the motion of objects that touch as they move past each other. four main types of friction: static friction sliding friction rolling friction fluid friction.

Static Friction Static frictionis 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.

Rolling Friction Rolling frictionis the friction force that acts on rolling objects. • about 100 to 1000 times less than the force of static orsliding friction.

Sliding Friction Sliding frictionis a force that opposes the direction of motion of an object as it slides over a surface. Sliding friction is less than static friction.

Fluid Friction • Fluid frictionopposes the motion of an object through a fluid. • increases as the speed of the object moving through the fluid increases. • Fluid friction acting on an object moving through the air is known as air resistance.

Gravity Earth’s gravity acts downward toward the center of Earth.

How do gravity and air resistance affect a falling object? Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.

Gravityis a force that acts between any two masses. • an attractive force. • can act over large distances.

Free Fall • Free fall – motion of an object when gravity is the only force acting on it • All objects near earth accelerate at 9.8 m/s2 in the absence of air resistance

Terminal Velocity • Terminal Velocity – when air resistance balances weight and the object stops accelerating and reaches its maximum velocity • 320 km/h (200 mi/h)

Projectile Motion A thrown ball follows a curved path. Projectile motion is the motion of a falling object (projectile) after it is given an initial forward velocity. • Air resistance and gravity are the only forces acting on a projectile.

REVIEW • If an object is at rest, which of the following statements must be true? • There are no forces acting on the object. • There is no friction acting on the object. • The forces acting on the object are unbalanced. • The net force acting on the object is zero.

REVIEW • Which of the following is not a type of friction? • static friction • sliding friction • fluid friction • pull friction

REVIEW • In which direction does Earth’s gravitational force act? • opposite the direction of motion • downward toward the center of Earth • upward away from the center of Earth • in the direction of motion

REVIEW • A ball thrown into the air follows a projectile course due to the initial velocity and the • force of gravity. • effect of air resistance. • motion of Earth beneath it. • mass of the ball.

REVIEW 5. The SI unit for force is 1 kg•m/s2, also called one kepler. TrueFalse

REVIEW • The SI unit for force is 1 kg•m/s2, also called one kepler. False: The SI unit for force is 1 kg•m/s2, also called one Newton.

Chapter 12Forces and Motion 12.2 Newton’s first and second laws of motion

Newton Newton built on the work of scientists such as Galileo. • Newton first defined mass and force. • He then introduced his laws of motion.

Newton’s First Law of Motion According to Newton’s first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero.

Unless acted upon by an unbalanced force, an object at rest remains at rest and an object in motion remains in motion with the same speed and direction. Inertiais the tendency of an object to resist a change in its motion.

Newton’s Second Law of Motion Newton’s second law of motion: the acceleration of an object is equal to the net force acting on it divided by the object’s mass.

F = ma Newton’s Second Law of Motion The acceleration of an object is directly proportional to the net force acting on it. • also depends upon its mass. • Mass is a measure of the inertia of an object.

2nd Law Example • How much force is needed to accelerate a 1400 kg car 2 m/s2?

2nd Law Example • How much force is needed to accelerate a 1400 kg car 2 m/s2? F = 1400kg x 2 m/s2 F = 2800N (Remember: 1 N =1 kg•m/s2) F = ma

Newton’s Second Law An automobile with a mass of 1000 kilograms accelerates when the traffic light turns green. If the net force on the car is 4000 newtons, what is the car’s acceleration?

Newton’s Second Law F = ma 4000N = 1000kg x m/s2 4000 / 1000 = 4 m/s2 Acceleration = 4 m/s2

Newton’s Second Law of Motion 1.A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N?

Newton’s Second Law of Motion • A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N? F = ma 60 = 40kg x m/s2 60 / 40 = 4.5 m/s2 acceleration = 4.5 m/s2

2nd Law Practice • How much force is needed to accelerate a 66 kg skier 1 m/s2? • What is the force on a 1000 kg elevator that is falling freely at 9.8 m/s2? • A baseball accelerates down at 9.8 m/s2. If the force is 1.4 N, what is the baseball’s mass?

2nd Law Practice • How much force is needed to accelerate a 66 kg skier 1 m/s2? 66N 2. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/s2? 9800 N 3. A baseball accelerates down at 9.8 m/s2. If the force is 1.4 N, what is the baseball’s mass? 0.143kg

Mass and weight are related but are not the same. • Mass is the measure of the amount of material an object contains. • Weight is the force of gravity acting on an object. • Weight is the product mass and acceleration due to gravity.

Weight and Mass W = mg is a different form of Newton’s Second Law, F = ma. The value of g in the formula is 9.8 m/s2.

Weight and Mass If an astronaut has a mass of 112 kilograms, what is his weight on Earth where the acceleration due to gravity is 9.8 m/s2?

Weight and Mass If an astronaut has a mass of 112 kilograms, what is his weight on Earth where the acceleration due to gravity is 9.8 m/s2? Weight = 112 kg x 9.8 m/s2 Weight = 1097.6 N

Weight Calculations • A 10 kg mass would weigh __ • A 50 kg mass would weigh __ • A 100 kg mass would weigh __

Weight Calculations • A 10 kg mass would weigh _98 N_ • A 50 kg mass would weigh _490 N_ • A 100 kg mass would weigh _980 N_

Chapter 12 Forces and Motion