Chapter 8 New Book Chapters 12 and 13 Purple Book Motion and Forces Chapter 8 New Book Chapters 12 and 13 Purple Book

Speed and Velocity Speed describes how fast an object is moving Speed equals distance divided by time The SI unit for speed is m/s (meters/second) Constant speed is when an object covers equal distances in equal amounts of time

Speed Graphs The relationship between speed, distance, and time can be seen by plotting a time and distance graph The x axis represents time The y axis represents distance D I S T A N C E (m) 600 500 400 300 200 100 2.5 5 7.5 10 Cruising jet speeding race car Flying eagle galloping horse Wheelchair racer person walking T I M E (s)

Speed Formula Speed = Distance/time (represents average speed) d t s

Practice Problems A wheelchair racer finishes a 132 m race in 18 s. What is the racer’s average speed? Calculate the distance in meters a cyclist would travel in 5.00 hours at an average speed of 12 km/h.

Velocity The ONLY difference between speed and velocity is that velocity gives a direction. Example of speed 60 mph Example of velocity 60 mph east Velocity is calculated the same way speed is calculated Velocity changes if either speed or direction changes

Practice Problems Find the velocity in meters per second of a swimmer who swims exactly 110 m toward the shore in 72 s. Find the velocity in meters per second of a baseball thrown 38 m from third base to first base in 1.7 s.

Momentum Everyday example: A train is more difficult to stop than a car moving along the same path at the same speed. The train is more difficult to stop because it has more mass. An object with more momentum is more difficult to stop. Momentum depends on velocity and mass

Momentum Formula The SI unit for momentum is kg ● m/s Momentum = mass x velocity p = mv p m v

More on Momentum An object with more mass has more momentum. A semi-truck has more momentum than a sports car The faster an object is moving the more momentum it has. A fast moving train has more momentum than a slow moving train If an object is not moving, it has no momentum

Practice Problems Calculate the momentum of the following objects: A 75 kg speed skater moving forward at 16 m/s A 135 kg ostrich running north at 16.2 m/s A 5.0 kg baby on a train moving eastward at 72 m/s An 0.8 kg kitten running to the left at 6.5 m/s A 48.5 kg passenger on a train stopped on the tracks

Law of Conservation The law of conservation of momentum states that the total amount of momentum in a system is conserved. Can you predict what will happen with two cars traveling with different velocities collide head on? The amount of momentum before the collision is the same as the total momentum after the collision.

Acceleration and Force Acceleration is any change in velocity For an object moving in a straight line, acceleration is calculated by dividing the change in the object’s velocity by the time in which the change occurs. The SI unit for acceleration is m/s2

Acceleration Formula Acceleration = final velocity – initial velocity time a = Δ v t Positive acceleration indicates speeding up Negative acceleration indicates slowing down

Practice Problems Natalie accelerates her skateboard along a straight path from 0 m/s to 4.0 m/s in 2.5 s. Find her average acceleration. A turtle swimming in a straight line toward the shore has a speed of 0.50 m/s. After 4.0 s, its speed is 0.80 m/s. What is the turtle’s average acceleration?

Acceleration Graphs A straight line on a velocity-time graph means that the velocity changes by the same amount each time. This is called constant acceleration. Positive slope indicates speeding up. Negative slope indicates slowing down.

Force Force is the cause of acceleration or change in an object’s velocity Net force is the combination of all of the forces acting on an object An object accelerates in the direction of the net force It will not accelerate if the net force is zero

Balanced and Unbalanced Force Balanced forces are forces acting on an object that combine to produce a net force equal to zero. The object does not move. Unbalanced forces are forces acting on an object that combine to produce a nonzero net force. The object moves.

Friction and Air Resistance Friction is the force between two objects in contact that opposes the motion of either object. Frictional forces are relatively great when both surfaces are rough. Frictional forces are relatively low when both surfaces are smooth. Air resistance is a form of friction that occurs by the interaction between the surface of a moving object and the air molecules.

Gravity Gravity is the attraction between two particles of matter due to their mass The force of gravity between to objects depends on the mass of each object Gravitational force is proportional to the product of their masses The greater the mass the larger the gravitational force Gravitational force changes as the distance between the objects changes. If the distance is doubled, the gravitational force decreases to ¼ its original value. If the distance is tripled, the gravitational force decreases to 1/9 its original value, etc.

Newton’s Laws of Motion Newton’s First Law An object at rest remains at rest and an object in motion remains in motion unless it experiences an unbalanced force—inertia Example: As a car comes to a stop, you can feel your body continue to move forward. Your seatbelt and friction make you stop your forward motion

Newton’s Second Law An unbalanced force acting on an object equals the object’s mass times its acceleration Force = mass x acceleration F = ma Example: It takes more force to push a full shopping cart than an empty one.

Formula for Newton’s 2nd Law Force = mass x acceleration F = ma F m a

More on Newton’s 2nd Law The SI unit for force is the newton (N) Free fall is the motion of a body when only the force of gravity is acting on it Because free-fall acceleration results from gravity, it is often abbreviated with the letter g The force of gravity on Earth is 9.8 m/s2

More on Newton’s 2nd Law The force of an object due to its gravity is known as weight Weight = mass x free fall acceleration w = mg w m g

More on Newton’s 2nd Law Weight and mass are different! Mass is a measure of the amount of matter in an object. Weight is the gravitational force an object experiences due to its mass. The weight of an object depends on gravity.

More on Newton’s 2nd Law Both air resistance and gravity act on objects moving through Earth’s atmosphere. When the force of air resistance equals gravitational force, the object stops accelerating—terminal velocity

Practice Problems What is the force necessary for a 1600 kg automobile to accelerate forward at 2.0 m/s2? A baseball accelerates downward at 9.8 m/s2. If the gravitational force acting on the baseball is 1.4 N, what is the baseball’s mass?

Newton’s Third Law For every action there is an equal and opposite reaction Example: When you kick a soccer ball, the soccer ball exerts an equal and opposite force on your foot. The force exerted on the ball by your foot is the action force. The force exerted on your foot by the ball is the reaction force.

More on Newton’s 3rd Law Action/Reaction forces are at work with rockets. The push of the hot gases through the nozzle is matched by an equal push in the opposite direction on the combustion chamber. This accelerates the rocket forward.