Motion and Forces Physical Science Chap 2 and 3

Describing Motion 2.1

What is your current speed?  Using the sun as a reference point, you are moving about ______ km/s. 30

Motion  Motion occurs when an object changes its position relative to a reference point.  You are not moving relative to your desk or your school building, but you are moving relative to the other planets in the solar system and the Sun.

The Tortoise and the Hare  Who wins the race?  Why?  What is speed?  What is average speed?  What are some units used to describe speed?

Describing Motion  Distance – how far something has moved  In science, we use meters ( m, km, cm, mm )

Describing Motion  Speed – how fast something moves  distance/unit of time (such as m/s)

Describing Motion  Average Speed – the total distance traveled divided by the total time of travel.  If the total distance traveled was 5 km and the total time was 1/4 h, or 0.25 h. The average speed was:

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Distance vs. displacement  Pg. 39 – Figure 2  How far did the runner run?  80 meters (don’t forget units)  What is her distance from the starting line?  20 meters  Can you determine the runners speed?  Why not?  Time is not given

Speed vs. Velocity  Question: A storm is 10 km away and is moving at a speed of 60 km/h. Should you be worried?  Answer: It depends on the storm’s direction…

Describing Motion  Velocity – includes the speed of an object and the direction of its motion  The speed of a car might be constant while its velocity changes.

Who skates faster? Your neighbor skates at a speed of 4 m/s. You can skate 100 m in 20 s. GIVEN: d = 100 m t = 20 s v = ? WORK : v = d ÷ t v = (100 m) ÷ (20 s) v = 5 m/s You skate faster! v d t

D. Calculations Sound travels 330 m/s. If a lightning bolt strikes the ground 1 km away from you, how long will it take for you to hear it? GIVEN: v = 330 m/s d = 1km = 1000m t = ? WORK : t = d ÷ v t = (1000 m) ÷ (330 m/s) t = 3.03 s v d t

Distance-time Graph  Who started out faster?  A (steeper slope)  Who had a constant speed? AA  Describe B from min.  B stopped moving  Find their average speeds.  A = (2400m) ÷ (30min) A = 80 m/min  B = (1200m) ÷ (30min) B = 40 m/min A B

Speed-time Graph Specify the time period when the object was... slowing down 5 to 10 seconds speeding up 0 to 3 seconds moving at a constant speed 3 to 5 seconds not moving 0 & 10 seconds

Motion over the years – gradual  Moving Plates  Cause mountains, earthquakes, etc.  Pacific ocean is getting smaller  From 1 cm/yr (in CA) to 17 cm/yr (Australia)

Self-Check At 8:00 AM you leave home and walk 0.5 km to a friend's house. At 11:30 AM you return home, then travel by car to the mall, which is 10 km away, and arrive at 11:45 AM. What is the total distance traveled? 2. If a student leaves home at 7:30 AM to walk to his school 2 km away, stops at a neighbor's house for 15 min, then arrives at school at 8:00 AM, what is the student's average speed for the trip to school? v d t

Acceleration 2.2

Acceleration  Acceleration – the rate of change of velocity (occurs with change in speed or direction)  Positive acceleration - if the acceleration is in the same direction as the velocity  Negative acceleration – if the acceleration is in the opposite direction from the velocity

Calculating Acceleration a v f - v i t

Application: Roller coasters  Engineers use the laws of physics to design amusement park rides that are thrilling, but harmless.

A roller coaster starts down a hill at 10 m/s. Three seconds later, its speed is 32 m/s. What is the roller coaster’s acceleration? GIVEN: v i = 10 m/s t = 3 s v f = 32 m/s a = ? WORK : a = ( v f - v i ) ÷ t a = (32m/s - 10m/s) ÷ (3s) a = 22 m/s ÷ 3 s a = 7.3 m/s 2 a v f - v i t

How long will it take a car traveling 30 m/s to come to a stop if its acceleration is -3 m/s 2 ? GIVEN: t = ? v i = 30 m/s v f = 0 m/s a = -3 m/s 2 WORK : t = (v f - v i ) ÷ a t = (0m/s-30m/s)÷(-3m/s 2 ) t = -30 m/s ÷ -3m/s 2 t = 10 s a v f - v i t

Airliner acceleration  Suppose a jet airliner starts at rest at the end of a runway and reaches a speed of 80 m/s in 20 s.

Stop Skating?  Now imagine that a skateboarder is moving in a straight line at a constant speed of 3 m/s and comes to a stop in 2 s.  a = (0 m/s – 3 m/s) / 2 s = –1.5 m/s 2

Videos and animations  orcesandtime/acceleration/zoom.weml orcesandtime/acceleration/zoom.weml  s/mmedia/kinema/acceln.html s/mmedia/kinema/acceln.html

Self-Check If a car is traveling 100 km/h and comes to a stop in 3 min, what is the car's acceleration? 2. If a runner maintains a constant speed of 12 km/h, how long will it take to complete a marathon race of 26.2 miles? 3. List 3 ways you can accelerate while walking.

Motion and Forces 2.3

Force  A force is a push or pull.  The force of the moving ball causes a ball at rest to move in the direction of the force.

Net Force  Net force – the combination of two or more forces acting on an object at the same time

Inertia  Inertia - the tendency of an object to resist any change in its motion  An object will keep moving at the same speed and in the same direction unless an unbalanced force acts on it.

Newton’s First Law of Motion  Newton's first law of motion states that an object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it.  If an object is at rest, it stays at rest unless an unbalanced net force acts on it.  This law is sometimes called the law of inertia.

Inertia in a car crash  When a car traveling about 50 km/h collides head-on with something solid, the car crumples, slows down, and stops within approximately 0.1 s.  Any passenger not wearing a safety belt continues to move forward at the same speed the car was traveling.  dia/newtlaws/cci.cfm dia/newtlaws/cci.cfm

Videos  orcesandtime/force/ orcesandtime/force/

Self-Check Can there be forces acting on an object if the object is at rest? 2. What is the net force on a refrigerator if you push on it and it doesn’t move? 3. Two students push on a box in the same direction, and one pushes in the opposite direction. What is the net force on the box if each pushes with a force of 50 N ?

Newton’s Second Law 3.1

Force and acceleration  What is different about throwing a ball horizontally as hard as you can and tossing it gently?  A hard-thrown ball has a greater acceleration than a gently tossed ball.

Mass and acceleration  If you throw a softball and a baseball as hard as you can, why don’t they have the same speed?  The softball would have less acceleration because it has a greater mass.

Newton’s Second Law  The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.  The acceleration is in the same direction as the net force on the object. m F a

Force to accelerate  What force would be required to accelerate a 40 kg mass by 4 m/s 2 ?

Friction  Friction is the force that opposes the sliding motion of two surfaces that are touching each other.  Depends on the kind of surface and the force pressing the surfaces together

Pinewood Derby Cars  What are some sources of friction in the cars that may hinder movement?  List ways to reduce friction.

Air Resistance  Air resistance is a friction-like force that opposes the motion of objects through air Depends on speed, size, and shape of object Note: Air resistance, not the object’s mass, is why feathers, leaves, and pieces of paper fall more slowly than pennies, acorns, and apples.

Motion vs. Acceleration?  Could this block be moving horizontally?  Yes  How?  If the forces acting on an object are balanced and the object is in motion, then it will continue in motion with the same velocity.  Remember: Forces do not cause motion. Forces cause accelerations.

Videos and Animations  s/mmedia/newtlaws/efff.html s/mmedia/newtlaws/efff.html

Self-check What 3 variables does Newton’s second law of motion connect?

Gravity 3.2

Gravity  Gravity is an attractive force between any two objects that depends on the masses of the objects and the distance between them.

Newton  Isaac Newton formulated the law of universal gravitation, which he published in 1687.

Range of Gravity  According to the law of universal gravitation, the gravitational force between two masses decreases rapidly as the distance between the masses increases.  No matter how far apart two objects are, the gravitational force between them never completely goes to zero.

Earth’s Gravitational Acceleration  Close to Earth’s surface, the acceleration of a falling object in free fall is about 9.8 m/s 2.  This acceleration is given the symbol g and is sometimes called the acceleration of gravity.

Weight  The gravitational force exerted on an object is called the object’s weight.

Weight and Mass Relationship

Projectile Motion  If you’ve tossed a ball to someone, you’ve probably noticed that thrown objects don’t always travel in straight lines. They curve downward.  Earth’s gravity causes projectiles to follow a curved path.

Horizontal and Vertical Distance  If you were to throw a ball as hard as you could from shoulder height in a perfectly horizontal direction, would it take longer to reach the ground than if you dropped a ball from the same height? Click image

Centripetal Acceleration  Centripetal acceleration is acceleration toward the center of a curved or circular path.  When a ball enters a curve, even if its speed does not change, it is accelerating because its direction is changing.

Centripetal Force  According to the second law of motion, when a ball has centripetal acceleration, the direction of the net force on the ball also must be toward the center of the curved path.  Centripetal force is the net force exerted toward the center of a curved path

Gravity can be a centripetal force  In the same way, Earth’s gravity exerts a centripetal force on the Moon that keeps it moving in a nearly circular orbit.

Videos  orcesandtime/gravity/ orcesandtime/gravity/

Self-check How do weight and mass differ?

Newton’s Third Law 3.3

Newton’s Third Law  Newton’s third law of motion: When one object exerts a force on a second object, the second one exerts a force on the first that is equal in strength and opposite in direction.

Action and Reaction  When a force is applied in nature, a reaction force occurs at the same time.  When you jump on a trampoline you exert a downward force on the trampoline. The trampoline exerts an equal force upward, sending you high into the air.  Even though the forces are equal, they are not balanced because they act on different objects.

Action / Reaction Forces Don’t Cancel  A swimmer “acts” on the water, the “reaction” of the water pushes the swimmer forward.  In a rocket, the hot gases are forced out the back of the rocket therefore pushing it forward.

Momentum  A moving object has a property called momentum that is related to how much force is needed to change its motion. Notice that momentum has a direction because velocity has a direction. m p v

Force and Changing Momentum  Recall that acceleration is the difference between the initial and final velocity, divided by the time.  Also, from Newton’s second law, the net force on an object equals its mass times its acceleration.  By combining these two relationships:

Law of Conservation of Momentum  The momentum of an object doesn’t change unless its mass, velocity, or both change.  Momentum, however, can be transferred from one object to another.  The law of conservation of momentum states that if a group of objects exerts forces only on each other, their total momentum doesn’t change.

When Objects Collide  The results of a collision depend on the momentum of each object.  When the first puck hits the second puck from behind, it gives the second puck momentum in the same direction.  If the pucks are speeding toward each other with the same speed, the total momentum is zero.

Bumper Car Momentum  Find the momentum of a bumper car if it has a total mass of 280 kg and a velocity of 3.2 m/s.

Bumper Car Velocity  The momentum of a second bumper car is 675 kg·m/s. What is its velocity if its total mass is 300 kg?

Videos  orcesandtime/newtonslawsofmotion/zoom. weml orcesandtime/newtonslawsofmotion/zoom. weml

Self-Check The momentum of an object is the product of its ________ and ________. 2. When two objects collide, what happens to their momentum?