Forces & Motion Chapter 12

What is a Force? Section 12.1 A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction. EX: The force of the wind pushing against the man slows his speed. A strong gust could even change the direction in which he was moving.

More Examples of Forces think of some and add them below Small forces . . . Medium forces . . . Large forces . . .

Measuring Forces Forces can be easy to measure. The stretch of the scale depends on the amount of weight acting upon it. Forces are usually measured in newtons, or (N). A 1 N force causes a 1 kilogram mass to accelerate at a rate of 1 meter per second each second.

Combining Forces Sometimes, the net force acting on an object is zero, so the forces are balanced and there is no change in motion. When an unbalanced force acts on an object, the object accelerates.

Balanced or Unbalanced What do you think? Put a “yes” or “no” next to each picture.

Friction Friction is a force that opposes (is opposite to) motion. Four Types: give an example of each static friction Sliding friction Rolling friction Fluid friction

Static and Sliding Friction Static friction is a force that acts on objects that are not moving. When you can’t get a large object to move, it is due to static friction. Once the object begins to move, the force that opposes the object’s motion is sliding friction. Sliding friction is less than static friction. Isn’t it easier to move a heavy object once it starts moving?

Rolling and Fluid Friction Rolling friction is about 100 to 1000 times less than the force of static or sliding friction. Have you ever tried to run in a swimming pool? Fluid friction opposes your motion. Fluid friction increases as the speed of the object moving through the fluid increases.

Air Resistance Air resistance is a type of fluid friction. At higher speeds, air resistance can be quite significant. What are some ways that bicyclists and speed skaters reduce air resistance? And what about competitive swimmers?

Gravity Gravity is a force that acts between any two masses. Gravity is an attractive force that acts downward toward the center of the Earth. An upward force usually balances the downward force of gravity.

Falling Objects Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration. Remember the whirly-bird activity? Gravity pulled the object downward, while air resistance pushed it upward. Teacher demo – falling objects

What happens when there is no air resistance? Teacher demos Apollo 15 demo http://www.youtube.com/watch?v=-4_rceVPVSY&feature=player_detailpage

Terminal Velocity As objects fall to the ground, they accelerate and gain speed. With this increasing speed comes increasing air resistance. If an object falls for a long time, the upward force of air resistance becomes equal to the downward force of gravity and the forces are balanced. This is called Terminal Velocity. Sky diving demo from CD

What does the graph look like?

What would the graph look like if there was no air resistance? Draw one! Think about it.

Projectile Motion If you throw a ball forward, you’ll notice it follows a curved path. The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow this curved path.

What About Gravity and Projectiles? What happens if you drop a ball and launch another horizontally at the same time from the same height? Will they both hit the ground at the same time? Teacher demo myth busters

Newton’s First and Second Laws Section 12.2 Aristotle, Galileo, and Newton all studied forces! Insert video about these guys

Newton’s First Law of Motion “Objects in motion stay in motion; objects at rest stay at rest unless an unbalanced force acts upon them.” This law is often called the “law of inertia”. (see Paul Hewitt video.) (demo)

Newton’s Second Law of Motion This one is actually an equation! “The acceleration of an object is equal to the net force acting on it divided by the object’s mass.” http://www.youtube.com/watch?v=iwP4heWDhvw&feature=player_detailpage Or, simply put, a = F m

Do the Math 1. With what force will a rubber ball hit the ground if it has a mass of 0.25 kg? 2. What is the mass of a falling rock if it hits the ground with a force of 147 N? 3. What is the acceleration of a softball if it has a mass of 0.50 kg and hits the catcher’s glove with a force of 25 N?

Weight and Mass Weight is the force of gravity acting on an object. Mass is a measure of the inertia of an object. So, an object’s weight is its mass X gravity, Or, simply put, W = mg This is basically the same thing as Newton’s second law, because W is a force!

Practice Problems If an astronaut has a mass of 112 kg, what is her weight on earth? What is her weight on the moon if the moon’s gravity is 1.67 m/s/s?

More Problems See additional handout for more practice with Newton’s second law. Do Newton’s second law lab. (handout) top gun intro how to launch an airplane

Newton’s Third Law of Motion Section 12.3 “For every action, there is an equal and opposite reaction.” http://www.youtube.com/watch?feature=player_detailpage&v=PrJnWTcW55s Newton’s cradle

Momentum Momentum is the product of an object’s mass and its velocity. Small objects moving slowly have a small amount of momentum. Large objects moving rapidly have a large amount of momentum.

Momentum Formula Momentum = Mass X Velocity 1.What is the momentum of a 0.5 kg puppy traveling at a velocity of 2 m/s? 2. What is the momentum of a 2,000 kg. elephant traveling at a velocity of 2 m/s? Which one would be easier to stop? Why?

Data Analysis The graph shows how the momentum of a given mass changes during a period of motion. According to the information, what is the momentum in kg m/s at 2.75 secs? What is the velocity of the 5 kg. mass at the 2 second interval?

Conservation of Momentum In physics, conservation means that something has a constant value, that is, the amount of momentum doesn’t change. See pg. 376 in your textbook.

Universal Forces Section 12.4 Electromagnetic: associated with charged particles. Electric: act between charged particles or objects. Magnetic: act on certain metals, on the poles of magnets, and on moving charges. Nuclear: forces holding the nucleus of an atom together.

Gravitational Force attractive force that acts between two masses Newton’s Law of Universal Gravitation: Says that every object in the universe attracts every other object. So, is that why students sleep in class?

Universal Gravitation The law really has two parts (mass & distance) 1. As mass increases, gravitational force increases. 2. As distance increases, gravitational force increases.

Think Planets! Bigger planets have a bigger gravitational force. Planets that are closer together are more influenced by each other’s gravitational force.

The Earth, Moon, Tides & Satellites The centripetal force provided by gravity and inertia to keep things in orbit.