Unit 1 AMSTI: Forces & Motion

Matter Everything in the universe is either matter or energy. Matter is anything that has mass and volume. Examples of matter: Your body, air Not matter (energy): Light, sound, fire

Mass Mass is the amount of matter in an object. Mass is measured in grams and kilograms. Mass is not weight!

Weight Weight is the downward pull on an object due to gravity. Weight is the product of gravity and mass. The unit for weight is the Newton (N).

If you go to the moon, will your mass or your weight change? The Moon has less gravity than Earth. You would weigh less on the moon. Your mass will stay the same.

Force A force is a push or a pull that causes an object to speed up, slow down, or turn. Some forces aren’t easily seen Gravity Magnetism Section 1 Notes: Ask them to name some forces they encounter or make each day. Sit very still. Are you applying any forces? Yes, your weight is pushing down on the chair and the chair is pushing down on the floor. So this means that forces don’t always cause a change in motion. Do you apply a force every time you move? Yes

Types of forces Contact force – a push or pull by an object. Gravity – The force of attraction between two masses. Friction – a force that resists motion between two surfaces.

The combination of all forces acting on an object is the net force. The net force on this box is 10 Newtons. 35 N 25 N - Notice: 10 Newtons The Newton is the unit of force (what force is measured in). - The net force will be acting in the direction to the right and the box will move to the right.

Balanced and unbalanced forces Balanced forces cancel each other out. No net force No change in an object’s motion Ex: two people of equal strength pulling on a rope in opposite directions Unbalanced forces don’t cancel each other There is a net force on the object The object changes its motion What examples can you think of? - Other examples of unbalanced forces: football (contact between players), seesaw - When unbalanced forces cause changes in motion, the object either speeds up, slows down, begins to more, or stops moving.

Gravity The force that objects exert on each other because of their masses. Present between all objects Downward pull on objects on Earth. Gravity depends on: The mass of the objects. The greater mass, the greater the gravity. The distance between the objects. The farther objects are apart, the less the gravity.

Gravity on Earth Gravity accelerates falling objects. Acceleration due to gravity is called g g is equal to 9.8 m/s2 See Acceleration Due to Gravity on p. 79

FRICTION Friction is a force that acts to resist sliding between two touching surfaces. Friction works to slow an object down and eventually stop it. Friction acts in the opposite direction of motion.

Friction Between two surfaces. Smoother surfaces have less friction than rough surfaces. The harder the surfaces are pressed together, the stronger the friction. What are some examples of beneficial friction and harmful friction?

Types of friction: Sliding friction Rolling friction is less than sliding friction. Examples: wheels, ball bearings Fluid friction – involved a fluid such as any liquid or gas. Examples: a falling leaf darts from side to side as it falls because of the fluid friction of the air; also known as air resistance.

Static friction You need a larger force to start something moving than you do to keep something moving. Static friction is the force you must overcome to start an object moving. See “Friction and Motion” and “Friction and weight” on pages 86 & 87.

Air resistance The friction due to air. Depends on: surface area and speed of an object Air resistance increases with increased surface area and increased speed of the object.

Weight Weight and mass are not the same. Mass is the amount of matter in an object. Weight is the force of gravity on an object’s mass. Weight = mass X gravity (9.8m/s2 ) What is the weight of a 20Kg mass on Earth?

How would that child’s weight change if they went to the moon? Why? What is the difference between weight and mass? Answer: the child’s weight would decrease because the moon has less gravitational pull and would thus accelerate the child less than on Earth. ( In other words, the acceleration due to gravity on the moon is much less than 9.8. Answer: Mass is the amount of matter (or stuff) in an object and is measured in kilograms (Kg). Weight is measured in newtons and is the result of gravity on an object. Therefore, an object’s weight could change when its mass does not.

Terminal velocity Falling objects encounter air resistance When the air resistance equals the weight of the object, the net force is zero. The speed of the falling object no longer increases and is called the terminal velocity. What type of friction is air resistance? Fluid friction. Is terminal velocity the result of balanced or unbalanced forces? Balanced.

Thought-provoking question… If you are pushing a book across a table, what are the four forces acting on the book? Your push Sliding friction Gravity Upward force of the table. Cover the answers until the students either figure them out or give up. Then discuss.