Force, motion, and machines

Force of gravity

• Gravity is a force that pulls objects toward each other.

• All matter has gravity

Force of gravity• 2 main things affect how

gravity pulls on an object:– The objects mass (more

mass = more pull from gravity)• Example: Why do the

planets orbit the sun.– The distance between the

two objects (The further the distance = less pull from gravity.)• Example: Why does the

moon orbit the earth?

Weight and mass

• Mass is a measurement of how much matter is in something.

• Weight is a measurement of how gravity is pulling down on something.

• Weight is affected by gravity, mass is not!

Laws of motion

• Isaac Newton came up with 3 laws of motion in the late 1600s.

Laws of motion• The first law of motion,

(sometimes called the law of inertia), says that an object at rest will stay at rest, and an object in motion will stay in motion unless it is acted upon by a force.

• Inertia is when an object resists change in motion.

• The more mass something has, the more inertia it has.

Balanced and unbalanced forces• Newton’s second law of motion can

be summarized by the equation F=ma.

• More mass takes more force to move. (Kick a wall or a ball?)

• Newtons second law of motion explains why an unbalanced forces cause an object to accelerate in the direction of the greatest force.

• Balanced forced lead to NO acceleration – or constant speed

Which way will it accelerate?

Newton’s third law of motion

• Newton’s third law of motion says that for every force there is an equal and opposite force.

• (When you push on the wall, the wall pushes back on you!)

• Newtons laws of motion quick quiz

Friction

• Friction is the force created when two surfaces rub against each other.

Friction

• There are 4 main types of friction– Static friction– Sliding friction– Rolling friction– Fluid friction

Friction

• Some devices use friction to control the motion of an object.

Simple Machines

• A machine makes work easier by changing one of 3 things:– The amount of force you

have to put out.– The distance you use the

force over.– The direction of the

force.

Simple Machines

• A machine’s mechanical advantage is the number of times that a machine increases a force.

• (Example: if it makes your job 3 times easier, it would have a mechanical advantage of 3)

Simple Machines

• There are 6 types of simple machines:– Inclined plane– Wedge– Screw– Lever– Wheel and axel– Pulley

Simple Machines – inclined plane

• An inclined plane is a flat sloped surface. (Like a ramp).

• An inclined plane lets you exert your force over a longer distance.

• Where in the school would you find inclined planes?

Simple Machines – inclined plane

• You can find the mechanical advantage of an inclined plane by using the formula:– Length of incline/height

of incline.

Simple Machines – wedge and screw

• A wedge and a screw are other simple machines that are made from an inclined plane.

• Where are some places in the school that you might find wedges and screws?

Simple machines - Levers• A lever is a rigid bar that is

free to rotate or pivot on a fixed point.

• The point that the lever rotates on or around is called the fulcrum.

• Levers change the amount of force you have to put out, and they can change the direction of the force.

Simple machines - Levers• First class lever:– A first class lever has the

fulcrum in the middle.– Examples of first class

levers include:• Prying open a paint can• Scissors• Rowing a boat• Pliers

Simple machines - Levers

• A second class lever has the resistance (load) in the middle.

• Examples of second class levers include:– Wheelbarrows– Nutcrackers

Simple machines - Levers

• A third class lever has the effort in the middle

• Examples of third class levers include:– Brooms– Tweezers– Tongs– Fishing poles– Hockey sticks

What type of lever is this?

Simple machines - Levers

• You can find the mechanical advantage of a lever using the formula:– Distance from the fulcrum

to the input force / Distance from the fulcrum to the output force.

– Input force is sometimes called “Effort”

– Output force is sometimes called “load” or “resistance”

Calculate the mechanical advantage for the levers

?

Simple machines – Wheel and axle

• A wheel and axle is made of 2 cylinders of different sizes that rotate together.

• A wheel and axle changes the distance over which a force is exerted.

• Examples include:– Screwdriver– Doorknob– A faucet

Simple machines – pulleys

• A pulley is made of a grooved wheel with a rope wrapped around it.

• Pulleys change the amount of force needed and the direction of the force.

• Where might you find pulleys at the school?

Simple machines – pulleys• The mechanical

advantage for a pulley is found by counting the number of rope segments (not counting the one that is attached to the effort.)