unit 20 - forces and friction

Forces

Types of Forces

Lift and Drag Magnetism Thrust

PullPush Gravity

What Forces DoIf forces are unbalanced the object will do one of the following things:

Begin to move

Stop moving

Change direction

Speed up

Slow down

Change shape

Balanced ForcesThe pulling force is equal on both sides, resulting on the rope remaining stationary

Balanced ForcesEach sumo wrestler is applying equal pushing force,

resulting in them remaining stationary

Balanced ForcesThe downward for of gravity is equal to the lifting force of the air under the

wings, resulting in the aircraft remaining aloft

Balanced Forces

The ship is floating on the water, the forces are balanced.

Gravity is pulling the ship downward, but the water is forcing the ship upwards due to its displacement of

lesser density

Equal and Opposite Forces

If substances are sufficiently strong, they can not be moved when forces are applied, because they will act

in an equal and opposite manner

Equal and Opposite Forces

In the case of the balancing rocks, the upward force of the lower rocks make up for the downward force of

gravity While the balloon pulls upward, the child’s hand pulls downward

Equal and Opposite Forces

In addition, the forces being applied by gravity can overcome upward forces, with sometimes unfortunate

results

Equal and Opposite Forces

If substances are sufficiently strong, they can not be moved when forces are applied, because they will act

in an equal and opposite manner

Unequal Forces

Those unequal forces will cause movement whether it is horizontal or vertical

Unequal Forces

If upward thrust or lift exceeds the force of gravity,

the spaceship or aircraft will take off.

Unequal Forces

Drag

In sky diving, initially gravity far exceeds drag, and thus you free fall

very rapidly (over 100 mph).

Once the chute is released, the amount of drag increases

dramatically and you slow to under 10 mph

Unequal Forces

Using drag and lift, wingsuit flyers and paragliders can control where they go and soar around for long periods before eventually landing.

Unbalanced Forces

The magnetic attractive force is pulling the metal upwards

Gravity is pulling the metal to the ground

The magnetic force is greater than the force of gravity therefore the metal can be lifted up

Unbalanced Forces

The magnetic repulsive force is pushing the train upwards

Gravity is pulling the metal to the ground

The magnetic force is greater than the force of gravity therefore the train can be levitated

Maglev technology has potential, because the train is not physically in contact with the track. It hovers and thus there is no friction

Balanced and Unbalanced Forces

The rubber band is exerting an opposite force to the person pulling on it.

It will remain in this state until one of two things happen:a) The material snaps (pulling force exceeds material strength)b) Person releases force and rubber band rebounds and launches

FRICTION

Friction occurs when two substances are in contact with each other

The greater the friction, the more force must be applied to move

5 mph

24 mph

35 mph

What is friction?Friction is a force that acts

when two objects rub together.When the two objects appear to

be smooth, there is less friction.When the two objects are

rough, there is a greater amount of friction.

Friction Road bikes have tiny thin

wheels to reduce contact with the ground and friction. In addition, the bikers wear tight fitting clothing to reduce air friction.

Mountain bikes have thicker tires to increase contact with the ground, using friction to prevent the biker from slipping and falling in rough terrain.

“Friction-less Movement”

Some things can be done to reduce friction.

Using lubrication such as water or oil can reduce the contact between solid surfaces

Air hockey uses air to lift the puck up and allow it to travel rapidly.

Friction at the Microscopic Level

Even though some objects look smooth, microscopically they are very rough & jagged.

As they slide against each other they grind and drag against each other.

Flies use microscopic hairs on their feet to grip these jagged edges on the surfaces

Different types of Friction:

Static Friction: Objects not moving.

Different types of Friction:

Rolling Friction: Round objects rolling.

Different types of Friction:

Sliding Friction: Objects moving.

Different types of Friction:

Fluid Friction: Force through fluids or air. When it is through air it is called air resistance.

Fluid Friction

In this case, the waterfall is encountering air resistance on the way down, causing it to spread out and slow down.

It also experience friction upon striking the pool surface, causing it to splash and spread.

Friction is good… Without friction we wouldn’t be able to walk, sit in a

chair or drive safely. Everything would keep sliding and falling over.

We need friction between our feet and the ground or we would not be able to stand up.

When you are riding a bike and press the brakes, friction slows your bike down.

Friction is bad… If you want to push something heavy, friction

makes it harder to move. Friction produces heat. Causing heat to build up

is not always a good thing, especially inside a car engine.

We try to reduce the amount of friction in a car engine by adding oil.

Gravity

The attractive force between all objects in the universe

GravityThe greater the mass, the more

gravity it has