Gravity, Friction, and Pressure

Demonstration• I’m going to drop a ping pong ball and a golf

ball from the same height at the same time. • Which one is going to hit the desk first? What

do you think?

• What did you observe?

Gravity• GRAVITY = the force between two objects.• As an object falls to the ground, it accelerates,

moving faster and faster.• The strength of the gravitational force between two

objects depends on two factors:– Mass- the more mass, the greater the gravity that is

exerted on the other object• If the mass of an object is doubled, the force between the

two objects is doubled– Distance- as distance increases, gravity decreases• If the distance is doubled, the gravity is one-fourth as

strong as before

Earth’s Gravitational Pull• The Earth’s gravity exerts a downward pull

downward pull on a dropped coin.• The coin exerts an equal upward force on earth.• Because the coin has such a small mass compared

to Earth, the coin can easily be accelerated.• This acceleration due to the Earth’s gravity is called

“g” and is equal to 9.8 m/s2 at Earth’s surface.• You calculate this using F = ma, where a = g, so the

formula for calculating force due to gravity on Earth is F = mg

Falling Objects• Two objects with different masses fall at the

same rate.• This is because the object with greater mass

has more gravity exerted on it, but greater mass means more inertia so the greater force doesn’t produce a larger acceleration.

Weight vs Mass• Weight and mass aren’t the same things!• Mass = how much matter something contains• Weight = force of gravity on an object• Weight can change based on location (on the

Earth, on the moon, in space, etc)

Friction• FRICTION = force that resists the motion between

two surfaces in contact.• Friction between two surfaces depends on:– The materials that make them up. Different

combinations create different frictional forces. – Motion. It’s easier to apply force to an object once it’s in

motion than to apply force when it is stopped. MORE MOTION = LESS FRICTION

– Pressure. The harder two surfaces are pushed together, the more difficult it is for them to slide over each other. MORE PRESSURE = MORE FRICTION

Friction and Heat• Friction between surfaces produces heat.• The more friction, the more heat.• This is because the molecules are speeding up.

The faster they move, the temperature increases.

• This can be intense- you can rub two sticks together and start a fire if you rub them fast enough. If there isn’t oil to lubricate (reduce friction) parts of your car, it can overheat and stop working.

Fluids• Falling objects in a vacuum accelerate at the same

rate, but can have different accelerations in the air. This is because air is fluid.

• FLUID = a substance that can flow easily (gases and liquids)

• When an object moves through a fluid, it pushes those fluid molecules out of the way. At the same time, the molecules of the fluid exert an equal and opposite force on the object that moved them. Friction depends on the shape of the moving object.

Air Resistance• Air Resistance = the friction due to air• Air resistance depends on surface area and the

speed of an object:– An object with larger SA comes into contact with

more molecules as it moves than an object with smaller SA. INCREASED SA = INCREASED AIR RESISTANCE

– The faster an object moves through the air, the more molecules it comes into contact with in a given amount of time. INCREASED SPEED = INCREASED AIR RESISTANCE

PRESSURE• PRESSURE = measure of how much force is acting on a

certain area (how concentrated a force is). • You can increase pressure by increasing force.• Pressure = Force P = F

Area A

• Remember, force is measured in Newtons. Area is measured in m2.

• Pressure is measured in PASCALS (Pa). 1 pascal = the pressure exerted by 1 N of force on an area of 1 m2. So 1 Pa = 1 N/m2

Sample Problem• A winter hiker weighing 500 N is wearing

snowshoes that cover an area of 0.2 m2. What pressure does the hiker exert on the snow?

• USE KQS!• What do we know? A = 0.2 m2 F = 500 N• What is the question? Pressure?• Solve! P = F/A P = 500 N P = 2500 N/m2

0.2 m2 N/m2 = Pa so…. P = 2500 Pa

Practice ProblemIf a winter hiker weighing 500 N is wearing boots that have an area of 0.075 m2, how much pressure is exerted on the snow?

Pressure in fluids and depth• Pressure in fluids depends on depth.• Pressure in air: – Changing elevation- Air has weight. The higher you go in

elevation, there is less air pressure– Changing density- Air at lower elevations is more

compressed and denser than air at higher elevations.• Pressure in water:– Water molecules are closer together than air molecules.– Water exerts more pressure on you than air and the

deeper you go the more pressure is exerted on you.

Buoyant Force• Buoyant force = the upward force on objects in a

liquid.• Whether or not an object floats in a fluid

depends on the densities of both the object and the fluid.

• Density = mass/volume, measured in g/cm3

• If an object floats in a fluid it is LESS DENSE than the fluid.