# Science - Motion/Forces

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### Aristotle

He thought that the rate at which an object falls depended on the object's mass.

### Galileo

He argued that the mass of an object does not affect the time the object takes to fall to the ground.

### Objects fall to the ground at the same rate

because the acceleration due to gravity is the same for all objects.

### Acceleration depends on

both force and mass

### Acceleration

this is the rate at which velocity changes over time.

### All objects accelerate towards Earth at a rate of

9.8 meters per second per second.

### 9.8 m/s/s/

This means 9.8 meters per second per second.

### In the second second an object falls, the downward velocity is

19.6 m/s downward

### In the third second an object falls, the downward velocity is

29.4 n/s downward

9.8 m/s downward

∆v = g × t

### In ∆v = g × t, the g stands for

acceleration due to gravity on earth

### In ∆v = g × t, the t stands for

the time the object takes to fall.

### 29.4 m/s

The speed of an object when it hits the ground after a period of 3 seconds.

### 19.6 m/s

The speed of an object when it hits the ground after a period of 2 seconds.

10 seconds.

5 seconds

### air resistance

force that opposes the motion of objects that move through the air

### the force of gravity

this pulls down on a falling object

air resistance

### net force on a falling object

the force of air resistance subtracted from the force of gravity.

### free fall

This occurs only if gravity is pulling down an object with no other forces acting on it. It is usually accompanied by the playing of a Tom Petty song. :)

no air

### two places there is no air

space and a vacuum. A third place is also potentially inside an 8th graders brain. :)

### weight

this is a measure of gravitational force.

### terminal velocity

the constant velocity of a falling object when the force of air resistance is equal in magnitude and opposite in direction to the force of gravity

### why is terminal velocity a good thing

because if there was no maximum speed, things like hailstones would destroy everything in their path.

### parachute

this increases air resistance and slows one to a safe terminal velocity.

### It is impossible for

any object to be weightless anywhere in the universe.

### orbiting

when an object moves in a regular path around another object

### centripetal force

any force that causes an object to move in a circular path

### centripetal means

towards the center

### Why does the moon stay in orbit

because the Earth's gravitational force provides a centripetal force on it.

### projectile motion

the curved path that an object follows when thrown, launched, or otherwise projected near the surface of Earth

### hopping grasshopper

The motion of this activity is an example of projectile motion.

### leaping frog, water sprayed by a sprinkler, an arrow shot by an archer

These are all examples of projection motion.

### horizontal motion

motion that is parallel to the ground

### which two forces combine to cause projectile motion

horizontal motion and vertical motion.

### vertical motion

motion which is perpendicular to the ground.

### a ball dropped without a horizontal push wll

fall straight to the ground

### a ball which is given a horizontal push will

fall to the ground with projectile motion

### Does gravity affect horizontal motion?

No, it only affects vertical motion.

### If a rock at rest falls off a tall cliff and hits the valley below after 3.5 s, what is the rock's velocity when it hits the ground?

34.3 m/s downward

### Newton's First Law of Motion

Objects in motion will remain in motion unless acted upon by another force.

### unbalanced force

This is what changes the velocity of an object as defined in Newton's First Law.

friction

### Objects at rest

tend to stay at rest until an unbalanced force acts upon it.

### law of inertia

The other name for Newton's First Law of Motion

### inertia

the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts on the object.

### Newton's Second Law of Motion

The acceleration of an object depends on the mass of the object and the amount of force applied.

### Newton's Second Law of Motion: Part 1

Acceleration Depends On Mass

### Newton's Second Law of Motion: Part 2

Acceleration Depends On Force

### Why do newer cars pollute less than older cars?

They have less mass, so it requires less force to move them.

### Acceleration will increase when

a larger force is exerted.

### F = m × a

The correct formula for Newton's second law of motion.

### Newton's second law explains why objects fall to Earth

with the same acceleration.

5 m/s

4 m/s

6 m/s

9 m/s

### Newton's Third Law of Motion

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

### Swimmer action force

Hands pushing through the water.

### Swimmer reaction force

Water pushing on the hands.

### For a swimmer, which force pushes them forward

The reaction force.

### A force is always exerted by one object

on another object.

pair of forces

### Space shuttle action and reaction force pairs

Thrusters pushing the exhaust games downward as gases push the shuttle upward with equal force.

### Action and reaction forces example

A rabbit's legs exerting a force of Earth with Earth exerting an equal force on the rabbit's legs causing the rabbit to accelerate forward.

### Equal and opposite force

whenever an object exerts a force on another object, the second object exerts this...

### momentum

a quantity defined as the product of the mass and velocity of an object.

p = m × v

19,200 kg m/s

16,000 kg m/s

32,000 kg m/s

24,000 kg m/s

12,000 kg m/s

18,000 kg m/s

### law of conservation of momentum

this means that anytime objects collide, the total amount of momentum stays the same.

### when does the law of conservation of momentum apply

then objects stick together or bounce off each other after they collide.

### billiard balls bouncing off one another

This is an example of the law of conservation of momentum

### The more momentum an object has.....

the harder it is to stop the object or change its' direction.

### If a truck has more mass than a car....

a larger force is needed to stop the truck than the car.

### If a fast moving car has a greater velocity than a slow moving car....

a larger force is needed to stop the faster moving car.

### Objects sticking together

After this happens, they move as one. the masses of the objects remain the same. The velocity changes. total momentum before and after collision stays the same.

### Objects sticking together - examples

When one football player tackles another player or when a dog catches a ball.

### Objects bouncing off one another - examples

Bowling balls and pins, billiard balls

### Objects bouncing off one another - what is happening

Momentum is transferred from one object to another object.

### Objects sticking together - what is happening

The mass of the combined object is equal to the mass of the two objects added together. Together, the one object has a velocity which is different from the velocity of the two individual items.

### Conservation of momentum is explained by

Newton's 3rd Law of Motion

### When a cue ball hits a billiard ball.......

the cue ball creates the action force, which makes the billiard ball move.

### What is the reaction force when a cue ball hits a billiard ball.......

this is the equal but opposite force exerted by the billiard ball on the cue ball. It make the cue ball stop moving.

moving objects.

Example: