Motion and ForcesMotion and Forces

Welcome to the Physics part of Welcome to the Physics part of Physical SciencePhysical Science

• Have you ever watched a rocket launch – like the space shuttle? Did you ever wonder what powered the rocket or how fast it went?

• Both of those questions can be related to motion, forces and mechanics.

SpeedSpeed

• The speed of an object is the distance the object travels per unit of time. Speed is a rate which tells you the amount of something that occurs or changes in one unit of time.

• Speed=distance over time

• Speed can be divided into two subtitles constant speed & average speed.

Constant & Average SpeedConstant & Average Speed

Constant speed is the speed that does not change.

Average speed is the total distance divided by time.

Speed is not the same as Velocity!

Speed is how fast something is going but no given or known direction – although often we can assume we know from the information given.

VelocityVelocity

• Velocity is Speed in a given direction.• If you know the velocity of an object you

know its speed and its direction.• Velocity is a vector quantity. It’s also

“direction-aware”. The direction of the velocity vector is simply the same as the direction which an object is moving. If going downward the velocity is downward. If going upward the velocity is upward.

VelocityVelocity• V1 represents the initial or starting velocity

– If the object starts from a rest, V1 will = 0

• V2 represents the final velocity of an object

– If the object ends with a stop, then V2 right at the end will be a zero, but not just a millisecond before that!

– V = d / t– And this means d = vt;

and t = d / v

AccelerationAcceleration

• The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

• Acceleration (a) = ΔV / Δt -or-

• Acceleration = force over mass

Let’s do an example. . .Let’s do an example. . .

Superman Leaps To the Top of a Building. . . . . . .Superman Leaps To the Top of a Building. . . . . . .

• Let’s say the building has a height of 660 feet

• His final velocity (V2) at the top is equal to 0 (cause he stopped to admire the view)

• We don’t know his starting velocity (V1) or how long it took to get to the top. . . .

• So – modifying the equations you are going to learn – we can get:– V2 = 2 g h where g is the acceleration due to gravity– This means V2 = 2 (32 ft/s2) (660 ft) = 205 ft/s = 140

mph!• Superman’s acceleration is definitely unearthly!

In addition to speed and velocity, In addition to speed and velocity, there is a difference between there is a difference between

Weight & Mass Weight & Mass

Mass of an object

will not change even

if the force of gravityon its changes. Mass is a

useful physical property for describing and measuring matter. Mass is a measurement of inertia. The SI unit of mass is Kilograms.

Weight is a measureOf the force of gravityon you. Weight is a useful measurementof how much matter an object has. Wt = mg

Mass and WeightMass and Weight

• Your mass will remain the same – it’s how much stuff that makes you up

• Weight is affected by the force of gravity – so on different planets, your mass will remain the same – but your weight will vary

• (Sounds like a good diet plan for me)

• Weight (Wt or W) = mass x gravity

Forces and MotionForces and Motion

• Isaac Newton quantified the Laws of Motion in his 1687 work “Mathematical Principles of Natural Philosophy”

• He equated motion to

forces and how they

interact on objects

• He developed three

laws

Newton's 1Newton's 1stst Law of Motion Law of Motion

• An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.  

• Sometimes referred to as the “Law of Inertia." – Inertia is the state of rest or resisting a force that may cause

motion or a change in velocity– Galileo First theorized about this using a ball on an incline

• There are two parts: one which predicts the behavior of stationary objects; and the other which predicts the behavior of moving objects . The two parts are summarized in the following diagram.

For example:For example:• You and the car are traveling the same

velocity – and if you brake suddenly – the car stops but you continue forward until the seatbelt stops you!

• If there were no friction – such as air resistance, gravity, etc. – then an object will continue in motion forever at the same velocity! This is how things move in space!

Example of 1Example of 1stst Law of Motion Law of Motion

– Pendulum is a example for Newton's first law• This thing is often called a Newton’s Cradle

– Once its in motion its always in motion.– Once its at rest its always at rest.

Newton's 2Newton's 2ndnd Law of Motion Law of Motion

• The second law states that the acceleration of an object is dependent upon two variables - the net force acting upon the object and the mass of the object.

• It explains the relation of force, mass & acceleration.

• Force=mass x acceleration (F = ma)• The net force on an object is equal to the

product of its acceleration and its mass.

ForceForce

• Force is measured in the SI unit called a Newton (N)– 1 N = 1 kg x 1 m / s2

1 N = .225 lbs1 lb. = 4.448 N

• Forces usually are in equilibrium (balanced)

Force

• Is a push or pull

• It can be divided into two subsets: unbalanced and balanced

• Unbalanced force can cause an object to start or stop moving; or change its acceleration, velocity or direction

• A balanced force is equal forces on an object that will not change the object’s motion

• The shuttle on the Fnet

launch pad is in a state of force equilibrium – the forces are balanced• The shuttle is in inertia or rest

with v1 = 0

Fgravity/Weight

Forces in balance. . . Forces in balance. . .

When forces become When forces become unbalanced – such as unbalanced – such as when the thrust is applied. . .when the thrust is applied. . .• Then you have

lift-off• The applied force

causes acceleration• The two SRB have a

total of 23,600,000 N of

force

(they separate at 45 km

and are recovered and

reused)

AccelerationAcceleration• a = Δ v

Δ t• This means the acceleration rate is the change

of velocity (v2 – v1) divided by the change in time (t2 – t1).

• Any of these variables can be a zero• Acceleration can be either negative or positive!• Acceleration due to gravity (g) is basically the

same concept, except it is up or down!

Acceleration Due to GravityAcceleration Due to Gravity• agrav or just plain a, has a value of 9.80665 m/s2

– We’ll round this off to 9.81 m/s2

• Use 10 for guesstimating!

– This value is for earth – your home planet may vary!

– Believe it or not – agrav at the equator is 9.7804 m/s2 and at the poles it is 9.8321 m/s2

• Can you figure out why?

Free Fall AccelerationFree Fall Acceleration

• If v1 (initial velocity is zero or the object is at rest then falls): – V2 = gt

– V2 = √2gh

– H = ½ gt2

– H = v2 t

2

If vIf v11 does not equal zero. . . does not equal zero. . .

• V2 = v1 + g t

• V22 = v1

2 + 2 g h

• H = v1t + ½ g t2

• H = v2 + v1 t

2

Momentum (Momentum (ρρ))

• Momentum is the product of an object’s mass and velocity

• It is directly proportional to mass and velocity

• It’s the tendency for an object to keep in motion– p = m v– F t = m v; where F t is the impulse or

change in momentum

Newton’s 3Newton’s 3rdrd Law of Motion Law of Motion

• A force is a push or a pull upon an object which results from its interaction with another object.

• A rocket launching is a prime example of this law

• Action force and

reaction force

Newton’s 3Newton’s 3rdrd Law. . . Law. . .

• Basically – the law means that for every action there is an equal and opposite reaction

• A rocket launch – the Fthrust downwards (action) forces the rocket upwards (reaction) against the Fgravity

• Of course, there are factors such as drag (friction) which must be overcome

Now – it’s time for. . .Now – it’s time for. . .

Some Rocket Science!!!!!