Motion and Forces Welcome to the Physics part of Physical 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.

Speed 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 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.

Velocity 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.

Velocity V 1 represents the initial or starting velocity –If the object starts from a rest, V 1 will = 0 V 2 represents the final velocity of an object –If the object ends with a stop, then V 2 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

Acceleration 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...

Superman Leaps To the Top of a Building Let’s say the building has a height of 660 feet His final velocity (V 2 ) at the top is equal to 0 (cause he stopped to admire the view) We don’t know his starting velocity (V 1 ) or how long it took to get to the top.... So – modifying the equations you are going to learn – we can get: –V 2 = 2 g h where g is the acceleration due to gravity –This means V 2 = 2 (32 ft/s 2 ) (660 ft) = 205 ft/s = 140 mph! Superman’s acceleration is definitely unearthly!

In addition to speed and velocity, there is a difference between Weight & Mass Mass of an object will not change even if the force of gravity on 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 measure Of the force of gravity on you. Weight is a useful measurement of how much matter an object has. Wt = mg

Mass 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 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 1 st 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. 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: 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 1 st 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 2 nd 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. net force 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.

Force Force is measured in the SI unit called a Newton (N) –1 N = 1 kg x 1 m / s 2 1 N =.225 lbs 1 lb. = 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 F net launch pad is in a state of force equilibrium – the forces are balanced The shuttle is in inertia or rest with v 1 = 0 F gravity /Weight Forces in balance...

When forces become unbalanced – such as 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)

Acceleration a = Δ v Δ t This means the acceleration rate is the change of velocity (v 2 – v 1 ) divided by the change in time (t 2 – t 1 ). 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 Gravity a grav or just plain a, has a value of m/s 2 –We’ll round this off to 9.81 m/s 2 Use 10 for guesstimating! –This value is for earth – your home planet may vary! –Believe it or not – a grav at the equator is m/s 2 and at the poles it is m/s 2 Can you figure out why?

Free Fall Acceleration If v 1 (initial velocity is zero or the object is at rest then falls): –V 2 = gt –V 2 = √2gh –H = ½ gt 2 –H = v 2 t 2

If v 1 does not equal zero... V 2 = v 1 + g t V 2 2 = v g h H = v 1 t + ½ g t 2 H = v 2 + v 1 t 2

Momentum (ρ) 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 3 rd 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 3 rd Law... Basically – the law means that for every action there is an equal and opposite reaction A rocket launch – the F thrust downwards (action) forces the rocket upwards (reaction) against the F gravity Of course, there are factors such as drag (friction) which must be overcome

Now – it’s time for... Some Rocket Science!!!!!