Unit 2 Forces & Motion

Forces Force- Ability to change motion(push or pull) Units of lb, N=kg. m/sec 2 If forces are balanced then the object won’t move and it is said to be in equilibrium Forces cause an object’s velocity to change & can therefore cause acceleration. Unbalanced forces=move Forces always occur in pairs!!!!!

Categories of Forces Contact forces- force exerted from an external source Applied force Friction Support force Field forces- forces exerted without contact Magnetic Electrical Gravitational

Newton’s Laws of Motion Newton’s First Law: “AKA” the Law of Inertia Inertia- property of an object to resist change in state of motion an object will stay at rest until acted on by an unbalanced force In other words, things tend to keep on doing what they were doing in the first place unless you apply a force.

Newton’s Laws of Motion Newton’s Second Law: force causes an object to accelerate while the object’s mass resists acceleration F net =ma a=acceleration (m/sec 2 ), F= force (N), m=mass (kg) F net= Right – Left F net= Top - Bottom EX. Lighter cars go faster than heavier ones pushed with equal force. Lighter cars resist the force of acceleration less allowing them to move faster than heavier ones.

Net Force Net Force - Vector sum of all the forces that act on an object In the same direction - add In opposite directions – subtract largest vector wins direction At right angles (a 2 + b 2 = c 2 ) When net Force = 0, there is no acceleration

3.3 Newton’s Third Law Newton’s 3 rd Law: For every action there is a reaction force equal in strength & opposite in direction For every action there is an equal & opposite reaction Can be + (move to right) or – (move to left) Examples: rockets, stepping into a boat, throw a ball on a a skate board

Gravity, Weight & Mass Gravity- The downward pull on you by the earth Depends on mass Mass- amount of matter in an object compared to a standard Changes on other planets On Earth= -9.8m/s 2 Weight- force created by gravity depends on mass It is a force acting on object in units of lb or N F w =mg F w = weight force (N) m=mass (kg) g=acceleration due to gravity (-9.8 m/sec 2 ) F w =mg F w = weight force (N) m=mass (kg) g=acceleration due to gravity (-9.8 m/sec 2 ) Fw m g

Drag Force Drag force – the force exerted by a fluid (liquid or gas) on an object moving thru the fluid. -We refer to this as air resistance when objects move thru the air The faster an object goes the greater the drag force. -When the drag force equals the force of gravity there is no acceleration. -A constant velocity – known as terminal velocity. -Large surface areas have a lower terminal velocity -Small surface areas have a higher terminal velocity

Law of Universal Gravitation Speaking of gravity… There’s earthly gravity (the earth & objects are attracted to each other) There’s universal gravity (attraction between heavenly bodies like the Sun and moon) No matter what kind of gravity you speak, two variables influence the strength of this attractive force: Object’s mass Distance between objects F = G m 1 m r 2

Free Fall- Drop straight down in the absence of air resistance. Acceleration due to gravity -9.8 m/sec 2 Object’s will experience uniform acceleration and uniform increases in velocity when in free fall! Terminal Velocity- Highest velocity reached by a falling object When an object stops accelerating, but continues to fall terminal velocity is achieved. When air resistance balances the weight of the object pushing down on it.

Friction: A force that… Reduces acceleration Works against motion Occurs when surfaces move against on another Causes wear on parts Examples of friction: Air friction- (air resistance)- aka – “drag” Sliding friction (rub hands together) Viscous friction- (oil in car engines & joint fluid) Rolling friction- (wheel on road, ball bearings) What is friction?

Static Friction (F fs ) Sliding Friction (F fk ) The frictional force that must be overcome to get an object moving μ s = F fs μ=coefficient of friction F N Fw: Weight Force = mass(gravity) F N : Normal Force Is always = to the Weight Force (Fw) μ s is always greater than μ k The frictional force that exists once an object is in motion. AKA kinetic friction μ k = F fk F N

Rules of Friction If an object is not moving, then the frictional force (Ff) is greater than the pulling force If an object is moving at a constant speed, then the frictional force (Ff) is equal to the pulling force If an object is accelerating, the frictional force (Ff) is less than the pulling force. F p =pulling force = F a =applied force F p or F a FfFf