Unit 3 Forces & Motion

Forces Force- an action applied to an object to change its motion(push or pull) Units of lb, N (equal to kg. m/sec 2 ) If forces are balanced then the object won’t accelerate and it is said to be in equilibrium Unbalanced forces=acceleration Forces always occur in pairs!!!!!

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: Unbalanced 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.

3.3 Newton’s Third Law Newton’s 3 rd Law: For every action force there is a reaction force equal in strength & opposite in direction Can be positive or negative Examples: rockets, stepping into a boat, throw a ball when on a skate board

Net Forces Net Force - Total sum of all the forces that act on an object In the same direction – add forces together In opposite directions –subtract forces largest vector wins when forces act in opposite directions At right angles (a 2 + b 2 = c 2 ) When net Force = 0, there is no acceleration. This means the object is: Not moving OR Moving at a constant velocity

Gravity, Weight & Mass Gravity- Force pulling toward mass of planet Depends on mass and distance from center Mass- amount of matter in an object compared to a standard Changes on other planets On Earth= 9.81m/s 2 we round it to 10 m/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 =ma g F w = weight force (N) m=mass (kg) a g =acceleration due to gravity (9.81 m/sec 2 ) F w =ma g F w = weight force (N) m=mass (kg) a g =acceleration due to gravity (9.81 m/sec 2 ) F w m a g Weightlessness!!

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 (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. Occurs when force of air resistance balances the weight of the object.

Falling without air resistance – feather and elephant

Falling with air resistance

Skydiver

Friction: Occurs when 2 surfaces are in contact Depends on types of surfaces the force pushing them together (normal force) Opposes motion (usually) Types of friction: Static friction – friction at rest Kinetic friction – friction of motion What is friction?

Two types of friction Static Friction (F fs ) The frictional force that must be overcome to get an object moving ( F fs ) Kinetic Friction (F fk ) The frictional force that exists once an object is in motion ( F fk )

Free-body diagrams It is a diagram that visually represents the various forces being applied on an object: F f = friction force F A = applied force (push or pull) F N = the normal or support force; perpendicular to surface F W = weight of object The sum of all of the forces involved is called the F NET FNFN FNFN FWFW F A FfFf

μ=coefficient of friction (ratio of frictional forces to the normal force) F N = Normal Force (support force) Often = to the Weight Force (Fw) F f = μ F N Coefficient of Friction

Rules of Friction If an object is slowing down, then the frictional force (F f ) is greater than F a If an object is moving at a constant speed, then the frictional force (F f ) is equal to F a If an object is speeding up, the frictional force (F f ) is less than F a. F f =friction force F a =applied force F a FfFf

Sample Problem A 8-kg box is being pushed at a constant speed along a tile floor with an applied force of 40 N. 1.Draw a free-body diagram. 2.Determine the value for the weight force, friction force and normal force. 3.What is the coefficient of friction between the tile and the box?

Sample Problem A 10-kg box is being pushed at a constant speed along a tile floor with a frictional force of 20 N. 1.Draw a free-body diagram. 2.Determine the value for the weight force, applied force and normal force. 3.What is the coefficient of friction between the tile and the box?

Sample Problem A 5-kg box is sitting on a tile floor that has a coefficient of friction of Draw a free-body diagram. 2.Determine the value for the weight force, friction force and normal force. 3.What is the minimum applied force needed to move the box?