Forces and the Laws of Motion

Changes in MotionForce is a push or a pull exerted on an objectForce causes a change in velocityA force acting on a mass causes an acceleration change in velocityIf 1 kg of mass is accelerated by 1 m/s2 then 1 Newton (N) of force is needed

Some forces act when two objects are in contact. These are contact forces

Some forces act from a distance (the objects are not in contact). These are Field Forces

Newtons First Law of MotionInertia The tendency of an object to maintain its present state of motionGalileo conducted experiments and concluded that if no friction is present, an object in motion will remain in motion at constant velocity foreverMass Measure of matter in an object

Inertia, Mass and MotionInertia is a property of mass. The more mass an object has the greater its inertia.

Newtons 1st Law of MotionLaw of InertiaAn object will remain at rest or in motion in a straight line at constant velocity (not accelerating) unless an UNBALANCED FORCE acts on the object.At RestUnless an unbalanced force acts on the objectIn Motion

Because of Inertia, the car stops (force of the wall) but the driver keeps on moving! Seat belt anyone?

Gravity, Mass and WeightThe weight of an object is the force that is applied to an object by gravityFWeight = m x gSince g has been measured to be the same for all objects, neglecting air resistance, the greater the mass of an object the greater the objects weight

A scale measures the amount of support force needed to balance the downward force of gravityYour weight (or anythings weight), is the product of your mass times the acceleration of gravity at your locationFWeight = m x gAt That Location

Special Applications of Newtons LawsWeight and FrictionWeight The force between too bodies, usually between a large mass and a much smaller massWeight is not an inherent property of an object (like mass or inertia) but is location dependentThe farther an object is from the center of the mass, the less the objects weightThe acceleration of gravity by a body is the same for all objects

What keeps the arrow moving after it is released from the bow?Early scientist thought that an invisible force called impetus pushed the arrow as it moved through the air.

Force Diagrams Free Body DiagramA force diagram shows all of the forces acting on an object using force vectorsFirst, analyze the problem

Net or Unbalanced Force The force that remains when all of the force vectors are addedAdding all of the forces on the car equals 0 (No net force)

Finding the Net or Unbalanced Force1. Draw a force free body diagram and identify all of the forces acting on the object2. Determine an appropriate coordinate system and sketch the force vectors using that coordinate system3. Find the x and y components of the vectors4. Find the net force acting in the x and y direction5. Determine the net force (magnitude and direction)6. If the net force is 0 the object is in equilibrium.

Newtons Second and Third Laws

Force, Mass and AccelerationNewtons Second Law The net force acting on an object equals the mass of the object times the objects acceleration.

F = m x aForce is a vector. The vector property of direction is in the direction of the acceleration. Mass is a scalar and changes the magnitude of the force.

Solving Newtons Second Law for acceleration gives:

a = F/mThe acceleration of an object is directly proportional to the net force on an object and inversely proportional to the mass of the objectNet Force causes acceleration

The weight of the mass is the force that accelerates the systemWhat TOTAL mass is being accelerated? The weight of the hanging mass is accelerating the total mass of the system. Adding a mass on the cart increases the mass of the system. Did the force increase? How will it affect the acceleration.

Newtons Second Law and Falling ObjectsNeglecting air resistance, all objects fall at the same rate of acceleration, gF/m = g = F/mThe greater the mass, the greater force needed to accelerate the object. So the ratio of F/m is g

FFNewtons Third Law of Motion

Newtons Third Law of Motion Forces always act in pairs. For every force there is an equal and opposite forceFFThe cannon applies a force to the cannon ball, and the cannon ball applies an equal but opposite force to the cannonaaa = F/Ma = F/m

Gravity, Force and Newtons Third LawSince the earths mass is MUCH greater than the mass of the apple, the acceleration of the earth up is VERY small-FApple on Earth = FEarth on Apple

Application of Newtons Laws & Free Body Diagramq = 0OWhen the angle of the incline is 0O FW = FN

q is not 0OyxFWFW is the weight of the wagon and is toward the center of the earthThis is the parallel component down the incline F=

A Closer Look At The Forces on an Incline

The Static and Kinetic Forces of Friction and Free Body DiagramIn figure a, there is no frictional force because there is no force trying to set the jug in motionIn figure b, a force is being applied, but not enough to set the jug in motionIn figure c the applied force is greater than the maximum frictional force so the jug acceleratesFnFgFnFnFgFgFaFa

Friction acts to oppose the applied force. The frictional force increases until it reaches a maximum. The frictional force BEFORE the jug moves is called the Static Force of Friction FS, the force that is opposing the motion.FgFnFa

After the jug is in motion, the force needed to maintain constant motion is the Kinetic Force of Friction FK For most surfaces FS > FKFnFgFk= FaFSFk

The nature of friction is due to the irregularities of the surfaces in contact. Friction is really a microscopic effectFS

A graph of the applied force vs. frictional force. The peak in the graph represents the maximum static frictional force and the horizontal line is the kinetic force that maintains constant motionThe frictional force equals the applied force until the maximum frictional force between the surfaces is reached. At that point the object begins to move. For most surfaces, the applied force to maintain constant motion is less than needed to begin the motion

Calculating the Frictional ForceThe frictional force is determined by the Normal force of the surface on the object and the type of surfaces that are in contactFf = mFN and m = Ff / FNWhere m (mu) is the coefficient of static (mS) or kinetic (mK) friction. Since m is a ratio of two forces, it lacks a unit

Objects in Equilibrium When the FNet on an object is 0, it is in Equilibrium Objects in Equilibrium are not acceleratingObjects in Equilibrium may or may not be moving

If M1 and M2 have the same mass, then the system is in equilibriumIf M1 and M2 do not have the same mass, then the system is not in equilibrium and the masses will accelerateM2 > M1

q is not 0OyxFWThe Frictional Force, Ff is acting on the wagon and is opposite the motionSF = FNet

Pulling a Wagon Up an Incline Free Body DiagramSF = FNetFW is the gravity acting on the wagons mass.Fn is the normal upward force from the incline.