Chapter 4 Force and Motion

Force and MotionA force is a push or a pull exerted

on an object. Can cause objects to speed up,

slow down, or change directionPg 89 “A force exerted on an

object causes the object’s velocity to change; that is a forces causes an acceleration (or deceleration)”

Forces and MotionEverything around the object that

exerts forces on it is called the external world.

Contact and Field ForcesField Forces are exerted without

contact◦Gravity◦Magnetism

Contact forces are exerted with contact.◦Push, pull, drag, touch

An agent is the thing that is exerting the force.

Free Body DiagramsHelp you analyze how forces

affect motionCreate a Pictorial Model

◦Go to Page 89Read “ To make a physical

representation…..” paragraph

Drawing Free-body Diagrams1. Represent the object with a dot2. Represent each force with a arrow that

points in the direction that the force is applied. Make the length of each arrow proportional to the size of the force.

3. Draw the force arrows pointing away from the particle. Use the symbol F with a subscript label to identify both the agent and the object on which the force is exerted

Combining ForcesNet Force: the vector sum of all

the forces on an object.Pg 92 Figure 4-5

Newton’s 2nd LawF = ma

The sum of the forces acting on the object is equal to the mass of the object times the acceleration of an object.

F = force a = acceleration m = mass

Force = Newtons or (kg * (m/s2))

accel = m/s2

mass = kg

Newton’s 1st LawAn object at rest will remain at rest, and an

object in motion will continue moving in a straight line with a constant speed until the net force of the object isn’t zero.

AKA: Law of Inertia

Inertia: an object’s ability to resist change

Equilibrium◦ When net force = 0◦ Object is at rest or moving at a constant velocity

Different Forces we will usePg 94 table 4-2

Do Pg 93 7 and 8

What is the magnitude of a force on a 3 kg that is acceleration a 2 m/s2

A force of 15 N is applied to a chair that weighs 10 kg. What is the acceleration of the chair? What is the acceleration if there is an opposing force of 10 N too?

Free Falling ObjectNo touching anythingAir resistance is neglectedGravity is the only forceBall’s acceleration is g, 9.8 m/s2

F = maThink about how g is difference in

other places.

Scales (bathroom)Based on springsThe scale exerts an upward force

on your b/c you are in contract with it. You are not accelerating so spring force must equal your weight (another type of force due to gravity)

Scales measure weight

Mass vs. weightWeight is dependent on gravityMass isn’tExample: Your mass is the same

on every planet because the amount of matter inside you stays the same.

Weight, however, depends on the magnitude of the gravity

15-18

Apparent WeightEx. Standing on a scale in an

elevator

How would you feel when you start moving up? Would you scale increase in force or decrease?

How would you feel when you start moving down? Or if you slow down? What would the scale say?

Apparent WeightThe force an object experiences as a

result of all the forces acting on it, giving the object an acceleration

What if the cable breaks? You and the scale would accelerate with a = -g. You would be weightless

Weightlessness does not mean that an object’s weight is actually zero. But no contact forces so no apparent weight.

19-20

Drag Force and Terminal VelocityAir exerts a balanced force on all sides-

no net forceDrag Force is the force exerted by a

fluid on the object moving through fluid. As the speed of the object increases, so

does the magnitude of the drag force.Size and shape of the object also affects

the drag forceIt is also affected by the properties of

the fluid (ie. Viscosity and temperature)

Terminal VelocityPg 101 Read first paragraphThe constant velocity that is reached

when the drag force equals the force of gravity is called terminal velocity.

How can we apply this idea to life situations?

Competitive skiers, sky divers, bikers21-27

Interaction ForcesForces come in pairsInteraction pair is two forces that

are in opposite directions and have equal magnitudes.◦Forces exist together or not at all.◦Ex. Roller Bladers

Newton’s 3rd LawAll forces come in pairs.

F A on B = -F B on A The force of A on B is equal in magnitude and opposite in

direction of the force on B on A

Example: Holding a bookPg 104 28-31

Forces of Ropes and StringsTension is the force exerted by a

string or ropeStrings/ropes are considered

masslessEx bucket tried to string in

equilibrium. The tension in the rope is equal

to the weight of the bucket (like a scale)

Example: Tug-of-WarPg 106 32-33

Normal ForceAny time two objects are in

contact, they each exert a force on each other.

The normal force is the perpendicular contact force exerted by a surface on another object.

Always equal to weight of the object