ENERGY IN MOTION FORCES Chapter 13 FORCES Chapter

ENERGY IN MOTION: FORCES Chapter 13

FORCES Chapter 13 A push or a pull Forces have 2 properties § Direction § Magnitude § Newton (N) is used to describe the magnitude, or size § 1 lb. = 4. 48 N A force can be exerted without the object moving. § Ex. Force exerted when sitting on a chair

GRAVITY AND WEIGHT The force of gravity on an object is called weight. Gravity exerts 9. 8 N on every kg of mass. Mass and weight are not the same thing! § Weight is less on the moon § W = mg; m=W/g; g=W/m

FRICTION Section 3 The force that opposes the motion between two surfaces that touch The surface of any object is rough § Even surfaces that feel smooth are covered with tiny hills and valleys § Friction is greater when the surface is rougher

TYPES OF FRICTION Kinetic/sliding friction- when an object moves § kinetic = moving § Less friction between surfaces that roll than surfaces that slide Static friction- when an object doesn’t move § The greater the force squeezing two surfaces together; § the greater the Friction force.

FRICTION: HARMFUL AND HELPFUL Friction cause holes in your socks or erosion Without friction your pencil wouldn’t leave a mark on you paper or you would slip and fall when you tried to walk Sometimes it should be increased § Rougher surfaces (sand/salt added to icy roads) § Increasing force (scrubbing a dirty pan in the sink) Sometimes it should be decreased § You can apply a lubricant between two surfaces (motor oil, wax, grease) § Roll rather than slide § Smoother surfaces

FORCE VECTORS To show force we use vectors. § § Vectors have a magnitude (size) and a direction Units = Newtons Can be positive or negative Must have a scale (ex: 1 cm = 1 N)

COMBINED EFFECT OF FORCES When two or more forces act on an object the result is combined or cumulative = net force These pictures don’t show it, but the length of the arrows (vectors) should represent the magnitude of the force § The longer the vector, the larger magnitude the force § And the combined length of the force vectors should equal the length of the net force vector.

BALANCED FORCES Objects that are not moving are called static Net force = 0 aka equilibrium Normal force = the force a surface exerts on an object that is pressing on it. In many cases forces acting on static objects are tension or compression § Tension- a force that acts on an object when it is pulled or stretched § Compression- a force that is exerted when matter is pushed or squeezed

UNBALANCED FORCES Unbalanced forces cause a change in velocity of an object § May be a change in speed, direction, or both § Net force is not zero Moving objects § Can change speed or direction Nonmoving objects § Start moving Direction of movement

SUMMARY Focus your summary on motion, forces, and friction.