Building Science Champions Gravity and Friction

In thought Write a paragraph describing the picture. Words to include in your paragraph: Gravity Free fall Wind resistance Speed Acceleration Motion Velocity Terminal Velocity

Objectives Describe friction and identify the factors that determine the friction force between two surfaces Explain how mass differs from weight State the universal law of gravitation Describe the effects of gravity and air resistance on an object in free fall

Key terms Force Contact force Noncontact force Net force Friction Fluid friction Rolling friction Static friction Sliding friction Gravity Free fall Projectile Air resistance Terminal velocity

Types of force Force – is a push or a pull Non-contact Force – is a push or a pull Some forces are only applied when objects are in contact with one another; while others are applied when objects do not touch Contact force is a push or pull on one object by another that is touching it – someone playing a guitar Are the following pictures contact or non-contact force Noncontact force is a force that one object can apply to one another object without touching it – electric force Using the force Contact Force

Strength of forces SI unit for force is the newton (N) What is the net force of the squares below? SI unit for force is the newton (N) Forces can be added together if they are working in the same direction Forces can be subtracted if they are working against one another 5 N 5 N 10 N 15 N 15 N 10 N 10 N 10 N 5 N 0 N 10 N 5 N

Net Force and Direction of forces The net force is the sum of all the forces added together Vector analysis – is the analysis of the different forces on an object resulting in the calculation of net force Net force is given with a direction – in the example below 27 N right 10 N 17 N

Friction Surfaces may seem smooth but they have many irregularities When two surfaces rub the irregularities of one surface get caught on those of the other surface Friction – the force one surface exerts on another when the two rub against each other Friction

Nature of friction The strength of the force of friction depends on two factors: the types of surfaces involved and how hard the surfaces push together. Rough surfaces produce greater friction than smooth surfaces Objects with greater mass and weight produce greater friction than lighter mass and weight Friction releases energy in the form of heat

Types of friction Fluid Friction – friction that occurs when an object moves through a fluid Rolling Friction – friction that occurs when an object rolls over a surface Sliding Friction – friction that occurs when an object slides over a surface. Static Friction – prevents surface from sliding past one another

Fluid Friction Sliding Friction Static Friction – prevents you from sliding. If the box doesn’t move when force is applied it is because there is not enough force to overcome the static friction Fluid Friction

Gravity Gravity – is the force that pulls objects towards Earth Gravity – is an attractive force that exists between all objects that have mass Gravity is calculated on Earth as 9.8 m/s/s or 9.8 m/s2

Free Fall Free fall – when the only force acting on a falling object is gravity Objects in free fall accelerate at a constant rate of speed – 9.8 m/s/s 1 sec – 9.8 m/s 2 sec – 19.6 m/s 3 sec – 29.4 m/s The mass of an object does not affect the rate of speed at Which the object falls Flashphoto

Projectile motion Projectile – an object that is thrown or shot Flash photo of a projectile Projectile – an object that is thrown or shot An object that is dropped and one that is thrown horizontally are both in free fall Horizontal motion does not interfere with free fall and both objects will hit the ground at the same time Both cannon balls will hit the ground at the end of 6 seconds. B will travel a further distance.

Air resistance Air resistance – a type of fluid friction that all objects encounter while falling through the air Friction is the direction opposite to motion, so air resistance is an upward force This why some objects fall faster than others Eventually air resistance will equal out the force of gravity – when this happens the object is no longer increasing in velocity Terminal velocity – the greatest velocity that an object reaches

Weight Weight – is the measure of force of gravity on an object, and mass is the amount of matter in that object Weight = mass (kg) x acceleration due to gravity Weight = mass x 9.8 m/s/s

Universal gravitation The law of universal gravitation states: that the force of gravity acts between all objects in the universe The moon has a gravitational effect on Earth, the Earth has a gravitational effect on the moon Gravitational pull on objects compared to the gravity rate on Earth

Gravitational force & Mass When the mass of one or both objects increases, the gravitational force between them also increases F = G*m1*m2 / r2 F = Force G = Gravity M1 = Mass of first object M2 = Mass of second object r = Distance between the objects 1 kg 1 kg 2 kg 2 kg Gravitational force increases if the mass of at least one objects increases.

Gravitational Force & Distance The attraction between objects decreases as the distance between objects increases Force Distance

Review Weight and mass are not equal Weight = mass * gravity All objects have a gravitational force/attraction to one another There are multiple types of friction

References Anderson, M. et all (2012) Physical Science. McGraw-Hill: Columbus Frank, D.V et al (2001). Physical Science. Prentice Hall: New Jersey