GRAVITY AND FRICTION Building Science Champions

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

Contact Force

Non-

conta

ct

Push Pull

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 touchContact force is a push or pull on one

object by another that is touching it – someone playing a guitar

Noncontact force is a force that one object can apply to one another object without touching it – electric force

Are the following pictures contact or non-contact force

Using the force

STRENGTH OF FORCES

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

10 N

5 N10 N

10 N

5 N

10 N

10 N

What is the net force of the squares below?

15 N 15 N

5 N

0 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

17 N10 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

Sliding Friction

Fluid 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

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/s2 sec – 19.6 m/s3 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

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 timeBoth cannon balls will

hit the ground at the end of 6 seconds. B will travel a further distance.

Flash photo of a projectile

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 = ForceG = GravityM1 = Mass of first objectM2 = Mass of second objectr = 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