Applied Technology Schools G09 Advanced Physics PDF

Mastery Programme G09 Advanced Physics Term 1 Key Concept 6: Free Fall Learning Outcomes Key Concept 6: Freefall  State that mass is a measure of the quantity of matter in an object at rest relative to the observer  State that weight is a gravitational force on an object that has ma...

Mastery Programme G09 Advanced Physics Term 1 Key Concept 6: Free Fall Learning Outcomes Key Concept 6: Freefall  State that mass is a measure of the quantity of matter in an object at rest relative to the observer  State that weight is a gravitational force on an object that has mass  Define gravitational field strength as force per unit mass; recall and use the equation g = W/m and know that this is equivalent to the acceleration of free fall  Know that weights (and masses) may be compared using a balance  Describe, and use the concept of, weight as the effect of a gravitational field on a mass  State that the acceleration of free fall g for an object near to the surface of the Earth is approximately constant and is approximately 9.8 m/s2  Describe the motion of objects falling in a uniform gravitational field with and without air/ liquid resistance (including reference to terminal velocity)  Know that friction (drag) acts on an object moving through a liquid  Know that friction (drag) acts on an object moving through a gas (e.g. air resistance) Mass and Weight Mass: Weight: Mass is a measure of the quantity of matter in an object at Weight is a gravitational force on an object with mass rest relative to the observer Weight is a force, so it is a vector quantity Mass is a scalar quantity The SI units for force are newtons (N) The SI unit for mass is the kilogram (kg) Weight is the effect of a gravitational field on a mass Consequently, mass is the property of an object that resists change in motion The weight of a body is equal to the product of its mass (m) and The greater the mass of an object, the more difficult it is the acceleration of free fall (g) to speed it up, slow it down, or change its direction Gravitational Field Strength Gravitational field strength is defined as: The force per unit mass acting on an object in a gravitational field On Earth, this is equal to 9.81 N/kg Gravitational field strength is also known as acceleration of free fall, or acceleration due to gravity In this context the units are m/s 2 The value of g (gravitational field strength) varies from planet to planet depending on their mass and radius A few examples of varying gravitational field strength are shown in the fiure.:  Using a Balance Mass Vs Weight An object’s mass always remains the same, however, its weight The weight of two objects can be compared using a balance. will differ depending on the strength of the gravitational field on different planets Because the gravitational field strength (g) is constant everywhere on Earth, this also allows us to measure the mass of For example, the gravitational field strength on the Moon is 1.63 an object. N/kg, meaning an object’s weight will be about 6 times less than on Earth Friction in Fluids Gases and liquids are known as fluids. Fluids are different to solids because the particles in fluids can move around Friction acts on objects moving through gases and liquids as the particles collide with the object. This type of friction is called drag Air resistance is a type of friction that slows the motion of an object Particles bump into the object as it moves through the air As a result, the object heats up due to the work done against the frictional forces Acceleration of Freefall (g) In the absence of air resistance, all objects fall with the same acceleration, regardless of their mass. This is called acceleration of freefall.(or acceleration due to gravity) This acceleration is equal to the gravitational field strength and is approximately 9.8 m/s2 near the Earth’s surface. This means that for every second an object falls, its velocity will increase by 9.8 m/s. So long as air resistance remains insignificant, the speed of a falling object will increase at a steady rate, getting larger the longer it falls for. Terminal Velocity Objects falling through fluids (fluids are liquids or gases) in a uniform gravitational field, experience two forces: Weight (due to gravity) Friction (such as air resistance) A skydiver jumping from a plane will experience: A downward acting force of weight (mass × acceleration of freefall) An upward acting force of air resistance (frictional forces always oppose the direction of motion) The force of air resistance increases with speed. This is illustrated in the image. Terminal Velocity (cont’d) Initially, the upwards air resistance is very small because the skydiver isn't falling very quickly. Therefore, there are unbalanced forces on the skydiver initially As the skydiver speeds up, air resistance increases, eventually growing large enough to balance the downwards weight force Once air resistance equals weight, the forces are balanced. This means there is no longer any resultant force Therefore, the skydiver's acceleration is zero - they now travel at a constant speed This speed is called their terminal velocity When the skydiver opens the parachute, the air resistance increases. This is due to the increased surface area of the parachute opening The upward force of air resistance on the skydiver increases, slowing the acceleration of the skydivers fall. The skydiver decelerates Eventually, the forces balance out again, and a new slower terminal velocity is reached  Part (b) When an object falls, initially it accelerates. The resultant force on the object is very large initially, so it accelerates This is because there is a large unbalanced force downwards (its weight) - the upward force of air resistance is very small to begin with Part (c) As the object falls faster, the force of friction acting upon the object increases. The force of air resistance is due to friction between the object's motion and collisions with air particles Collisions with air particles slow the object down, so air itself produces a frictional force, called air resistance (sometimes called drag) Part (d) Eventually the object falls at a steady speed when the force of friction equals the force of weight acting on it. When the upwards air resistance increases enough Part (a) to balance the downwards weight force, the resultant force The weight of an object is the product of the object's mass and on the object is zero the gravitational field strength. This means the object isn't accelerating - rather, it is moving The weight force is due to the Earth's gravitational pull on the object's at a steady (terminal) speed mass as it falls through a uniform gravitational field