Physics
Free Falling Object
A free falling object is an object that is falling under the sole influence of gravity, with no other forces acting upon it. In this state, the object experiences a constant acceleration due to gravity, typically denoted as 9.81 m/s^2 near the surface of the Earth. The motion of a free falling object can be described using equations of motion derived from Newton's laws of motion.
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eBook - ePubThe Really Useful Science Book
A Framework of Knowledge for Primary Teachers
- Steve Farrow, Amy Strachan(Authors)
- 2017(Publication Date)
- Routledge(Publisher)
Contrary to the belief of Aristotle (Greek philosopher and scientist), Galileo (Italian scientist) proved that, in the absence of air resistance, all objects fall to the ground at the same rate. He tested this by dropping two balls, one light and the other heavy, to show they both hit the ground at the same time. This was further proved by astronauts on the Moon, who dropped a feather and hammer, simultaneously, that touched the surface at the same time.An object falling through the Earth’s atmosphere will not continue to accelerate indefinitely. At some stage, the force of air resistance, or drag, will equal the weight of the falling object. At this point, the forces on the object will balance, the object will stop accelerating and it will continue to fall at a constant speed, known as its terminal velocity. As an example, the terminal velocity of free-fall parachutists is about 120 mph (193 kph).WORKING SCIENTIFICALLY
Paper helicopters
Children can investigate the factors affecting air resistance on paper helicopters. By changing the weight (adding paper clips), the size of the body or wings, or the shape of the wings, they can time how long it takes the spinner to land.
The third law of motion states that every action has an equal and opposite reaction.The third law of motionImagine a 1-kg mass placed on a table. Because of the acceleration due to gravity, the 1-kg mass is exerting a downward force of 9.8 N on the table. The table is solid and rigid and is not moved by the force of the mass pushing down on it. Instead, it provides a reaction or contact force, exactly equal and opposite to the force applied by the 1-kg mass, and this reaction force supports the mass in a stationary position on the table surface. It is conventional, as in Figure 4.21 , to represent the size and direction of the forces concerned by arrows of appropriate length and direction. The forces in this instance constitute a balanced pair, and the 1-kg mass remains stationary.This is a difficult concept. Some accounts describe the table in the first example as ‘pushing back’ on the 1-kg mass, and many people find it hard to accept that the table actively ‘pushes’ the mass with exactly enough force to keep it in position. An easier concept, in my opinion, is the idea that the force provided by the 1-kg mass produces an equal and opposite reaction force on contact that supports
- Paul Grimshaw, Neil Fowler, Adrian Lees, Adrian Burden(Authors)
- 2007(Publication Date)
- Routledge(Publisher)
The pages of this book and the person who is reading it will be exerting an attractive force on each other. However, because of the relatively small masses involved (i.e., the pages of the book and the human body) the force will be very small and it will not be possible to observe its effects. The planet Earth (due to its very large mass) exerts a significant force on the human body. This force produces the weight of an individual or object. The attractive force of the Moon on your body is less than the attractive force you experience on the planet Earth (because the Moon has much less mass than the Earth). Hence you will weigh less on the Moon although your mass will be exactly the same. This is the reason that astronauts are able to jump large distances when they are on the surface of the Moon. The force of gravity is an external force that acts on all bodies.The force of gravity is constantly acting on all bodies. The effect of the force of gravity (in the balance of all external forces that are acting, i.e., the net force) results in an acceleration of the body or object. The acceleration on the planet Earth is given as –9.81 m/s2 (presented as a minus value because the force of gravity from the Earth acting on the body will be trying to pull the body downward toward the center of mass of the Earth).Acceleration caused by the force of gravityOn the planet Earth the effects of the force of gravity due to the mass of the planet act on all objects that have mass. However, this effect will only act on the vertical component of any movement. Any horizontal component of movement will be independent of the external force of gravity. For objects that are at or close to the surface of the Earth the acceleration of –9.81 m/s2 is considered to be constant. This acceleration (because of the position of the object in relation to the center of the Earth and because of the Earth’s relative large mass compared to the object’s small mass) will act on all objects with the same rate regardless of their mass. Hence, dropping a hammer and a pen from the same height while on or at the surface of the Earth will result in both objects hitting the floor at the same time (neglecting air resistance).Air resistanceIn some situations, within human movement and sport, the effects of air resistance (as an external force) are not
eBook - ePub- John Bird(Author)
- 2019(Publication Date)
- Routledge(Publisher)
section 18.1 :thena =v - utwherea × t = v - u from which v = u + a t- u is the initial velocity in m/s,
- v is the final velocity in m/s,
- a is the constant acceleration in m/s2 and
- t is the time in s.
When symbol a has a negative value, it is called deceleration or retardation. The equation v = u + at is called an equation of motion.Problem 3. A stone is dropped from an aeroplane.Determine (a) its velocity after 2 s and (b) the increase in velocity during the third second, in the absence of all forces except that due to gravityThe stone is free-falling and thus has an acceleration a of approximately 9.8 m/s2 (taking downward motion as positive). From above:final velocity v = u + atfinal velocity v = u + at- (a) The initial downward velocity of the stone u is zero. The acceleration a is 9.8 m/s2 downwards and the time during which the stone is accelerating is 2s. Hence, final velocity, v = u + at = 0 + 9.8 χ 2 = 19.6m/s, i.e. the velocity of the stone after 2 s is approximately 19.6 m/s
- (b) From part (a), the velocity after two seconds u is 19.6 m/s. The velocity after 3 s, applying v = u + at,is v = 19.6 + 9.8 χ 3 = 49 m/s. Thus, the change in velocity during the third second is (49 – 19.6) = 29.4 m/s
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