SUHAILA MOHAMED NOR ADILAH SITI SARAH SYAZWANI ROSLAN
Gravity• A Universal Force a)Until 17th century, - a heavy object would fall faster than a light object as Aristotle once taught. b)After research by Galileo Galilei - if air resistance can be ignored, all bodies fall with an equal acceleration.
1. Newton- studied the force of gravity. He got the inspiration when apple fell from a tree, hitting his head.2. In Newton’s opinion, the falling apple was acted on by a force of attraction (i.e. the force of attraction due to gravity from the centre of the earth)3. Any object which falls only under the influence of the force of attraction due to gravity and without any influence of other forces is said to experience free fall.4. All object which experience free fall will fall with the same acceleration (i.e. the acceleration due to gravity)
Gravitational Field1. The force of gravity acts through space and can cause a body which is not in contact with any surface to fall freely to the ground – the earth is surrounded by a gravitational field which exerts a force on any body in the field.2. The strength of a gravitational field – the gravitational force acting per unit mass on an object in the field. It is denoted by the symbol ‘g’3. On the earth, the gravitational field strength, g is 9.8 N kg-14. Gravitational field strength: g = Gravitational force, F Mass of body, M
The non-uniform gravitational field of theEarth, g is represented by radial linesdirected towards the centre of the earth.The field is strongest where the linesare close.
ExampleQuestion :• A body of mass 2 kg has a weight of 20 N. Find the value of the gravitational field strength
Gravitational acceleration Mass , m Weight , w The amount of The force of gravitymatter in the object. definition on the object. Varies with the The mass of an magnitude of object is constant Changing of value gravitational field everywhere. strength, g of the location. Scalar quantity Physical quantity Vector quantity Base quantity Type of quantity Derived quantity Kilogram, kg SI unit Newton, N
FORMULAForce, F = ( mass, m ) ( acceleration, a ) Weight, w = ( mass, m ) ( gravitational acceleration, g )
Gravitational field strenghGravitational field = the region in which an objectexperiences a force due to gravitational attraction.Gravitational field strength = the ratio of the weight tothe mass of the object / weight per mass / W mRearrange the formula : W = ( m ) ( Gravitational fieldstrength ) When compared to : W = ( m ) ( g ) so : g = Gravitationalacceleration = Gravitational field
FREE FALLING OBJECT☻ Is an object falling under the force of gravity only☻ Does not encounter other force (example : air resistance or friction that would oppose its motion
The difference between a fall in air and a free fall in a vacuum of a coin and a feather
When a girl stands When a girl stands on the platform of a on the platform of a weighing scale, there are two weighing scale, there are two forces acting on her :: forces acting on her b) The upward normal reactiona) The girl’s weight, w(=mg) force, R exerted on her feet Acting downwards By the platform of the scale The reading of the scale gives the value of the normal reaction Force, R
Different situation in the lift Note : when the lift is accelerating upwards or downwards, the reading on the scale gives the ‘apparent’ weight, which is equal to the normal reaction force on the feet of the girl
1 A box of mass 1.6 kg is suspended from a spring balance hanging from the ceiling of a lift What is the reading on the spring balace if : (a) The lift is stationary (b) The lift moves upwards at an acceleration of 2 ms-2? (c) The lift moes downwards at an acceleration of 3ms-2 [take g = 10 ms-2]
SOLUTION(a) If the lift is stationery :T = mg = 1.6 x 10 = 16 N(b) When the lift is accelerating upwards :T1 - mg = ma T1 = (1.6 x 10) + (1.6 x 2) = 19.2 N(c) When the lift is accelerating downwards : mg – T2 = ma T2 = (1.6 x 10) – (1.6 x 3) = 11.2 N
2 The weight of a student on earth’s is 450N. What is this weight on the moon if earth’s gravitational strength is six times the gravitational strength of the moon ?
SOLUTION(Symbol with subscript 1 represent the situation on earth while symbols with subscript 2 represent the situation on the moon) w1 = 450 N g1 = 6g w1 = mg w2 = mg w1 mg1 g1 w2 = mg2 = g2 w2 = w 1 g 2 g1 = 450 x g2 6g2 = 75 N
3 A ball is thrown vertically upwards with an initial velocity of 20 ms-1. If g = 10 ms-2 and air resistance can be neglected, find: (a) the maximum height reached (b) the time taken before it reaches the ground
SOLUTION a)The velocity of the ball is zero when it reaches itsmaximum height. The acceleration of the ball when it isthrown upwards is -g as it experiences a deceleration u = 20 ms-1 v = 0 a = -10 ms-2 v2 = u2 + 2as 0 = 202 + 2(-10)s s = 400 m 20 = 20 m (maximum height reached = 20 m)
(b) The displacement of the ball on reaching the ground is zeros = ut + 1 at2 20 = 20 t + 1 (-10)t2 25 t - 20 t = 0 25 t (t-4) = 0 t = 0 or 4( time taken before it reaches the ground = 4 s)