EXERCISE 3.2WEIGHT AND GRAVITY


Take G  6.7  1011 N m2 kg2, mass of Earth  6  1024 kg, mass of Moon  7  1022 kg, mass of Mars  6.4  1023 kg, radius of Moon  1.7  106 m, radius of Mars  3.4  106 m, distance between the centres of the Moon and the Earth  3.8  108 m).


Q1 Calculate the force due to gravity between the Moon and the Earth. Q2 Calculate acceleration due to gravity on (i) the Moon and (ii) Mars. Q3 An astronaut throws a stone straight up from the surface of the Moon with an initial speed of 25 m s1. Calculate the highest point reached by the stone. Calculate how high the astronaut can throw the same stone on Earth given the same initial speed as on the Moon.Take acceleration due to gravity on the Earth and Moon as 9.8 m s2 and 1.6 m s2 respectively.


Q4 An astronaut carries out an experiment to measure the acceleration due to gravity on the surface of Venus. He drops an object from a height of 1.6 m and the object takes 0.6 s to fall. Use this data to calculate acceleration due to gravity on the surface of Venus.


Q5 In August 2003, at the birth of their baby, a couple argued about the influence of the planets on the newborn child.The mother, being a believer in astrology, thought Mars was in a particularly auspicious place in the sky that day; the father was not so sure.The midwife intervened in the discussion and claimed she was more influential than the planet and told them to stop talking nonsense.The midwife had a mass of 72 kg and her centre of gravity was 45 cm from the baby’s at the time of delivery; Mars, with a mass of 6.4  1023 kg, was 56 million km away, the closest it had been to Earth in 50 000 years! What exerted the stronger gravitational attraction on the 4 kg baby, the midwife or Mars?


Q6 A satellite has a mass of 1 200 kg.Taking acceleration due to gravity as 9.8 m s2 on the surface of the Earth, calculate: (i) its weight on Earth (ii) its weight when it’s a distance of ‘double the radius of the Earth’ above the equator (iii) its mass when it is at this altitude.


Q7 A man of mass 80 kg takes a flight from Ireland to New York.The maximum altitude of the plane is 11 km above sea level.Taking radius of Earth as 6 371 km, calculate the man’s greatest temporary loss in weight during the flight. Give your answer to one decimal place.


Friction


FRICTION Friction is a force that opposes the motion of a body.


It may be due to two surfaces in contact sliding over each other, or it may be where a body is moving through a fluid, such as with air resistance.


Surfaces that appear smooth to the naked eye are usually imperfect on closer inspection. A lubricant can reduce friction by separating the surfaces and therefore reducing the contact between the rough surfaces.


When answering questions involving friction, a surface described as being ‘smooth’ means we assume there is no frictional force when an object slides over it.


Microscopic view Friction (a)


Force making body tend to move


Friction (b) Fig 3.10: Illustration of microscopic view of surfaces (a) without lubricant and (b) with lubricant FORCE AND MOMENTUM 53 Microscopic view


Force making body tend to move


Lubricant


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