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Mass and weight questions
Everyday English uses mass and weight interchangeably, and physics never does. One is an amount of matter that goes everywhere with you; the other is a force that depends on where you are standing.
18 original questions · 53 marks · the mass and weight notes · Mechanics
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Distinguish between the mass and the weight of an object, including the unit of each.
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Mass is the amount of matter in an object, a scalar measured in kilograms (1). Weight is the gravitational force on it, a vector measured in newtons (1).Calculate the weight of a 5.0 kg object on Earth, where g = 9.81 m s−2.
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W = mg = 5.0 × 9.81 (1)
W = 49.1 N (1)State what a chemical balance and a newton meter (spring balance) each measure.
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A balance compares mass, in kg (1); a newton meter measures force, so it reads weight, in N (1).The same 5.0 kg object is taken to the Moon, where g = 1.62 N kg−1. State its mass there and calculate its weight.
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Mass is unchanged at 5.0 kg (1)
Weight = mg = 5.0 × 1.62 (1)
Weight = 8.1 N (1)An astronaut has a mass of 80 kg. Calculate their weight (a) on Earth (g = 9.81 m s−2) and (b) on Mars (g = 3.70 m s−2).
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(a) W = 80 × 9.81 (1)
W = 784.8 N (1)
(b) W = 80 × 3.70 (1)
W = 296 N (1)A person of mass 60 kg stands on a set of scales in a lift. Taking g = 9.81 m s−2, calculate the reading (the normal force) when the lift accelerates downwards at 3.0 m s−2, and state how the reading compares with the person's weight.
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mg − N = ma (1)
N = m(g − a) = 60 × (9.81 − 3.0) (1)
N = 408.6 N (1)
This is less than their true weight of 588.6 N, so they feel lighter (1)A planet has mass 6.0 × 1024 kg and radius 6.4 × 106 m. Using g = GM/r2 with G = 6.67 × 10−11 N m2 kg−2, calculate the gravitational field strength at its surface.
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g = GM/r2 (1)
GM = 6.67 × 10−11 × 6.0 × 1024 (1)
g = GM / (6.4 × 106)2 (1)
g = 9.77 N kg−1 (1)In the absence of air resistance, a hammer and a feather dropped together on the Moon land at the same time. Explain why, referring to mass and weight.
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Acceleration = weight/mass = mg/m = g, independent of mass (1). Both objects fall with the same acceleration g (1), so with no air resistance they land together, even though the hammer has far more mass and weight (1).A student says 'the object weighs 5 kg'. Explain what is wrong with this statement and give the correct way to express it.
The gravitational field strength g may be quoted as 9.81 N kg−1 or as 9.81 m s−2. Show that these two units are equivalent.
A parcel has a weight of 34.3 N on Earth, where g = 9.81 N kg−1. Calculate its mass.
State the direction in which an object's weight acts.
The value of g varies slightly over the Earth's surface: it is 9.78 N kg−1 at the equator and 9.83 N kg−1 at the poles. A scientist of mass 70.0 kg travels from the equator to the South Pole. State what happens to her mass, and calculate the change in her weight.
An airline limits cabin baggage to a mass of 7.0 kg. A passenger's bag has a mass of 1.9 kg and contains a laptop of mass 1.6 kg, a camera of mass 850 g and books of mass 2.3 kg. Deduce whether the baggage is within the limit, and calculate its total weight. g = 9.81 N kg−1.
A stone hangs from a newton meter on Earth, where g = 9.81 N kg−1; the meter reads 12.0 N. The same stone and meter are taken to Europa, where g = 1.31 N kg−1. Calculate the reading on Europa.
A probe lands on the surface of a planet and hangs a 6.00 kg calibration mass from a newton meter, which reads 53.2 N. Use the data below to deduce which planet the probe has landed on.
Surface gravitational field strength in N kg−1: Mercury 3.70; Mars 3.71; Venus 8.87; Earth 9.81.A student stands on a set of bathroom scales on the floor of a drop-tower ride. During the ride the cabin falls freely. Explain why the scales read zero during the fall, and state whether the student's weight has changed.
A geologist checks a rock sample of mass 3.0 kg with two instruments: a beam balance and a newton meter. Both instruments are then taken to the Moon, where g = 1.62 N kg−1 (on Earth g = 9.81 N kg−1). Describe and explain the reading each instrument gives on the Moon.
The same practice on paper: the printable workbook for this topic, questions and a worked answer book.
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