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Saturday, June 26, 2010

LESSON NOTES! :D 8:07 PM

Topic: Weight, Mass, Density

Lesson Notes - Weight, Mass

Definition: Weight is the gravitational force acting on an object.
SI unit: Newton (N)

Definition: Mass is a measure of the amount of matter in a body.
SI unit: Kilogram (kg)

Definition: The gravitational field strength at any point in a gravitational field is the gravitational force per unit mass exerted on any bodies.
SI unit: G

What is gravitational field?
A gravitational field is a region in which a mass experiences a force due to gravitational attraction.

Formula for Weight & Mass:
Weight = mass x gravitational field strength
W = mg
Formula for Gravitational field strength:
Gravitational field strength = Weight / Mass
G = N / kg

Mass
-The mass of an object is constant. It does not change even when the object is on the Moon surface in which the gravitational pull on the object changes.

-The mass of an object is a measure of the inertia of the object.

-The inertia of an object is its resistance to change in its state of rest or motion.

-Hence the larger the mass of the object, the greater is its inertia.

-Below are situations which can be explained using the idea of inertia the resistance to change in the state of rest or motion of an object.
(a) When a car suddenly speeds off, its passenger is pushed back against the seat.
- Initially the passenger is at rest.
- When the car suddenly speeds off, the passenger tends to remain in a state of rest.
- He is pushed by the seat.
(b) A passenger in a fast moving car is thrown forward when the car suddenly brakes.
- Initially the passenger is moving.
- When the car suddenly stops, the passenger tends to continue moving because of his
inertia.
- Hence he is thrown forward.

Gravitational field and field strength
- When a ball is released, it drops to the floor due to the gravitational pull of the Earth.
- There is a gravitational field around the Earth. A gravitational field is the region within which a mass experiences a force of attraction.
- The gravitational field strength g is the gravitational force per unit mass.
- On the Earth surface, the gravitational field strength g is approximately 10N/kg
- The gravitational force on a mass is known as its weight. Hence the weight of a mass m is given by (FORMULA)
- The gravitational field strength of the Earth decreases as the altitude increases.
- On the surface of the Moon, the gravitational field strength is about 1/6 the value of g on the Earth surface.
- Hence the weight W (= mg) of an object is not constant but changes when the value of g changes.
- The acceleration of an object that falls freely under the action of gravitational force is known as the acceleration due to gravity g.

Density

Definition: The density of a substance is defined as its mass per unit volume.
SI unit: kilogram per cubic metre (kg / m3)

Formula for Density:
Density = Mass / Volume
p = m / V

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Multiple Choice Question

1) Which of the following experiences the same gravitational field strength?

(1) A 50 kg mass resting on the floor.
(2) A 50 kg mass hanging on a 20 m tall tree.
(3) A 50 kg mass dropping down from a 20 m tall tree.
(4) A 100 kg mass resting on the floor.

A (1) and (2) only.
B (1), (2) and (3) only.
C (1), (2) and (4) only.
D (1), (2) , (3) and (4).

2) The weight of a body is given by

A weight = density x volume.
B weight = mass x gravitational field strength.
C weight = mass x acceleration.
D weight = velocity x time.

3) The gravitational field strength on the Moon is 1/6 that on the Earth. When a ball is thrown vertically upwards on the Earth, the maximum height reached is H. When the ball is thrown vertically upwards with the same speed on Moon, the maximum height reached is

A 1/6H. C 6H
B H D 12H

4) When 2.0 cm³ of a liquid vaporised, the volume of the vapour is 2400 cm³. What is the ratio of the density of the liquid to the density of the vapour?

A 1/1200 C 240
B 1/2400 D 1200

5) A steel ball bearing of radius 5 mm is of weight W. What is the weight of a steel ball bearing of radius 10mm?

A 2W C 8W
B 4W D 16W

6) A body that is moving resists changes to its motion. This resistance increases when

A the mass of the body increases.
B the gravitational field strength increases.
C the speed of the body increases.
D the external force on the body increases.

7) The gravitational field strength is the ratio

A mass / volume .
B gravitational force / weight .
C gravitational force / mass .
D mass / weight .

8)A body of mass 500g has a volume of 100cm³. If a hole of 20 cm³ was drilled through the body, what is the density of the body?

A 0.20 g/cm³
B 5.00 g/cm³
C 6.25 g/cm³
D 25.0 g/cm³

9) Five identical iron alls, each of mass 25g, were immersed in a measuring cylinder containing 30 cm³ of water. The reading of the water level rose to 50 cm³. What was the density of the iron?

A 0.50 g/cm³
B 1.25 g/cm³
C 5.00 g/cm³
D 6.25 g/cm³

10) The density of a gold bar is 19.0 g/cm³ . When a hole of volume 1.0 cm³ is drilled into the bar, the density of the gold bar, the density of the golf bar will be

A 18.0 g/cm³
B 19.0 g/cm³
C 19.5 g/cm³
D 20.0 g/cm³

Structured Question

1) The weight of a rock on Earth is 20N. [The gravitational acceleration on Earth and on the Moon are 10 m/s² and 1.7 m/s² respectively.]

(a) What is the mass of the rock on Earth?
(b) What is the mass of the rock on Moon?
(c) What is the weight of the rock on the Moon?

2) A metal alloy is made from a mixture of 500g of iron and 120g of cobalt. Iron has a density of 7.8g/cm³

(a) Calculate the volume of iron and cobalt used.
(b) What is the total volume of the mixture?

3) A metal sphere weighs 30N on the Moon. The gravitational field strength on the Moon is 1/ 6 of that of the Earth. The sphere was released from a height of 5m above the surface of the Earth.

(a) State the acceleration of the sphere.
(b) State the acceleration of the sphere if it was released at the same height above the surface of the moon.

4) The mass of an object on Earth is 10 kg but the object has a weight of 100 N.

(a) What is the difference between mass and weight?
(b) What is the gravitational field strength on Earth?
(c) List two separate equipment used to measure mass and weight of an object respectively?