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Chapter 4

Chapter 4. Mass, Weight and Density. Newton’s first law of motion. An object at rest will remain at rest and an object in motion will continue in motion at constant speed in a straight line in the absence of a net force acting on it (when the net force acting on it is zero). Forces. force.

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Chapter 4

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  1. Chapter 4 Mass, Weight and Density

  2. Newton’s first law of motion An object at rest will remain at rest and an object in motion will continue in motion at constant speed in a straight line in the absence of a net force acting on it (when the net force acting on it is zero). Forces force A force is a push or a pull. It is a vector and has both magnitude and direction.

  3. Forces force A force is a push or a pull. It is a vector and has both magnitude and direction. Newton’s first law of motion An object at rest will remain at rest and an object in motion will continue in motion at constant speed in a straight line in the absence of a net force acting on it (when the net force acting on it is zero).

  4. Mass State of Inertia • When a body of matter is stationary, it needs a force to make it start moving • The bigger the mass, the bigger the force needed • Masses have inertia, a reluctance to start moving

  5. Mass State of Inertia • When a body of matter is moving, it needs a force to stop it • The bigger the mass, the bigger the force needed • Masses have inertia, a reluctance to stop moving Question: Do you know why you need to put on a seat belt in a car?

  6. (Straits Times 8 May 2002)(5)  Drivers who do not belt up will be fined S$120 and given three demerit points from today. Passengers who fail to do so will be fined S$120. The tough stand comes after ten years of relying on education, rather than enforcement, to get people to comply with the law. The Traffic police will be on the lookout for such offenders. Traffic Police chief, Assistant Commissioner Rajoo Gopal said yesterday, "There will be no more warnings and no escapes." FINE

  7. Mass State of Inertia • When a body of matter is stationary, it needs a force to make it start moving • The bigger the mass, the bigger the force needed • Masses have inertia, a reluctance to start moving The mass (m) of a body of matter is a quantitative measure of its inertia, i.e., its resistance to a change in the state of rest or motion of the body, when a force is applied.

  8. Mass Amount of Substance Conventionally, Mass (m) of a body of matter is a measure of its amount of substance in the body

  9. Mass Amount of Substance - SI Unit: kg - Scalar quantity A sliding balance An electronic balance

  10. Gravitational Field & Field Strength Gravitational Attraction • - Any two bodies in the universe attract each other with a force. • This spectacle is called the • gravitational attraction. • This force of attraction is known as the • gravitational force or force due to gravity.

  11. Gravitational Field & Field Strength Newton’s Law of Gravitational Every two particles in the universe attract each other with a force proportional to the product of the two masses and inversely proportional to the square of their distance apart F : Force between two particles m1, m2: masses of the two particles r: separation between two masses G: Universal Constant of gravitational 6.67 x 10-11 Nm2kg-2 Note: Equation is NOT required in Pre-IB

  12. Gravitational Field & Field Strength Newton’s Law of Gravitational

  13. Gravitational Field & Field Strength Newton’s Law of Gravitational Radius of Earth: 6.4 x 106 m Mass of Earth: 6.0 x 1024 kg Radius of Moon: 1.74 x 106 m Mass of Moon: 7.4 x 1022 kg

  14. Gravitational Field & Field Strength Newton’s Law of Gravitational Note: Earth is not spherical but ellipsoidal (equatorial radius exceed polar radius by 21 km). Centrifugal effects not felt at pole due to rotation at pole axis. Hence, gravitational force is max at poles compare to equator

  15. Gravitational Field & Field Strength Gravitational Field A gravitational field is a region in which a mass experiences a force due to gravitational attraction • Represented by imaginary line of forces • For Spherical body, lines are directed radially inward towards the centre of body • Lines closer, strong field • Lines widely separated, weak field • Near surface of earth, lines of force almost parallel, hence constant field

  16. Gravitational Field & Field Strength Gravitational Field Strength Gravitational field strength at a point in free space surrounding any mass is the force per unit mass acting on any object placed there. g: Gravitational Field Strength Or numerically Acceleration due to gravity in the case of planet Unit: N Kg-1 or ms-2

  17. Weight Weight of a body is the gravitational force exerted on it by Earth. W: Weight of body, N m: mass of body, kg g: gravitational field strength, Nkg-1 10 Nkg-1 near the surface of the earth

  18. Weight Compression balance Spring balance

  19. Comparison Between Mass & Weight

  20. Weight Question: A satellite is launched from Earth into orbit. What happens to the mass and weight of the satellite? MassWeight A Decreases Decreases B Decreases Stays constant C Stays constant Decreases D Stays constant Stays constant

  21. Weight Question to ponder: What is the cause of low tide and high tide on Earth?

  22. free fall acceleration due to gravity All objects fall freely towards the centre of the earth and have the same acceleration (acceleration of free fall). • all objects fall freely at g 10 m s-2 when near the earth and air resistance is negligible • speed of a free-falling body increases by 10 m s-1 every second or when a body is thrown up, its speed decreases by 10 m s-1 every second Although the acceleration due to gravity is considered constant, it tends to vary slightly over the earth since the earth is not a perfect sphere.

  23. free fall acceleration due to gravity - Free-falling objects do not encounter air resistance. - All free-falling objects (on Earth) accelerate downwards at a rate of approximately 10 ms-2 (to be exact, 9.81 ms-2).

  24. Free Fall (no air resistance)

  25. Free Fall (no air resistance)

  26. Falling with air resistance

  27. Density Density (ρ) is defined as the mass (m) of a substance per unit volume (V) ρ: unit is kgm-3 m: mass of object V: volume

  28. Density

  29. V4 V2 object object V1 V3 sinker Volume of object = V2 – V1 Volume of object = V4 – V3 measurement of volume irregular volume Volume of an irregular-shaped object can be found with: • measuring cylinder alone for an object that sinks in water for an object that floats on water

  30. spout sinker object V5 Volume of object = V5 V6 Volume of object = V6 measurement of volume irregular volume Volume of an irregular-shaped object can also be found with: • displacement can and measuring cylinder for an object that sinks in water for an object that floats on water

  31. Floating & Sinking ρ = 0.65 gcm-3 • When an object is placed in a liquid of lower density, the object sinks • - If it is placed in a liquid of a greater density, it floats

  32. Floating & Sinking Hydrometer

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