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Mass, Weight, Normal Force

Middle School Physics

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Mass, Weight, Normal Force

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  1. Grade 8 - Physics Mass and Weight

  2. Massvs Weight Mass Weight Weight is the force of gravity working on a mass Weight of an object depends on the gravitational acceleration in that area Measured in newtons (N) Vector quantity • Mass is the amount of matter an object has • Mass of an object is constant everywhere • Measured in kilograms (kg) • Scalar quantity

  3. w = m . g Note: w = weight (N) m = mass (kg) g = gravitational acceleration (m/s2)

  4. An astronaut has a mass of 60 kg. What is his weight on Earth? (g = 10 m/s2) What would be his weight on the Moon, if the Moon’s gravitational acceleration is 1/6 of the Earth’s What would be his mass on the Moon? w = m . gearth w = 60kg . 10m/s2 w = 600 N The mass stays the same, 60kg Known m = 60kg gearth = 10m/s2 gmoon = gearth gmoon = m/s2 w = m . gmoon w = 60kg . m/s2 w = 100 N

  5. What is the weight of a 60kg-astronaut in outer space? In vacuum of outer space far away from any mass of objects, the astronaut is weightless. Because the gravitational acceleration is zero. g = 0 w = 0

  6. Sample Problems Calculate the weight of Goku in Planet Namec, if his mass is 80kg and the gravitational acceleration in that area is 15m/s2. The weight of an astronaut is 750N. One time, he went to Planet X6577-HY and weighing only 150N. What’s the gravitational acceleration on Planet X6577-HY?

  7. Grade 8 - Physics Normal Force (N)

  8. Normal Force (N) Ever turn too quickly and walk straight into a wall? Well it hurts for sure. We can blame the normal force for the pain we feel when running into solid objects. The normal force is the reaction force that surfaces give to prevent solid objects from passing through each other.

  9. Normal force is a contact force. • If two surfaces are not in contact, they can't exert a normal force on each other. • For example, the surfaces of a table and a box cannot exert normal forces on each other if they are not in contact.

  10. However, when two surfaces are in contact (e.g. a box and a table) they exert a normal force on each other, perpendicular to the contacting surfaces. In horizontal surface, • This normal force will be as large as necessary to prevent the surfaces from penetrating each other.

  11. Remember: “The normal force is always perpendicular to the contact surface” F N N N N w w w

  12. Sample problems A 2kg block is on the floor. The block is stationary. Determine the normal force given by the floor to the block! (g = 10m/s2) Solution Known m = 2kg g= 10m/s2 N The block is not moving in vertical direction (y axis), so Asked N = ? w

  13. Sample problems If the mass A is 1kg and mass B is 3kg, and gravitational acceleration is 10 m/s2, determine: the normal force acting on object A the normal force acting on object B Solution Both objects are not moving in vertical direction (y axis), so Object A Object B NA – wA = 0 NA = wA NA = mA . g NA = 1kg . 10 m/s2 NA = 10N NB – wA – wB = 0 NB = wA + wB NB = mA.g + mB.g NB = 1kg.10 m/s2 + 3kg.10 m/s2 NB = 10N + 30N = 40N Known mA = 1kg mB = 3kg g= 10m/s2 Asked NA = ? NB = ?

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