5.2 a weight problem

5.2 A weight problem 13 April 2023

How many forces do you know?

How do you measure a force?

How much do you weigh?

Aims

• Record measurements of mass and weight in appropriate units. [Level 4]

• Recognise that some forces act at a distance and others are ‘contact’ forces. [Level 4]

• Explain the difference between mass and weight. [Level 5]

• Explain how weight is caused by gravity acting on a mass. [Level 6]

Keywords

• Contact force• Friction• Gravity• Kilogram• Magnetism• Mass• Newton• Weight

mg

R

Contact VS non-contact

Weight

MagnetismFriction

Air resistance

Upthrust

Weight and mass

• If you buy a can of coke it has a mass – its made of stuff. If you take it to the Moon it still is made of the same stuff, its mass doesn’t change

• The weight of the coke depends on the gravity of the objects it is near to. The Earth has more gravity than the Moon so on the Moon it would have less weight

• You can tell they are not the same as they have different units!

Weight VS MassMass Weight

Symbol

Units

Unit symbols

Description

Weight VS MassMass Weight

Symbol m W

Units kilogram newton

Unit symbols kg N

Description How much stuff something is made of

The gravitational pull on an objects mass

Plotting mass against weightMass / kg Weight / N

0.1

0.2

0.3

0.4

0.5

Plotting mass against weightMass / kg Weight / N

0.1 1

0.2 2

0.3 3

0.4 4

0.5 5

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.550

1

2

3

4

5

6

Mass / kg

Wei

gh

t /

N

Converting mass into weight

• On earth you multiply the mass in kilograms by 10 N/kg

• On the Moon you multiply by 1.6 N/kg• On Jupiter you multiply by 26 N/kg

Measuring weight

• You use a spring balance or force meter or newton meter to measure weight

• It simply measures the stretch of a spring that has been calibrated in the factory – big stretch is a big force

5.2 A weight problemQuestions

1. Copy and complete the sentences below using these words:weight mass newtons kilograms

The _______ of an object is constant and depends on the amount of matter it has. Gravity gives an object its _______. Massis measured in _______. Weight is a forceand is measured in _______.

2. If you went to the Moon, how would your weight and mass change?

3. How do you think the Sun’s gravity compareswith the Earth’s? Explain your answer.

(Level 3)

(Level 3)

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(Level 4)

5.2 A weight problemQuestions

4. Rank these objects in terms of the amount of gravity they have: Earth, Sun, pencil sharpener, Moon, Jupiter.

5. Michelle says ‘If you go to the Moon, you’ll be slimmer because you lose weight’. Do you think she is right or wrong? Explain your answer.

6. A rocket travels from the Moon to the Earth. Describe how its weight will change during the journey.

7. Why is it important that a rocket has as little mass as possible?

(Level 5)

(Level 7)

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(Level 6)

(Level 7)