Section 5.7 Mass And Weight

Section 5.7 Mass and weight

• Weightlessness

© Manhattan Press (H.K.) Ltd.

1

5.7 Mass and weight (SB p. 100)

Mass and weight Measure weight of object: using a spring balance

Expt 5E Mass and weight spring balance mass hanger

© Manhattan Press (H.K.) Ltd.

2

5.7 Mass and weight (SB p. 100)

Data-logging set-up Expt. 5D Mass and weight (data-logging)

force sensor computer data-logging interface

The results

© Manhattan Press (H.K.) Ltd.

3

5.7 Mass and weight (SB p. 101)

Mass and weight Mass (m) — measure of quantity of matter inside a body

Earth

Weight (W) — measure of gravitational attraction on a body towards © Manhattan Press (H.K.) Ltd.

4

5.7 Mass and weight (SB p. 101)

Mass and weight Differences

Mass: • Unit: kilogram (kg) • Scalar quantity

© Manhattan Press (H.K.) Ltd.

Weight: • Unit: newton (N) • A kind of force • Vector quantity

5

5.7 Mass and weight (SB p. 101)

Mass and weight W = mg

Earth © Manhattan Press (H.K.) Ltd.

6

5.7 Mass and weight (SB p. 101)

Measuring devices

measuring mass: beam balance © Manhattan Press (H.K.) Ltd.

measuring weight: spring balance 7

5.7 Mass and weight (SB p. 102)

Weight at different positions

weight may change at different positions © Manhattan Press (H.K.) Ltd.

mass universal constant 8

5.7 Mass and weight (SB p. 102)

Weight at different positions Gravitational acceleration (g) changes from places to places

Earth g = 10

© Manhattan Press (H.K.) Ltd.

Weight is position dependent

Moon g = 1.67

9

5.7 Mass and weight (SB p. 103)

Weight at different positions Values of g on different planets Place

Mass (m) / kg

Earth Moon

10

Venus Jupiter

© Manhattan Press (H.K.) Ltd.

Gravitational acceleration (g) / m s

-2

Weight (W = mg) / N

10

100

1 of earth = 1.67 6

16.7

9 of earth = 9 10

90

2.6 of earth = 26

260

10

5.7 Mass and weight (SB p. 103)

Class Practice 8: 8 Write down the differences between mass and weight in the table below. Ans wer Mass

Weight

Unit

kg

N

Scalar or Vector?

scalar

vector

Measured by

beam balances

spring balances

Depend on position?

No

Yes

© Manhattan Press (H.K.) Ltd.

11

5.7 Mass and weight (SB p. 104)

Weightlessness What is “weightlessness”?

No weight ?

Lost our sensation of having weight (from the reaction by the ground)

Go to

© Manhattan Press (H.K.) Ltd.

Activity 3

12

5.7 Mass and weight (SB p. 105)

Edmond in a lift Forces: R = supporting force on Edmond (reaction from ground) mg = weight of Edmond = 62 × 10 = 620 N

“normal weight = 620 N”

© Manhattan Press (H.K.) Ltd.

13

5.7 Mass and weight (SB p. 105)

Edmond in a lift 1. Accelerating upwards, a = 1.4 m s−2 By F = ma R − mg = ma R= m (g + a) ∴ R= 62 × (10 + 1.4) = 707 N

a = 1.4 m s-2

feel “heavier”

© Manhattan Press (H.K.) Ltd.

14

5.7 Mass and weight (SB p. 105)

Edmond in a lift 2. Uniform motion, no acceleration no net force, a = 0 R − mg = 0 ∴ R= mg = 62 × 10 = 620 N feel “his own weight” © Manhattan Press (H.K.) Ltd.

15

5.7 Mass and weight (SB p. 106)

Edmond in a lift 3. Accelerating downwards, a = 1.4 m s−2 By F = ma mg − R = ma ∴ R= m (g − a) R= 62 × (10 − 1.4) = 533 N feel “lighter” © Manhattan Press (H.K.) Ltd.

16

5.7 Mass and weight (SB p. 106)

Edmond in a lift 4. cable breaks, FREE FALL, a = g By F = ma mg − R = ma ∴ R= m (g − g) =0 “I

feel no weight”

Actually W ≠ 0 but R = 0 only © Manhattan Press (H.K.) Ltd.

17

5.7 Mass and weight (SB p. 109)

Useful Website Physics Zone: Lessons on Forces (http://www.sciencejoywagon.com/physicszone /lesson/02forces.htm) Vector addition (http://www.edp.ust.hk/physics/explore/dswmedia/vector .htm) Resultant of Forces (Addition of Vectors) (http://www.walter-fendt.de/ph14e/resultant.htm) Test your response time (http://topendsports.com/testing/reactime.htm)

© Manhattan Press (H.K.) Ltd.

18

5.7 Mass and weight (SB p. 110)

Mind Map

Tension

5.2 Action of forces

Friction Gravitational force remain its state of motion

balanced, (net force = 0)

Force and motion

forces action unbalanced, (net force ≠ 0) on an object

how does it react back?

what happens when force add together? © Manhattan Press (H.K.) Ltd.

draw

5.1 Newton’s 1st law of motion

concept of inertia F = ma

5.4 Newton’s 2nd law of motion 5.6 Newton’s 3rd law of motion

5.5 Free body diagrams

by using

5.7 Mass and weight action and reaction pair

5.3 Addition and resolution of forces 19

The The End End

© Manhattan Press (H.K.) Ltd.

20

5.7 Mass and weight (SB p. 104)

Activity 3 Weight inside a lift

Let’s start: 1. Enter a lift which is resting on the ground floor. Place a balance on the floor of the lift. 2. Stand on the balance and record the reading (mo).

© Manhattan Press (H.K.) Ltd.

21

5.7 Mass and weight (SB p. 104)

Activity 3 (Cont) Weight inside a lift

3. Press the sixth floor button and close the lift door. The lift starts to move upwards. At that moment, record the reading (m1) again. 4. After a while, observe the changes in the reading when the lift is still moving upwards.

Ans A while later, the reading of the balance wer

decreases to the original reading gradually.

© Manhattan Press (H.K.) Ltd.

22

5.7 Mass and weight (SB p. 104)

Activity 3 (Cont) Weight inside a lift

5. At the moment when the lift is about to reach the sixth floor, record the reading (m2) again. 6. When the lift stops at the sixth floor, take the reading (m3).

© Manhattan Press (H.K.) Ltd.

23

5.7 Mass and weight (SB p. 104)

Activity 3 (Cont) Weight inside a lift

7. Are the readings mo, m1, m2 and m3 the same? Does your weight change? Explain Ans briefly. wer

They are not the same: m1 > mo = m3 > m2

© Manhattan Press (H.K.) Ltd.

24

5.7 Mass and weight (SB p. 104)

Activity 3 (Cont) Weight inside a lift

8. Repeat steps 2 to 6 when the lift is moving downwards. Describe the change in the readings.

Ans At first, the reading of the balance wer decreases slightly. Then the reading is equal to the original reading. When the lift stops, the reading increases slightly.

Return to

Text © Manhattan Press (H.K.) Ltd.

25