Section 5.1 Newton’s First Law Of Motion

Section 5.1 Newton’s first law of motion • Galileo’s experiments

• Inertia • Newton’s first law of motion © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 57)

Galileo’s experiments pushing, pulling, lifting things need

force

move, stop and change direction of objects

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Quiz © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 57)

Galileo’s experiments Ancient philosopher – Aristotle: “Force must be applied to make a body move.”

Is it correct?

Galileo Galilei “pin-and-pendulum” experiment © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 58)

“Pin-and-pendulum” experiment

Start

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End

Always reach the same height as A

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5.1 Newton’s first law of motion (SB p. 59)

“Pin-and-pendulum” experiment If a pin is fixed below the clamp, The bob also reaches the same height as the starting point.

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5.1 Newton’s first law of motion (SB p. 59)

“Pin-and-pendulum” experiment

When air resistance & friction are negligible → bob swings between two points of maximum height © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 59)

Galileo’s thought experiment

A

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A ball bearing is released on a curved rail with different curvatures

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5.1 Newton’s first law of motion (SB p. 60)

Galileo’s thought experiment If there is no friction, the ball bearing must reach the same height as the starting point.

How about if ……

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5.1 Newton’s first law of motion (SB p. 60)

Galileo’s thought experiment right end laid flat & infinitely long move at a uniform velocity forever tends to climb to its original height

A body can move without applying a force on it © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 60)

Inertia Inertia of a body — tendency for it to remain at rest / move at a uniform velocity — resists changes in its state of motion Go to

Activity 1

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5.1 Newton’s first law of motion (SB p. 63)

Inertia and mass

We use heavy bowling … why? Thinking 1 © Manhattan Press (H.K.) Ltd.

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Discussion 1 11

5.1 Newton’s first law of motion (SB p. 63)

Inertia and mass ↑ mass → ↑ inertia Heavy ball

wooden bottles (smaller mass)

greater tendency to maintain its state of motion

smaller tendency to remain at rest- the bottle fall down

Mass is a measure of the quantity of matter inside a body © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 63)

Inertia and mass Ping-pong ball

v

Easier to stop

basket ball v

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Harder to stop

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5.1 Newton’s first law of motion (SB p. 64)

Newton’s first law of motion Sir Isaac Newton modified Galileo’s idea of inertia:

Newton’s first law of motion: relation between force and motion

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5.1 Newton’s first law of motion (SB p. 64)

Newton’s first law of motion Newton’s first law of motion — a body will remain in its state of motion if no net force acting on it uniform velocity motion

at rest or

No net force force no net © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 64)

Newton’s first law of motion When a car brakes suddenly, occupant is thrown forwards tendency to keep on moving by Newton’s first law of motion

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5.1 Newton’s first law of motion (SB p. 64)

Newton’s first law of motion When a car starts to move, the head of occupant tends to at rest (thrown backwards) tends to at rest by Newton’s first law of motion cause neck injuries Thinking 2 © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 65)

Seat belt and head rest Safety measures in vehicles

Head rests Seat belt

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5.1 Newton’s first law of motion (SB p. 65)

Seat belt and head rest Without seat belt

With seat belt

The seat belts exert pulling force on the occupants

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5.1 Newton’s first law of motion (SB p. 65)

Seat belt and head rest Head rests

Cushioning forces exerted on both the head and body to protect the neck of driver © Manhattan Press (H.K.) Ltd.

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To section 5.2

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5.1 Newton’s first law of motion (SB p. 56)

Quiz

1. When the bus accelerates forwards form the rest, the passengers on the bus lean backwards slightly. Why? B. The passengers lean backwards due to their own inertia. © Manhattan Press (H.K.) Ltd.

Ans wer

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5.1 Newton’s first law of motion (SB p. 56)

Quiz (Cont)

2. Thy tyres of vehicles have tread patterns C. to increase friction with the road. © Manhattan Press (H.K.) Ltd.

Ans wer 23

5.1 Newton’s first law of motion (SB p. 56)

Quiz (Cont)

3. If the ball hit by the bus moves at a Ans uniform speed on a road, wer

C. there is no net force acting on the Return to ball. Text

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5.1 Newton’s first law of motion (SB p. 61)

Activity 1 Tricks with inertia

Let’s start: (A) Coin on a cardboard 1. Place a cardboard with a coin on it on your index finger (Fig. (a)).

Fig. (a)

2. Flick the cardboard with the other hand. Ans What happens to the coin? wer The coin remains at rest on the finger. © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 61)

Activity 1 (Cont) Tricks with inertia

(B) Stack of coins 1. Stack coins in one column on a table (Fig. (b)) . Fig. (b)

2. Strike the lowest coin with a steel ruler. What happens to the other coins? The rest of the stack of coins do not Ans collapse. They stay vertically. wer © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 61)

Activity 1 (Cont) Tricks with inertia

(C)Toilet paper 1. Place a roll of toilet paper on a rod (Fig. (c)). 2. Pull the toilet paper quickly by applying a jerk. What happens?

Fig. (c)

Ans wer

The piece of toilet paper is torn off and the roll remains stationary. © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 61)

Activity 1 (Cont) Tricks with inertia

Fig. (c)

3. Pull the toilet paper slowly this time. Ans What happens? wer The roll of toilet paper rotates continuously. © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 62)

Activity 1 (Cont) Tricks with inertia

(D) Ball bearing on a running trolley 1. Stick a small lump of plasticine onto a trolley (Fig. (d)).

Fig. (d)

2. Make a cup-shaped hollow in the plasticine to hold a ball bearing on it loosely. © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 62)

Activity 1 (Cont) Tricks with inertia

3. Set an obstacle on an inclined plane. 4. Push the trolley down the inclined plane to hit the obstacle. What happens to the steel ball when the trolley hits the obstacle?

Fig. (d)

Ans wer

The steel ball keeps on moving and flies off when the trolley stops © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 62)

Activity 1 (Cont) Tricks with inertia

Explain briefly the motions in the tricks (A), (B), (C) and (D).

Fig. (a)

Fig. (b)

Ans wer Fig. (c) © Manhattan Press (H.K.) Ltd.

Fig. (d) 31

5.1 Newton’s first law of motion (SB p. 62)

Activity 1 (Cont) Tricks with inertia

Tricks (A), (B), (C) demonstrate the tendency of a body to remain at rest. Trick (D) demonstrates the tendency of a body to maintain its state of motion.

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Fig. (a)

Fig. (c)

Fig. (b)

Fig. (d)

Text © Manhattan Press (H.K.) Ltd.

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5.1 Newton’s first law of motion (SB p. 63)

Discussion 1:

It is more difficult to push a heavy bag than a Ans light bag form rest. Explain briefly. wer

Hard to push © Manhattan Press (H.K.) Ltd.

Easy to push 33

5.1 Newton’s first law of motion (SB p. 63)

Discussion 1 (Cont):

As the heavy bag has a greater mass, it has a greater tendency to remain at rest. Therefore it is more difficult to move.

Hard to push © Manhattan Press (H.K.) Ltd.

Easy to push

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Text 34

5.1 Newton’s first law of motion (SB p. 63)

Thinking 1:

Hold a bottle of ketchup. Open the cap and turn it upside down. The ketchup does not flow out immediately. But if the bottle is thrust downwards at a high speed and then suddenly stopped, the ketchup is dislodged. Explain briefly. Ans

When the bottle is thrust downwards, both the bottle and the wer ketchup are moving downwards. When the bottle is stopped suddenly, the ketchup inside tends to continue its motion, so it flows out from the open end. Return to © Manhattan Press (H.K.) Ltd.

Text 35

5.1 Newton’s first law of motion (SB p. 64)

Thinking 2:

A car braked suddenly near a cliff as shown. Explain the motion of the driver. Ans wer

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Text Due to inertia, the driver and the hat are thrown forwards. They have the tendency to remain in motion. © Manhattan Press (H.K.) Ltd.

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