Newton’s Laws& Rocketry

Chapter 14

The Causes of Motion

• Galileo developed the concept of inertia.

• Inertia is that property that causes a body to RESIST changes in its state of motion.

• Inertia depends on mass.

• If no net force acts on an object, it stays at a constant velocity.

• Constant velocity can be 0 m/s too.

Newton’s 1st Law of Motion

• In the absence of a resultant external force acting upon it, a body will remain at rest or in uniform rectilinear motion (its acceleration will be zero).

• It takes an unbalanced force acting on an object to change its state of rest or motion.

• E.g. an apple on a table has no net force.• E.g. an apple dropping off the table has a

net force acting on it.

Exam Question•

A b l o c k g l i d e s o n a p e r f e c t l y s m o o t h s u r f a c e .

= c o n s t a n tv

T h e b l o c k i s s u b j e c t e d t o t h e s a m e f o r c e ( F ) i n f o u r d i f f e r e n t d i r e c t i o n s a s s h o w n b e l o w . I n a l l f o u r c a s e s , t h e f o r c e ( F ) i s l e s s t h a n t h e w e i g h t o f t h e b l o c k .

C a s e 2

F F F

F

C a s e 1 C a s e 3 C a s e 4

I n w h i c h c a s e w i l l t h e f o r c e ( F ) c a u s e a c h a n g e i n t h e s p e e d o f t h e b l o c k ?

A )

I n c a s e s 1 a n d 2

B )

I n c a s e s 2 a n d 4

C )

I n c a s e s 3 a n d 4

D )

I n a l l f o u r c a s e s

Exam Question

Which of the following situations show an object or person that is not acted on by a resulting force? 1. A cyclist slowing down 2. Someone sitting on a park bench 3. A sign fixed to the wall 4. A truck moving away after stopping at a stop sign 5. A rocket moving at a constant speed in space

A)

Situations 1, 2 and 4

B)

Situations 1, 3 and 5

C)

Situations 2, 3 and 5

D)

None of these situations

Exam QuestionT h e d ia g ra m b e lo w re p re s e n ts th e t r a je c to r y o f a n o b je c t m o v in g to th e r ig h t o n a h o r iz o n ta l p la n e . E a c h c i rc le r e p re s e n ts th e p o s i t io n o f th e o b je c t a t 0 .0 5 s in te rv a ls .

A

A fo rc e a c ts u p o n th e o b je c t a t p o in t A . W h a t w a s th e e f f e c t o f th is fo r c e o n i t s s p e e d ?

A )

I t o n l y c h a n g e d th e s p e e d o f th e o b je c t .

B )

I t o n l y c h a n g e d th e d i r e c t io n o f th e o b je c t .

C )

I t c h a n g e d th e s p e e d a n d th e d i re c t io n o f th e o b je c t .

D )

I t h a d n o e f f e c t o n th e s p e e d o f th e o b je c t .

Activity

• Page 308, Q. 1-3

Aerodynamics

•

Newton’s 2nd Law of Motion• If an unbalanced force acts on an object,

the object accelerates in the direction of the force.

• The acceleration varies inversely with the mass.

• The acceleration varies directly with the force.

• F = ma.

• 1 N is the force required to accelerate 1 kg by 1m/s2.

Newton’s 2nd Law

•

♫ ♪ I’m Free Falling…♫

• The acceleration of

both elephants due to

gravity is the same.

Activity

• What is the acceleration of a 70 kg skater pushed by a force of 161 N [W]?

• F = ma or a = F/m = 161 N [W]/ 70 kg

• a = 2.3 m/s2 [W]

Exam Question

A net force "F" causes a mass m1 of 4.0 kg to accelerate at a rate of 6.0 m/s2. This same force "F" causes a second mass m2 to accelerate at 8.0 m/s2. What is the value of mass m2?

A)

2.5 kg

B)

3.0 kg

C)

3.5 kg

D)

5.3 kg

Exam QuestionT h e d i a g r a m b e l o w s h o w s a c a r t w i t h m a s s M a c t e d o n b y a f o r c e F w h i c h p r o d u c e s a n a c c e l e r a t i o n a .

MF

S i t u a t i o n I

A s e c o n d c a r t w i t h a m a s s o f 5 M i s a t t a c h e d t o t h e f i r s t , a n d a f o r c e t h r e e t i m e s b i g g e r t h a n t h e f i r s t a c t s o n t h e s y s t e m . T h e d i a g r a m b e l o w s h o w s t h i s s i t u a t i o n .

M53

S i t u a t i o n I IF

M

H o w w i l l t h e a c c e l e r a t i o n i n S i t u a t i o n I I c o m p a r e w i t h t h a t o f S i t u a t i o n I ?

A )

I t m u s t b e 3 t i m e s i s l a r g e .

B )

I t m u s t b e 2 t i m e s i s l a r g e .

C )

I t m u s t b e 2 t i m e s s m a l l e r .

D )

I t m u s t b e 3 t i m e s s m a l l e r .

Exam QuestionIn the following mechanical system, block 1 has a mass of 200 g. As it falls, it drags cart 2 along the table top horizontally. The cart has a mass of 300 g. A string attaches the block to the cart over the pulley. Disregard friction and the mass of the string and the pulley.

2

1

What is the acceleration of the block?

A) 2.0 m/s2

B)

4.0 m/s2

C)

6.0 m/s2

D)

10.0 m/s2

Activity

• Page 314, Q. 1-4

Reaction Forces

•

Newton’s 3rd Law of Motion• Newton’s 3rd Law of Motion: For every

action force, there is an equal and opposite reaction force.

• OR If object A exerts a force on object B, then object B exerts a force equal in magnitude but opposite in direction on object A.

• OR You cannot touch someone else without being touched in return.

• OR Forces exist in pairs.

Reaction Forces

•

Examples of Reaction Forces

• E.g. A tractor exerts a force of 2000 N [W] on a trailer. The trailer exerts a force of 2000 N [E] on the tractor.

• When we push on a wall it pushes back.• When we kick a football, the same force

kicks back on us. Our foot decelerates a bit because our mass is greater. The football accelerates a lot because its mass is small.

Activity

• Page 317, Q 1-4

Exam Question

A car with a mass of 1000 kg and moving at a speed of 30 m/s comes to rest over a distance of 100 metres. What is the force of friction (acting on the wheels of the car) which causes the car to stop?

A)

3 000 N

B)

3 500 N

C)

4 000 N

D)

4 500 N

Activity

• Page 321, Q. 1-6

Summary• Newton’s 1st Law of Motion: If no net force

acts on an object, it maintains its state of rest or its constant speed in a straight line.

• Newton’s 2nd Law of Motion: If an unbalance force acts on an object, the object accelerates in the direction of the force. The acceleration varies inversely with the mass and directly with the force. F = ma.

• Newton’s 3rd Law of Motion: For every action force, there is an equal and opposite reaction force.

Activity

• Page 321, Q. 7-10