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Guess Who…

Dynamics

Dynamics is the study of how forces act on objects and their effect on the motion.

Starter

� Discuss with your partner, what do you know about Forces and Sir Isaac Newton?

Force: Any push or pull.

The units of force are Newtons (N)

There are four fundamental forces that make up all of the forces in the universe:

1) Gravitational Force

2) Electromagnetic force

3) Strong Nuclear Force

4) Weak Nuclear Force

Force

� Force is a vector quantity. To describe it fully we must have size and direction. We therefore use arrows to denote the direction of a force.

15N

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What happens to the ball when a force is applied?

1. Speed

2. Direction3. Shape

Newton’s first law (law of inertia)

� Newton’s 1st law: an object at rest tends to stay at rest and an object in motion tends to stay in motion, unless another force acts.

� in other words, and object will always continue doing whatever it is doing – if it is at rest it will stay at rest, if it is moving it will stay moving, unless a force acts to change its motion.

Think and then discuss with your partner

Mr Marsella is testing his new sports car when his brakes fail. Unfortunately he isn’t wearing a seat belt. What happens to him upon impact with the wall? Why?

Explain this in terms of Newton’s 1st law.

Newton’s first law (law of inertia)

� Newton’s 1st law: an object at rest tends to stay at rest and an object in motion tends to stay in motion, unless another force acts.

� in other words, and object will always continue doing whatever it is doing – if it is at rest it will stay at rest, if it is moving it will stay moving, unless a force acts to change its motion.

Balanced Forces

Balanced forces are EQUAL FORCES which act in OPPOSITE DIRECTIONS. They CANCEL EACH OTHER OUT.

FF

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If balanced forces act on a STATIONARY OBJECT, it REMAINS STATIONARY.

FF

If balanced forces act on a MOVING OBJECT, it

continues moving in the same direction with CONSTANT VELOCITY.

What happens if the forces are not balanced?

Inertia

� Inertia: the resistance an object has to a change in its state of motion.

� What does this mean?

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Balanced or Not?� What will happen in each case?

What is the resultant force, direction and motion?

35Kg

50 N120N

50N

50N

Starter

� What are the net forces on each box?

CTransport 1996:6

Free Body Diagrams worksheet

Starter� At the London Olympics Ivan

Ukhov won Gold with a high jump of 2.38m.

� If Ivan went to the moon how might this affect his jump? Why?

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What is Gravity?

� Discuss with the person next to you?

Fg =

MASS WEIGHT

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Mass

� Amount of matter in an object

� Constant (relatively)

throughout the universe

Weight

� Gravitational attraction (weight)

� Changes depending on

where you are in a gravitational field

What is gravity?

What would happen if we could escape gravity?

https://www.youtube.com/watch?v=OZY8279b7BU

https://www.youtube.com/watch?v=0iDyhg0BPZA

Notes – complete the notes.

Force of Gravity: Any two objects will feel a force of attraction that depends on their masses and distance apart.

Mass (kg): The amount of matter in an object.

Weight (N): The amount of gravitational force felt by an object.

Mass is constant throughout the universe whereas weight changes depending on where you are.

The formula for force of gravity is:

Fg = mg

Where:

m = mass (kg)

g = acceleration due to gravity (m/s2)

= gravitational field strength (N/Kg)

g varies depending on the size of the planet and distance to the object

For Example:

� On Earth at sea level g = 9.80 m/s2

� On the moon’s surface g = 1.60 m/s2

� On Jupiter g = 24.5 m/s2

� The Sun g = 274 m/s2

Determine your weight (in Newtons) on Earth, the moon and Jupiter.

What would the weight of a 25.0kg bag of cement be on earth?

Fg = mg Fg = 25 x 9.80 = 245N

What would the weight of a 5.0kg bag of potatoes on earth?

Fg = mg Fg = 5 x 9.80 = 49N

What is the mass 2555 N Grizzly bear?

m = Fg / g Fg = 2555 / 9.80 = 261kg

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Jumping on the moon and the sun!

Assuming you wouldn’t burn up and die in a hot fiery mess before getting anywhere near the sun, how high could you jump? Workbook

� Read and answer questions 77-80

Starter

A 3.00 kg mass suspended by a rope is moving upwards with a constant velocity of 2.00 m/s.

3.00 kg

2 m/s

Calculate the tension (force) in the rope.

Constant velocity means forces are balanced, Fg = Ft

g mF =g

9.81 3×=N 29.4F =g

Tension in rope is 29.4 N(down )

Draw FBD’s for both questions!

What upward force must be supplied by the rotor blades when the helicopter is travelling at the constant speed?

• https://phet.colorado.edu/sims/html/forces-and-motion-basics/latest/forces-and-motion-basics_en.html Net force

• Higher animation

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• NEWTON’S SECOND LAW states:

When an object is acted on by a constant UNBALANCED FORCE the body moves with constant acceleration in the direction of the unbalanced force.

Virtual Mechanics

Newton’s 2 nd Law “If a body is acted upon by an unbalanced force, it will accelerate.”

Stated as a formula:

Fnet = ma

Note that the units for force are N

Newton’s 2 nd Law “If a body is acted upon by an unbalanced force, it will accelerate.”

Stated as a formula:

Fnet = ma

Note that the units for force are N

Ex. A 5.0 kg block is pushed to the right along a frictionless track with a force of 10.0 N. What is its acceleration?

Ex. A 5.0 kg block is pushed to the right along a frictionless track with a force of 10.0 N. What is its acceleration?

Ex. A 650 kg car accelerates at 4.0 m/s2 south. What is the net force acting on it?

Ex. A 650 kg car accelerates at 4.0 m/s2 south. What is the net force acting on it?

Since Fnet can be determined using acceleration then you will sometimes be required to use kinematics in solving force problems.

Ex:

A 1500 kg ice cream truck accelerates from rest to a top speed of 45 km/h in 8.0 s. What was the net force acting on the truck?

Since Fnet can be determined using acceleration then you will sometimes be required to use kinematics in solving force problems.

Ex:

A 1500 kg ice cream truck accelerates from rest to a top speed of 45 km/h in 8.0 s. What was the net force acting on the truck?

Finding F net

To find F net when two forces work together simply add them.

Ex. Stan and Kyle are pushing a 75 kg Cartman along a frictionless ice rink. Stan pushes with 55 N and Kyle pushes with 45 N. Find Cartman’s acceleration.

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Complete worksheet What is the net force and acceleration?

35Kg

50 N 120N

50N

50N

Starter

A 3.00 kg mass suspended by a rope is moving upwards with a constant velocity of 2.00 m/s.

3.00 kg

2 m/s

Calculate the tension (force) in the rope.

Constant velocity means forces are balanced, Fg = Ft

g mF =g

9.81 3×=N 29.4F =g

Tension in rope is 29.4 N(down )

Draw FBD’s for both questions!

What upward force must be supplied by the rotor blades when the helicopter is travelling at the constant speed?

A 2 kg mass accelerates horizontally at 3 m/s2.

The mass is pulled by a force of 10 N.

Calculate the force of friction acting against the block.

kg 2m =-2ms 3a =

?Fnet =

a mFnet =3 2×=

right the to N 6Fnet =

2 kg10 NFriction

acceleration = 3 m/s2

6friction10 =−∴610friction −=

N 4friction =

Winner – Loser = Fnet

Starter

A 1000 kg car accelerates to the right at 4 m/s2. The force of friction acting on the car is 600 N. Calculate the force exerted by the car’s engine.

FENGINEFriction = 600 N

acceleration = 4 m/s2

a mFnet =kg 1000m =2m/s 4a =

?Fnet =4 1000 ×=

N 4000Fnet =

4000frictionF engine =−∴6004000Fengine +=

N 4600Fengine =

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What’s happening here?

CM1

http://www.bbc.co.uk/news/science-environment-21341485

• Why is felix accelerating?

• Why does he stop accelerating?

Terminal velocity

The air resistance acting on a moving object increases as it gets faster.

Terminal velocity is reached when the air-resistance (acting upwards) has increased to the same size as the force of gravity (fg) (acting downwards)

W = weight

Friction Ff(air resistance) = 0 N

time = 0s, velocity = 0 m/s, friction = 0 N

a = -9.81 m/s2

W = weight

Ff

a < -9.81 m/s2

v

Slide 55

CM1 Christopher Marsella, 12/6/2017

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W = weight

Ff

a = 0 m/s2

v

Equal & opposite forces=Zero Acceleration=Terminal velocity

Examples:1) You hit a baseball with a bat.The bat hits the ball - the ball hits the bat

2) A sprinter starts running.The sprinter pushes the ground - the ground

pushes the sprinter

3) A fish swims through water.The fish pushes the water backwards - the

water pushes the fish forwards

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Force of GUN on BULLET

Firing a gun

Force of BULLET on GUN

Force of RUNNER on BLOCKS

Starting a sprint

Force of BLOCKS on RUNNER

A Rocket

Force of ROCKET

pushing out Exhaust gas

Force of Exhaust gas on ROCKET

Imagine a bug hitting the windshield of a semi trailer.

What force pair occurs?

Which force is bigger?

Which object has a greater acceleration?

Hands up if you hate Newton!!!

Since, a = Fnetm

the truck has a much smaller acceleration because it is much, much more massive.

Bug hits truck –Truck hits bug

NEITHER!!!

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• ..\Physics 11 - Church\Dynamics\Other Resources\Zero-g.m4v