Connecting Force and Motion, and Newton’s First Law of …



Connecting Force and Motion, and Newton’s First Law of Motion

Question: If you were aboard the Starship Enterprise and you accelerated to light speed

in an instant what would happen to you? Also, what would happen to you if

you stopped in an instant?

A. Let’s first examine what happens when a jet or starship turns on its engine:

1. The hot air produces thrust for the jet, and fusion reaction pushes gas and

radiation backward propelling the starship forward.

a. What is thrust? Thrust is a force that propels/pushes an object

forward

- What is a force? Force is a push or pull one object exerts

on another

b. Why is force important?

- Force is what is going to determine what an object does

Example: If an object is moving does the force

always change an objects velocity?

- Sometimes

- 2 types of forces

1. Balanced Force – Forces on an object that are equal in

in size and opposite in direction

Example: You are having a tug of war match and you plant yourself firmly and lean back to push against the ground causing the ground to push back on you. Your opponent does the same. If you do not move forward or backward, the force of your opponent pulling the rope and must be balancing the force the ground pushing you back.

2. Unbalanced Force – Force on an object that are not

not equal in size and opposite in direction

- In this case the net force always changes the

velocity of an object

c. How is force calculated?

- It is calculated F = M x A where F = Force, M = Mass,

and A = Acceleration

- The triangle can be applied to this equation

- From this equation you can calculate Force, Mass, and

Acceleration.

- You must take note that you now have 2 ways to

calculate Acceleration. This is why it is so

important to recognize what information has been

given to you in order to choose the correct

formula

- Force is directly related to mass and acceleration

Example: If you had to push soccer ball or car up

a hill which one would you pick and why? A soccer because has less mass and you don’t have to exert as much energy

B. Newton’s First Law of Motion (also known as the Law of Inertia)

1. An object at rest tends to stay at rest and an object in motion tends to stay in

motion

a. What does this have to do with inertia?

- Inertia is the tendency of an object to resist any change in its

motion

b. Let’s think back to the starship questions. What made the starship go?

- The thrust of the engine: it was an unbalanced force that caused the ship to change velocity

c. When a jet lands what makes the jet stop?

- No more thrust being produced, air resistance, and friction from

the ground

- The forces go from being unbalanced to balanced once the jet

stops

d. How is mass related to inertia?

- The greater the mass of an object the greater the interia of the

object

e. Think about this: Ice is very slippery. If you push a hockey puck on the ice it goes a long distance. However it eventually stops. If

Newton’s First Law is true why does the puck stop?

- Friction: is the force that opposed motion between

two surfaces that are touching each other

Newton’s Laws of Motion

Newton’s First Law of Motion or Law of Inertia

- Objects in motion tend to stay in motion or objects at rest tend to stay at rest

unless acted upon by a force

Forces acting upon a moving object:

1. Gravity

2. Friction

3. Wind

4. Other objects

5. Water current

Forces acting upon a resting object:

1. Pushing

2. Pulling

Newton’s Second Law of Motion

- A net force acting on an object causes an object to accelerate in the direction of

the force

- Deals directly with accelerated motion

o Example: Hitting a baseball – the direction of the force of the swinging bat cause the ball to go in that direction

- Acceleration (change in speed and or direction) is determined by:

1. Size of the force

2. Mass of the object

-So, the larger the mass the greater the force needed to achieve the same

acceleration as an object with a smaller mass

- Calculating Force:

F = M x A Where F = Force, M = Mass, and A = Acceleration

Things you need to be aware of:

` 1. Mass is expresses in kg

. 2. Acceleration is expressed in m/s2

3. Force is expressed in kg x m/s2 or Newtons (N)

Example: How much force is needed to accelerate a 70 kg rider and her 200 kg motorcycle at 4 m/s2?

F = M x A M = 200 kg and 70 kg A = 4 m/s2 F = ?

F = (200 kg + 70 kg) x 4 m/s2

F = 1080 N or 1080 kg x m/s2

Example: A 63 kg skater pushes off from a wall with a force of 300 N. What is the skaters acceleration?

A = F/M A = ? F = 300 N M = 63 kg

A = 300N/63 kg

A = 4.76 m/s2

Lets Introduce Gravity: Answer the following question?

Which object would hit the ground first: A bowling ball or a

tennis ball?

Answer: Neither, they would hit at the same time

Why?

Answer: Earth’s gravity causes all falling objects to accelerate at

9.8 m/s2

Remember: 1 kg weighs 9.8 N on earth

If you take F = M x A ( W = M x A Where W = weight (remember weight is a measure of the pull of gravity on an object)

If acceleration due to gravity is 9.8 m/s2, then:

W = M x 9.8 m/s2

Which means 1 kg = 9.8 (kg x m/s2) or 9.8 N

Example: A weightlifter raises a 440 kg barbell with an acceleration of

2 m/s2. How much force does the weightlifter exert on the barbell?

F = M x A F = ? M = 440 kg A = 9.8 m/s2 + 2 m/s2

F = 440 kg x (9.8 m/s2 + 2 m/s2)

F = 5200 N

Note: This all depends if the object is freely falling. If the object is

thrown or some other force pushes or pulls the object you must

take this into account when making your calculation

Question: Which object would hit the ground first: A crumpled piece of

paper or a non crumpled piece of paper.

Answer: A crumpled piece of paper

Why is that if they both have the same mass and they are freely

Falling?

Answer: Air resistance!!!! – Air pushing up or against the

falling object will slow the acceleration

Terminal Velocity – Highest velocity that will be reached by a falling object

- Basically when an object falls it will eventually stop accelerating due to

the balancing force between gravity and air resistance. The object will

continue to fall however it will be at a constant velocity.

Newton’s Third Law of Motion

Question: What will happen if you try to leap from a row boat onto the dock?

Newton’s Third Law of Motion states when one object exerts a force on a

second object, the second one exerts a force on the first that is equal in size

and opposite in direction

1. What does that mean? For every action there is an equal but opposite

reaction

Example: The force you are exerting on your seat is the same force

your seat is exerting on you

- You are pushing the seat down and the seat is holding you up.

2. Back to the initial question: What happens? You fall in

a. Why? When you leap from the boat, the boat exerts a force on your feet, moving you forward. Your feet also exert an equal and opposite force on the boat sending it backward.

3. Is this how a rocket works? Yes!!!!

a. Balloon Rockets – When the air was released from the balloon the

force of the escaping air went one way the balloon went the other

1. The forces were equal and opposite

2. If you did not release the air from the balloon the force of the air inside the balloon is acting on the balloon. However there is nowhere for the air to go. When you open the balloon the air pushing against the opposite end of the opening of the balloon is pushing against the balloon (action force) causing the air to escape out the bottom thrusting the balloon forward

4.What is momentum?

a. Is a property of any moving object resulting from its mass and

Velocity

1. To calculate momentum:

p = mv p = momentum, m = mass, v = velocity

- The unit for momentum (p) is kg x m/s

- Momentum has a direction because velocity has a direction. However if an object is not moving, it has zero momentum.

- Question: If a motorcycle is traveling north at 100 mph and a small plane was traveling the same direction at 100 mph do they both have the same momentum?

o No, because the mass of the plan is greater than the mass of the motorcycle

Example: Which object has a greater momentum: a 3000 kg elephant traveling at 3 m/s or a 100 kg bike traveling at 25 m/s?

P = M x V Elephant p = 3000 kg x 3 m/s = 9000 kg x m/s

Motorcycle p = 100kg x 25m/s = 2500 kg x m/s

The elephant has the greater momentum

- So the momentum of an object doesn’t change unless

you change the velocity or mass of the object

b. Law of Conservation of Momentum

- States the total momentum of a group of objects does not change unless outside forces act on the objects

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