Unbalanced Forces and Motion

Unit: 06

Lesson: 01

Reviewing Motion Notes

• a. What is motion? • Motion - The act or process of changing position or

place. • Examples - moving vehicles, swinging a bat, ball

flying through the air.

• b. What is speed? • Speed - distance traveled per unit time. • Examples – 2 meters per second, 55 miles per hour,

22 kilometers per hour.

• c. What is displacement? • Displacement – the distance and direction of an

objects change in position from the starting point.

Force and Motion• d. What is distance?• Distance – how far an object has moved: uses units such

as meters, feet, inches, centimeters, kilometers etc.

• e. What is constant speed? • Constant speed - when an object in motion neither speeds

up or slows down.

• f. What is average speed? • Average speed – the total distance traveled divided by the

total time of travel.

• g. What is velocity? • Velocity – includes the speed of an object AND its

direction. • Examples – race car traveling at 55 mph due north, boat

traveling southeast at 22 km/h.

Review Newton’s 1st Law

• What is the relationship between mass and force?

As mass increases, force increases• Inertia- the tendency of a body at rest to remain

at rest or a body in motion to remain in motion unless acted upon by an outside force.

• Unbalanced forces- forces that act upon an object that cause the object to move in the direction of the greater force

Newton’s 1st Law This law is often called "the law of inertia".

• What does this mean?• This means that there is

a natural tendency of objects to keep on doing what they're doing.

• All objects resist changes in their state of motion. In the absence of an unbalanced force, an object in motion will maintain this state of motion.

Newton's 2nd Law F=ma

• Acceleration is produced when a force acts on a mass.

• The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).

Newton’s Second Law• F = ma • What does the equation mean? • F = force (unit is Newton’s) • m = mass (unit is Kilograms) • a = acceleration (unit is meters/second2) • Remember that velocity is the speed and direction of an object.

• Remember that speed is the rate of change of an objects position.

• Acceleration is the rate of change of an object’s velocity.

• Force is defined as a push or pull that one object exerts on another object.

• Mass is the amount of matter contained in an object.

• 1. You are pushing a friend on a sled. You push with a force of 20 N. Your friend and the sled together have a mass of 60 kg. Ignoring friction, what is the acceleration of your friend on the sled?

F = 20 N m = 60 kg

a = 0.33 m/s2

• 2. A student pedaling a bicycle applies a net force of 400 N. The mass of the rider and the bicycle is 25 kg. What is the acceleration of the bicycle and the rider?

• F = 400N m = 25 kg

• a = 16 m/s2

• 3. How much force will a tennis racket need to exert on a tennis ball, with a mass of 0.67 kg, to make it accelerate at a rate of 5,600 m/s2?

• m = 0.67kg a = 5,600 m/s2

• F = 3752 N

Newton’s 3rd Law

• For every action there is an equal and opposite re-action.

• This means that for every force there is a reaction force that is equal in size, but opposite in direction.

Other examples of Newton’s Third Law

• The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

3rd Law

• Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards.

3rd LawThe reaction of a rocket The reaction of a rocket is an application of the is an application of the third law of motion. third law of motion. Various fuels are burned Various fuels are burned in the engine, producing in the engine, producing hot gases. hot gases.

The hot gases push The hot gases push against the inside tube of against the inside tube of the rocket and escape out the rocket and escape out the bottom of the tube. the bottom of the tube. As the gases move As the gases move downward, the rocket downward, the rocket moves in the opposite moves in the opposite direction.direction.

A What?

Tribologist

What does a tribologist study in relation to force and motion.

Tribology comes from the Greek “tribo” , meaning “to rub”, and “logos” meaning “principle or logic.”

Is a scientist that studies the principle of friction, lubrication, and wear (of interacting surfaces that are in relative motion).

Friction

Explain the results from the shoe drag activity.

1. What is happening to the shoe before it begins to move?

It is experiencing static friction between it and the floor. Static friction is the force which keeps a motionless object from being pushed or pulled across a surface.

Friction (cont.)

2. Why is the spring not stretched as far once the shoe begins to move?

Once the shoe starts moving, the spring is not stretched as far as when the shoe was sitting still because when the shoe was sitting still, it had to overcome the static friction, as well as inertia, to get going. Once the shoe was going, it experiences kinetic friction. Kinetic friction acts to resist the motion of an object sliding across a surface.

Friction (cont.)

3. Why is there less friction to resist its movement once the shoe starts moving?

Kinetic friction is less than static friction, if everything else stays the same.

4. What happened to the shoe when the rocks were added?

When rocks were added, the increased mass increased inertia. The aluminum foil and wax paper also had different effects on the amount of friction that was present.

5. What has an impact on the amount of friction in a system?

The kind of material, weight, roughness and lubricants can all have an impact on the amount of friction in a system.

Friction (cont.)

Which of the following examples show how Newton’s First Law of Motion occurs in everyday life?

• A car suddenly stops and you strain against the seat belt.

• While riding a skateboard, you fly off the board when hitting a curb, a rock, or another object, which abruptly halts the motion of the skateboard.

• A magician pulls the tablecloth out from under a table full of dishes and the don’t fall off the table.

• The car turns left and you lean or slide to the right side of the car.

Newton’s 1st Law of Motion= Inertia

Newton's First Law of Motion= Inertia

• Why is Newton’s First Law of Motion called the law of inertia?

States that an object at rest tends to stay at rest, and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Newton's First Law of Motion= Inertia

• What is an unbalanced force?

Since these two forces are of equal magnitude and in opposite directions, they balance each other. The book is said to be at equilibrium. There is no unbalanced force acting upon the book and thus, the book maintains its state of motion.

No motion

Gravity Pulling Down on all objects

Table pushes upward on the book

Force applied to the book by your hand Friction