Newton’s Laws of Motion Grade 8 Science

Force Pop Quiz What is a force? What is the difference between a balanced and unbalanced force? What are the four types of friction? What is the law of universal gravitation? What two factors affect the gravitational attraction between object?

The First Law of Motion Newton’s first law of motion states that an object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless it is acted upon by an unbalanced force Example: Clothes on the floor of your room will stay there unless you pick them up Example: A tennis ball flies through the air once you hit it with a racket. If your friend doesn’t hit the ball back, the forces of gravity and friction will eventually stop the ball On Earth, gravity and friction are unbalanced forces that often change an object’s motion

Inertia An object’s resistance to a change in motion The amount of inertia an object has depends on its mass Objects with greater mass have more inertia, and require a greater force to cause a change in motion

The Second Law of Motion States that acceleration depends on the object’s mass and the net force acting on the object Written as: Acceleration = Net Force / Mass Force = Mass X Acceleration (F=mXa) Force is measured in kilograms x meters per second per second or in a unit called newton (N) 1 N = 1 kg x 1 m/s/s

The Second Law of Motion (Cont.) The acceleration of an object will increase if the force increases According to the equation, acceleration and force change in the same way – they both get larger The equation also shows that the acceleration will increase if the mass decrease

The Second Law of Motion - Examples What is the net force on a 1,000 kg object accelerating at 3 m/s/s? Force = Mass X Acceleration Force =

The Second Law of Motion - Examples What net force is needed to accelerate a 25 kg cart at 14 m/s/s? Force = Mass X Acceleration Force =

The Third Law of Motion Newton’s third law of motion describes the relationship between two forces States that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object “For every action, there is an opposite and equal reaction”

The Third Law of Motion (Cont.) Newton’s third law refers to forces on two different objects The action and reaction forces described by this law cannot be added together because they are each acting on difference objects Forces can be added together only if they are acting on the same object

Momentum Momentum is a characteristic of a moving object that is related to the mass and the velocity of the object Momentum = Mass X Velocity Unit for Momentum is kilogram-meters per second (kg-m/s) The momentum of an object is in the same direction as the velocity of the object The more momentum a moving object has, the harder it is to stop

Momentum Practice Problems A golf ball travels at 16 m/s, while a baseball moves at 7m/s. The mass of the golf ball is 0.045kg and the bass of the baseball is 0.14kg. Which has greater momentum? Momentum = Mass X Velocity

Momentum Practice Problems What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s? Momentum = Mass X Velocity

Law of Conservation of Momentum States that, in the absence of outside forces, the total momentum of the objects that interact does not change. It is the same before and after the interaction The total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the objects Friction would be an example of an outside force that might act on the objects