Force A force is a pull or a push.

The unit of measure for force is Newton (N). English scientist The unit of measure for force is Newton (N). English scientist. (1643 – 1727) Force and its direction can be represented by an arrow: The longer is the arrow, the greater is the force. F₁ F F₂

Combining forces The combination of all forces is called the net force. When forces act in the same direction we add forces. = 10N When forces act in the opposite direction we subtract forces. = Forces may cancel each other. = 0 5 N 5 N 5 N 10 N 5 N 5 N 5 N 5 N

Friction The force that 2 surfaces exert on each other when they rub against each other is called friction. There are four types of friction: Static friction – when you can’t move an object Sliding friction – when 2 solid surfaces slide over each other Rolling friction – when an object rolls across a surface Fluid friction – when an object moves through a fluid

Gravity Gravity is the force that pulls objects towards each other. Ex. The gravity of the sun keeps the planets orbiting around the sun. Ex. The gravity of the sun keeps the planets orbiting around the sun.

The gravity force on the Earth is 9. 8 N for each kilogram The gravity force on the Earth is 9.8 N for each kilogram. The gravity force on the Moon is 6 times less than on Earth. Ex. Mass in spacesuit is 165 kg on Earth and Moon. Weight on Earth is 1617 N (165 x 9.8) Weight on Moon is 270 N ( 1617 / 6)

Air Resistance Objects in free fall accelerate at 9.8 m/s regardless of their masses. Objects falling through the air experience a type of fluid friction which is called air resistance. That is why a fruit falls faster than a leaf. No air resistance No air resistance.

Projectile Motion When you throw an object it has a projectile motion.

Inertia Inertia is the tendency of an object to resist a change in motion. The heavier is the object the more inertia it has. Ex. The inertia of the objects on the table keeps them from moving.

The Second Law of Motion It states that acceleration depends on the object’s mass and on the net force acting on the object. Formula: Acceleration = Net Force Mass Ex. A motorcycle can accelerate faster than a car even if its engine is smaller than a car.

Calculating Force Example: A speedboat pulls a 55 kg water-skier. The force causes the skier to accelerate at 2 m/s. Calculate the net force. Solution: Fnet = Mass x Accelleration Fnet = 55 x 2 = 110 kg ∙ m/s² F = 110 N Answer: 110 N

Newton’s Third Law It 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.

Momentum Momentum is the quantity of motion. Formula: Momentum = Mass x Velocity The unit of measure for momentum is kg ∙ m/s². It is easier to stop a moving ball at 20 m/s than car having the same speed because of the mass differences. Example: Which has more momentum: a 3 kg sledgehammer swung at 1.5 m/s, or a 4 kg sledgehammer swung at 0.9 m/s? Solution: a) Smaller sledgehammer: Momentum = M x V = 3 x 1.5 = 4.5 kg ∙ m/s b) Larger sledgehammer: Momentum = M x V = 4 x 0.9 = 3.6 kg ∙ m/s Answer: Smaller sledgehammer has more momentum

Rockets and Satellites Rockets are often used to carry satellites into space. Artificial satellites orbit the Earth in a circular path because of centripetal force. Communications Satellite Communications Satellite