What is a Force? a push or pull on an object Unit is the newton (N) force has size and direction Force can be shown as an arrow

The amount and direction of force can be shown as an arrow

There are two main types of Forces Contact Forces Friction Air resistance Noncontact Forces Gravity Magnetism

Contact Force objects that are touching forces can be small or large

Friction resists the sliding motion of surfaces that are touching It acts in the opposite direction of motion rough surfaces and heavier objects produce more friction

To reduce the amount of friction, apply a lubricant between two surfaces. Motor oil, wax, and grease are examples.

Air Resistance the force between air and an object It tries to slow things down It works opposite gravity A falling object with less surface area will have less resistance and therefore move quicker. A falling object with more surface area has more air resistance and therefore moves slowly.

a person falling without a parachute has only the surface area of his body, therefore very little air resistance and is therefore falling quickly. When the parachute deploys it increases the surface area, increasing air resistance, and allowing him to slow down.

The windshield and front end of a jeep have a large surface area that comes in contact with the air when the jeep is in motion. This creates a high amount of air resistance The curved design of this car reduces surface area and therefore reduces air resistance

Noncontact Force The force between objects that are not touching Example :gravity and magnetism

Gravity According to the Law of Gravity developed by Sir Isaac Newton states that all objects are attracted toward each other by a gravitational force.

The strength of the gravitational force between objects depends on their mass and distance. As the mass of an object increases, the gravitational force increases between it and another object. As the distance between two objects increases, the gravitational force decreases between them

The force of gravity between us and Earth is strong because the mass of Earth is so large. Although there is a force of gravity between us and our book, both objects have a relatively small mass compared to Earth so we don’t feel it.

Use the worksheet we did in class to describe the affect of mass and distance on the strength of gravity.

Magnetism the force where objects are attracted or repelled to one another. Usually these objects are metals such as iron. A magnet creates an invisible area of magnetism all around it called a magnetic field.

The closer the opposite ends of magnets get the stronger the force becomes. A magnet has two ends called poles, a north pole and a south pole.

How do forces Combine? Often more than one force often acts on an object. The net force on an object is all the forces added together. What is the net force on the bike? 17N to the right

Forces in the Same Direction When forces are applied in the same direction, they are added to determine the size of the net force.

Forces in Different Directions When two forces act in opposite directions, you subtract the smaller force from the larger force to determine the net force. The net force will be in the direction of the larger force.

Balanced force Net force = 0 N Objects are not in motion Unbalanced Force Net force is not 0 N Objects are in motion

How much force is required to keep the dogs from moving? 160 N to the left

Three people are pulling on a wagon applying forces of 100 N,150 N, and 200 N. Which way is the wagon moving and with how much force? 50 N to the left

Newton’s Laws of Motion 1 st Law – An object at rest will stay at rest, and an object in motion will stay in motion, unless acted upon by an outside force. 2 nd Law – Force equals mass times acceleration. F=MxA 3 rd Law – For every action there is an equal and opposite reaction.

1 st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. The property of an object to resist change is known as Inertia

A golf ball on a tee will remain on the tee until the club makes contact with it.

Example: An astronaut in outer space will continue drifting in the same direction at the same speed indefinitely, until acted upon by an outside force.

Example If you are driving at 65 mph and have an accident, your car may come to a stop in an instant, while your body is still moving at 65 mph. Without a seatbelt, your inertia could carry you through the windshield.

2 nd Law of Motion Force = mass x acceleration An object with a smaller mass will take less force to move it. An object with a greater force will take more force to move it.

The mass of the large rock is 15kg. The mass of the smaller rock is 1kg. Which rock will require a greater force to accelerate? The larger rock because force=mass x acceleration

3 rd Law of Motion “For every action there is an equal and opposite reaction.” Newton’s third law discusses pairs of objects and the interactions between them.

Flying gracefully through the air, birds depend on third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.

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