Chapter 8 Lesson 3 Forces

What is Force? A force is a push or pull on an object An example would be pushing the classroom door open The unit for force is the newton (N)

Contact Forces A contact force is a push or pull one object applies to another object that is touching it This contact force can be small such as a finger pushing the button on an elevator or as big as a wrecking ball crashing in to a building

Noncontact Force A noncontact force is a force that one object applies to another object without touching it An example would be the attraction between metal and a magnet and gravity

Gravity Gravity is an attractive force that exists between all objects that have mass Mass is the amount of matter in an object The Law of Gravity was developed by Sir Isaac Newton This law states that all objects are attracted to each other by a gravitational force. The strength of the force depends on the mass of each object and the distance between them

Gravitational Force Depends on Mass If the mass of an object increases, so does its gravitational force between it and another object The gravitational force between you and Earth is large because Earth’s mass is large. That is what holds you to Earth. The GF between you and your pencil is small because your mass and the pencil’s mass is small compared to the Earth’s mass. You do not feel the attraction even though it is present

Gravitational Force Depends on Distance As 2 objects move apart, the GF between them decreases

Mass and Weight are DIFFERENT Weight is a measure of GF acting on an object’s mass Weight depends on the mass of the 2 objects and the distance between them When the weight of 2 objects on Earth are compared, the one with the greater mass will weigh more Those same two objects will have a different weight on the moon but their masses will be the same as they are on Earth but the weight will be different because the mass of the moon is less than the mass of the Earth.

Friction Friction is a contact force that resists the sliding motion of 2 surfaces that are touching Rough surfaces or materials tend to produce more friction

Effects of Friction Friction causes things to slow down The heavier the object, the more friction it experiences Friction acts in the opposite direction of the object’s motion

Air Resistance Air resistance is the frictional force between air and object’s moving through it The more surface area an object has, the more air resistance it experiences For example, a flat piece of paper compared to a crumbled ball piece of paper

Combining Forces When more than one force acts on an object, the forces combine and act as one force The sum of all the forces acting on the object is called the net force

Combining Forces (cont) When 2 forces are acting on an object but in opposite directions, the direction of the force must be included when you add them The positive direction is usually to the right The negative direction is usually to the left If the force to the right is the same as the force to the left, the object does not move

Balanced Forces If the net force on an object is 0N, the forces acting on the object are called balanced forces There is no movement of the object

Unbalanced Forces When the net forces acting on an object are not 0, the forces acting on the object are called unbalanced forces The object will move in the direction in which the force is greater. Unbalanced forces cause a change in the object’s velocity It will change the objects speed or direction

Newton’s 1st Law of Motion If the net force acting on an object is zero, the motion of the object does not change This is also known as the law of inertia The person in the car flies out because the car and the person are traveling at 70mph and then the car comes to a sudden stop because it hits the wall but the person in the car is still traveling at the 70mph

Newton’s 2nd Law of Motion States that acceleration of an object is equal to the net force exerted on the object divided by the object’s mass This means that the greater the mass, the greater the force needed to accelerate the object at the same rate Acceleration is the same direction as net force A=f/m

Newton’s 3rd Law of Motion States that for every action there is an equal and opposite reaction When one object exerts a force on a second object, the second object exerts a force of the same size but in the opposite direction These equal and opposite forces are called force pairs

Which one of Newton’s laws is also known as the law of inertia? Newton’s 2nd law Newton’s 1st law Newton’s 3rd law Newton’s 4th law

Your mass will change if you go to the moon True or false: Your mass will change if you go to the moon False, your weight will change but your mass will stay the same

A person pushes a cart forward with a force of 20N A person pushes a cart forward with a force of 20N. A friction force of 5N pushes the cart backwards. What is the net force on the cart? 15N backward 15N forward 25N backward 25N forward