Chapter 3, Section 3

Chapter 3, Section 3

3 What is force? Motion and Forces A force is a push or pull. Sometimes it is obvious that a force has been applied. But other forces aren t as noticeable.

What Is a Force? A force...... is a push or pull.... acts on an object.... requires an agent (the one exerting the force).... is a vector.... is a contact force or a long range (non-contact) force.

3 Motion and Forces Changing Motion A force can cause the motion of an object to change.

A Force Can a. Move a stationary object b. Stop a moving object c. Change the speed of an object d. Change its direction Force causes ACCELERATION!

Types of forces (brain storm!)

Contact Forces vs. Non-contact Forces (or long-range forces) ideas?

3 Motion and Forces Balanced Forces Force does not always change velocity. 3. (a) When two or more forces act on an object at the same time, the forces combine to form the net force. (b) The word net means added together.

3 Balanced Forces The net force on the box is zero because the two forces cancel each other. c. Forces on an object that are equal in size and opposite in direction are called balanced forces. Motion and Forces

result in no net force, and no acceleration

3 Motion and Forces Unbalanced Forces The students are pushing on the box in the same direction. These forces are combined, or added together, because they are exerted on the box in the same direction.

3 Motion and Forces Unbalanced Forces The net force that acts on this box is found by adding the two forces together.

3 Motion and Forces Unbalanced Forces 4. When two students are pushing with unequal forces in opposite directions, a net force occurs in the direction of the larger force.

3 Motion and Forces Unbalanced Forces The net force that moves the box will be the difference between the two forces because they are in opposite directions. They are considered to be unbalanced forces.

MORE on Types of Forces

3 Gravity Many types of forces act on objects. 2.2 Types of Forces Gravity is an attractive force between all objects that have mass.

3 2.2 Types of Forces Gravity (cont.) The ball does not travel in a straight line because of the unbalanced force of gravity acting on it.

3 Weight and Mass Mass is the amount of matter in an object and does not change with location. Weight is the gravitational force on a object and changes with location. Weight is a force and a vector. 2.2 Types of Forces WEIGHT is the same thing as the FORCE of GRAVITY

3 Friction 2.2 Types of Forces Friction is a force that opposes the movement between two surfaces in contact. A book pushed across a table slows down because of friction. Friction can be an unbalanced force acting on the book to slow it down.

3 2.2 Types of Forces 1. Static Friction (keeps things from sliding) Static friction is the force between two surfaces in contact that keeps them from sliding when a force is applied. A force is applied to a heavy box, but the box doesn t move. The forces are balanced, the force pushing the box equals the force of static friction pushing in the opposite direction.

3 2. Sliding Friction When the force pushing on the box is larger than the maximum static friction force, the box begins to slide. 2.2 Types of Forces The frictional force that acts on the sliding box is called sliding friction.

3 2.2 Types of Forces Sliding Friction (cont.) The book pushed across the table slows down because of sliding friction. Without sliding friction, the book would continue moving without a force being applied.

3 2.2 Types of Forces Sliding Friction (cont.) Usually friction is present and an unbalancing force must be applied to keep an object moving. When friction is greatly reduced, objects move with nearly constant velocity without an applied force. Another type of friction is 3) rolling friction. Rolling friction is usually less friction than sliding friction.

3 Tension 2.2 Types of Forces A tension force is a pulling force exerted by an object when it is being pulled by, or suspended from, a rope or string.

Air Resistance

Air Resistance What two forces are acting on an object when it falls? Gravity Air resistance

Air Resistance Imagine dropping two pieces of paper. One is crumpled and the other is flat. Which one will reach the ground faster and why? The crumpled one, because it has less surface area.

Air Resistance When something falls, air resistance acts in the opposite direction as the force of gravity. Air resistance acts in the opposite direction of the object s motion.

Air Resistance The amount of air resistance an object experiences depends on three things: Speed Size Shape

Air Resistance Why do leaves, papers, and feathers fall at different speeds than acorns, pens, and glasses? Because of Air Resistance Air resistance (not mass) is responsible for the difference in objects falling speed.

Terminal Velocity As an object falls, it accelerates and its speed increases. The force of air resistance increases with speed.

Terminal Velocity The force of air resistance increases until it becomes large enough to cancel the force of gravity. When the forces cancel each other out the object no longer accelerates. Terminal velocity is reached when air resistance is equal to the force of gravity. The object then falls at a constant speed, called terminal velocity.

Terminal Velocity Terminal Velocity is the highest velocity that a falling object will reach. A low terminal velocity allows the skydiver to land safely.

Terminal Velocity Why would a skydiver want to lay out flat versus falling standing up? Think about our paper example crumpled vs flat

Terminal velocity short video