Force

Goal of the class To understand what forces are and how they interact Question of the day: How does an unbalanced force act on an object?

Forces A force is a push or a pull Forces are what cause objects to move (dynamics) A force has a magnitude and a direction. Force is a vector.

Forces The SI derived unit of force is the Newton (N) 1 N = 1 kg·m/s2 A force of 1N acting on a mass of 1kg will produce an acceleration of 1m/s2 Sir. Isaac Newton

Forces Forces can act through a distance or through contact: Distance Contact Gravity Pushes + Pulls Electric Force Friction Magnetic Force

Fundamental Forces

Force Diagrams A force diagram or free-body diagram is used to show forces to help analyse a problem

Combining Forces Forces are vectors and can be combined in different ways. This combined force is the addition of all the forces and is called the net force. 5N 5N 10N = 5N 10N 5N = 5N 5N = 0

Combining Forces 안녕! Draw a free body diagram for the baby on the sled. The dog pulls horizontally with a force of 100N. The weight of the baby and sled is 150N.

Net Force A book is placed on a table with an incline of 35 degrees. The free-body diagram shows the forces acting on the book. Find the net force N = 18N Set up coordinate system Find all x&y components Solve Ffriction = 11N mg = 22N

Newton’s First Law An object with no resultant force is not moving

Newton’s First Law An object will remain at rest, or with a constant velocity unless it experiences an external net force. Sometimes called the Law of inertia Inertia is the resistance to a change in velocity Car example going around corner

Equilibrium Equilibrium means to be in balance. When the net force is zero an object is in equilibrium. Example: A lift stopped on the 11th floor.

Newton’s Second Law The acceleration of an object is directly proportional to the net force and inversely proportional to the mass. ΣF = ma Net force = mass x acceleration 1N = 1kg x 1m/s2 momentum

Newton’s Second Law A ball is kicked with a force of 15N and accelerates at 5m/s2 to the right. What is the balls mass?

Newton’s Third Law For every action there is an equal and opposite reaction. Rocket in outer space This does not mean that the net force is always 0 Imagine pushing a door

Questions

Homework set #5 Read Chapter 5 Answer questions from page 143 on Question numbers 19, 29, 33, 53, 61, 75