Forces & The Laws of Motion Ideas of Sir Isaac newton

Section 1 Objectives Describe how forces affect the motion of an object. Interpret and construct free-body diagrams.

What is a force? A force is a push or pull that may change an object’s state of rest or motion. Forces can cause acceleration. Forces can cause objects to start moving, stop moving, or change direction. Do you think a force can affect an object’s velocity?

The SI unit of force: Newton (N) The SI unit of force is the Newton, N, named after Sir Isaac Newton who studied force and motion. The Newton is the amount of force that, when acting on a 1-kg mass, produces an acceleration of 1m/s 2. The weight of an object measures the magnitude of the gravitational force exerted on it.

Classes of forces Contact forces result from physical contact between two objects. (e.g. pulling on a spring, F spring ) Field forces do not involve physical contact between objects. (e.g. gravitational force, F grav )

Force diagrams A force is a vector quantity. Force diagrams portray force vectors as arrows.

Free-body diagrams Physicists utilize free-body diagrams to help analyze situations involving forces.

Section 2 Objectives Explain the relationship between the motion of an object and the net external force acting on the object. Determine the net external force on an object. Calculate the force required to bring an object into equilibrium.

Newton’s First Law of Motion An object at rest remains at rest, and an object in motion continues in motion with constant velocity unless acted on by an external net force.

The first law is also known as inertia Inertia is the tendency of an object to resist changes in motion.

What is net force? The vector sum of forces acting on an object is the net force.

Sample Problem B: Determining Net Force Derek leaves his physics book on top of a drafting table that is inclined at a 35° angle. Find the net force acting on the book.

Equilibrium Objects that are neither at rest or moving with constant velocity are said to be in equilibrium. Here, the net force is zero.

Section 3 Objectives Describe an object’s acceleration in terms of its mass and the net force acting on it. Predict the direction and magnitude of the acceleration caused by a known net force. Identify action-reaction pairs.

Newton’s Second Law The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the object’s mass. F=ma

Sample Problem C Roberto and Laura are studying across from each other at a wide table. Laura slides a 2.2-kg book towards Roberto. If the net force acting on the book is 1.6-N to the right, what is the book’s acceleration?

Newton’s Third Law If two objects interact, the magnitude of the force exerted on the first object by the second object is equal to the magnitude of the force simultaneously exerted on the second object by the first object, and these two forces are in opposite direction. (Also called action- reaction forces)

Forces always exist in pairs. E.g. field forces are action-reaction forces.

Everyday forces Section 4 Objectives Differentiate among mass and weight. Find the direction and magnitude of normal forces. Describe air resistance as a form of friction. Use coefficients of friction to calculate frictional force.

Weight & Normal Force Weight is a measure of the gravitational force on an object. Its value can change with location. W=mg The normal force is a force that acts on a surface in a direction perpendicular to the surface. F n

Frictional Force Friction is the force that opposes motion, or the applied force, F app.

Static friction and Kinetic friction Static friction is the force that resists the initiation of sliding motion between two surfaces that are in contact and at rest. (E.g. a jug of orange juice and a counter.) Kinetic friction is the force that opposes the movement of two surfaces that are in contact and are sliding over each other.

The coefficient of friction The ratio of the magnitude of the force of friction between two objects in contact to the magnitude of the normal force with which the objects press against each other, µ=F f / F n

Sample Problem D Coefficients of friction A 24-kg crate initially at rest on a horizontal floor requires a 75-N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.

Sample Problem E Overcoming Friction A student attaches a rope to a 20.0-kg box of books. He pulls with a force of 90.0-N at an angle of 30° with the horizontal. The coefficient of kinetic friction between the box and the sidewalk is Find the acceleration of the box.