Forces and the Laws of Motion

Everyday ForcesChapter 4: Section 4

Learning Targets• Explain the difference between 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 forces

P3.1d, P3.2A, P3.2C, P3.2d, P3.4e, P3.6C

The Fundamental Forces• Four fundamental forces exist in

nature– They are the gravitational force, the

nuclear strong force, the weak nuclear force, and the electromagnetic force

– These are all field forces

Force Characteristics• Strong and weak force have very

small ranges, so their effects are not directly observable

• Electromagnetic and gravitational forces act over long ranges

• Strong force is the strongest while gravity is the weakest

Weight• Weight is the measure of the

gravitational force exerted on objects– The more mass an object has, the more

strongly it is pulled by the gravitational force– The gravitational force exerted on an object

by Earth, Fg, is a vector quantity

– The magnitude of this force is a scalar quantity called weight

Fg = mag

where g = 9.81 m/s2

• Weight depends on location– The values of gravity on other planets

depend on the planet’s mass and radius– Objects also weigh less at higher

altitudes than at sea level because the value of gravity decreases as distance from the surface of the Earth increases

• 50 kg = 110lbs• 1 N = 0.225 lbs

The Normal Force• The normal force, Fn, is a force

perpendicular to the contact surface – It is not always opposite the force of

gravity

• An object on a ramp will have a normal force perpendicular to the ramp, but not directly opposite the force of gravity

• In the absence of other forces, the normal force is equal and opposite to gravity

Calculating Normal Force

• The magnitude of the normal force can be calculated as:

Fn = mgcosΘ

• The angle (Θ)is the angle between the normal force and a vertical line and is also the angle between the contact surface and the horizontal

Frictional Force• Friction is an

electromagnetic force that opposes applied forces– Most surfaces are actually

quite rough – Even when surfaces are only

in contact at a few points, they stick together at these points

• When a milk jug is at rest, the only forces acting on it are gravity and the normal force – These forces are equal and opposite

(Newton’s 3rd Law) so the jug is at equilibrium

• When you push the jug with a small horizontal force, F, the table exerts an equal force in the opposite direction– The resistive force that keeps the jug from

moving is the force of static friction, Fs

• When the applied force is as great as it can be without causing the jug to move, the force of static friction reaches its maximum value, Fs,max

Kinetic Friction• When the applied force exceeds the

maximum static friction (Fs,max), the jug begins to move

• The frictional force on an object in motion is called the force of kinetic friction, Fk

– This force is less than the maximum static friction (Fs,max)

Friction and the Normal Force

• The magnitude of the force of friction is approximately proportional to the magnitude of the normal force that the surface exerts on an object

• A heavier object would experience a greater normal force and therefore greater friction

Calculating Friction• The force of friction depends on the

mass of the object as well as the composition of the surfaces in contact

• The quantity that expresses the frictional force on a particular surface is called the coefficient of friction– Represented by the Greek letter mu (μ)

Friction EquationsCoefficient of Kinetic Friction

μk = Fk / Fn

Coefficient of Static Frictionμs = Fs,max / Fn

• The coefficient of kinetic friction is always less than or equal to the coefficient of static friction

Force of FrictionFf = μ * Fn

Air Resistance• Air resistance (FR)is a form of

friction• When the upward force of air

resistance balances the downward gravitational force, the net force is zero – The object continues to move with a

constant maximum speed known as terminal velocity