Chapter 3: ForcesSection 1:Newton’s Second

LawForce, Mass, and Acceleration

Newton’s Second LawFriction

Air Resistance

Force, Mass, And Acceleration

• Force, mass, and acceleration are connected by using Newton’s Second Law.

• FORCE AND ACCELERATION: For any object, the greater the force that is applied, the greater its acceleration will be.

• FORCE AND MASS: The acceleration of an object depends on its mass as well as the force exerted on it.

Newton’s Second Law

• According to Newton’s second law of motion, the net force acting on the object causes the object to accelerate in the direction of the net force.

• Acceleration (m/s2) = net force (N)mass (kg)

Example Calculation• “a” = F/m

– Force is 40 N (newtons)– mass is 80 kg– Looking for…. acceleration (m/s2)

• “a” = 40 N

• Try one: A student pedaling a bicycle applies a net force of 200 N. The mass of the rider and the bicycle is 50 kg. What is the acceleration of the bicycle and the rider????

• YUP-----200 N/50 kg = 4m/s2

80 kg

so…. 0.5 m/s2

http://www.ic.arizona.edu/~nats101/n2.html

• Second Law SUMMARIZED:

– Objects accelerate in the direction… that you push it.

– If you push twice as hard…..it accelerates twice as much.

– If the object has twice the mass…..it accelerates half as much!

• Newton’s second law can be rearranged to find the other parts of the equation.

• Net force = m*a

• Mass (kg) = net force (N)Acceleration (m/s2)

N = kg * m/s2

Friction• Friction is the force that opposes

motion between two surfaces that are touching each other.

• What causes Friction?– The source of friction are microscopic

dips and bumps that stick together called microwelds.

• The stronger the force pushing the two surfaces together, the stronger these microwelds will be.

• 3 types of friction– Static Friction

• Friction between two surfaces that are not moving past each other.

– Sliding Friction• Caused by microwelds constantly breaking

and then forming again.

– Rolling Friction• Due partly to the microwelds between a

wheel’s rotation and the surface it rolls over. Microwelds break then reform as the wheels rolls over the surface.

Air Resistance• When an object falls toward Earth,

it is pulled downward by the force of gravity.

• However, another force called air resistance acts on objects that fall through the air.

• Like friction, air resistance acts in the opposite direction of gravity.

• Terminal velocity is reached when air resistance balances gravity – example: parachutes!!