Newton’s Second Law Examined

“A change in motion is proportional to the motive force impressed and takes place

along the straight line in which that force is impressed”

Another way to put it…

• Acceleration is proportional to the net force on an object and inversely proportional to its mass

Sample Problem 1

Rocket Guy weighs 905 N and his jet pack provides 1250 N of thrust, straight up. What is his acceleration.

Sample Problem 2

A helicopter of mass 3770 kg can create an upward lift force (F) when empty. It can accelerate straight upward at a maximum of 1.37 m/s2. A careless crewman overloads the helicopter so that it is just unable to lift off. What is the mass of the cargo?

Sample problem 3

Len pushes toward the right on a 12 kg box with a force of 31 N. Martina applies a 11 N force on the box in the opposite direction. The magnitude of kinetic friction between the box and the floor is 4.5 N as the box slides to the right. What is the box’s acceleration?

Newton’s Third law

• What is Newton’s Third Law of Motion? • What does it mean?

Normal Force

• When two objects are in direct contact, the force one object exerts in response to the force exerted by the other. This force is perpendicular to the objects’ contact surface.

• Huh???????????

Processing TimeWhat is the normal force of the desk on the boxes? What is theNormal force of the 9 kg box on the top twoBoxes?

Tension

Force exerted by a string, cord, twine, rope, chain, cable, etc

Often times, tension “takes the place” of the normal force.

Friction

• A force that resists the motion of one object sliding past another

• Varies by materials in contact• Proportional to normal force • What are the different types of friction?

Static friction

• Force opposing sliding when no motion• Balances “pushing” force until object slides• Max static friction proportional to:– Coefficient of static friction– Normal force

Equation?

Sample problem 1

• Anna is pushing a 7 kg box with 7 N of force to the left, but the box does not move. What is the force of static friction?

• What is the max static frictional force if the coefficient of static friction for these materials is 0.31?

Kinetic Friction

• Friction opposing sliding in motion• Force constant as object slides • Proportional to:– Coefficient of kinetic friction– Normal force

Equation?

Sample Problem 1

• What is the force of friction of a sliding box down a ramp if the box has a mass of 5 kg and the coefficient of kinetic friction is 0.67?

Sample problem 2

• While rearranging your living room, you push your couch across the floor at a constant speed with a horizontal force of 69 N. You are using special pads on the couch legs to help it slide easier. If the couch has a mass of 59.5 kg, what is the coefficient of kinetic friction between the pads and the floor?

Independent Practice