Announcement

• Exam 1 Next Thursday– Rooms CR 302 and 306

Static Forces Scenario

A block on a frictionless ramp is held by a rope. As the incline angle of the ramp increases,

How does the weight of the block depend on ?

Static Forces Scenario

A block on a frictionless ramp is held by a rope. As the incline angle of the ramp increases,

How does the component of the weight perpendicular to the ramp depend on ?

Static Forces Scenario

A block on a frictionless ramp is held by a rope. As the incline angle of the ramp increases,

How does the component of the weight parallel to the ramp depend on ?

Static Forces Scenario

A block on a frictionless ramp is held by a rope. As the incline angle of the ramp increases,

How does the normal force from the ramp depend on ?

Static Forces Scenario

A block on a frictionless ramp is held by a rope. As the incline angle of the ramp increases,

How does the tension in the rope depend on ?

Forces

That aren’t zero

§ 4.3

What’s the point?

• How do forces affect motion?

Experience Tells Us

The greater the net force on an object, the more it accelerates.

The greater the mass of an object, the harder it is to accelerate.

Newton’s Second Law

Fm

a =

equivalently,

F = ma

a = acceleration F = net force m = mass

Quantify Force

• Unit of force: 1 newton (N) = force needed to accelerate 1 kg at 1 m/s2

F = ma = (1 kg) (1 m/s2) =kg m

s2

• How much is a newton?

Board Work

A crate with mass 32.5 kg initially at rest on a warehouse floor is acted on by a net horizontal force of 140 N.

a. What acceleration is produced?

b. How far does the crate travel in 10.0 s?

c. What is its speed at the end of 10.0 s?

Whiteboard Problem

A bowl of petunias of mass m accelerates in free fall at rate g. What is the magnitude of the net force acting on the bowl?

Weight and Weightlessness

Mass and gravity

§ 4.4

What’s the point?

• What is weight?

Gravitational Force

• Magnitude of gravitational force is proportional to mass: F = mg.

• g = gravitational field; units N/kg.

• Direction of gravitational force is toward the center of the earth (down).

• At earth’s surface, g 9.8 N/kg.

Poll Question

Gravity is constantly pulling us downward, but we are not accelerating downward. This means that

A. Newton’s second law does not apply here.

B. Gravity does not apply a physical force.

C. Some other force exactly opposes the force of gravity.

D. Gravity stops at the earth’s surface.

A Physics Haiku

Elevator breaks

Susie falls and feels no weight

Gravity still there– Heidi Forbes

Example Problem

A box rests on a frictionless table, and is pushed by two people. One pushes with a force of F1 = 10 N at an angle of 30° below horizontal. If she stops pushing, what will be the acceleration of the box?

5 kg

F1 = 10 N

F2 = ?

2 = 60°1 = 30°