Types of Forces

• Contact Forces

• Forces at a distance.

Four Main Forces in Universe

• Gravity Fg.

• Electromagnetic EM

• Strong Nuclear Strong or Nuclear

• Weak Nuclear Weak

•All Forces fall into one of the above categories.

• Normal Force

(reaction force) –

• perpendicular to the surfaces in contact or touching.

Common Forces

What if you’re leaning on a wall. Where is the normal force?

Where is it now?

Sliding Friction – force acts at contact between surfaces. Direction of friction opposes velocity or attempted velocity.

Ff = Fn is constant

Which way is friction?

Which way is the book moving?

Right

Left

The box is not moving.

• Tension –

• transmitted through string, rope, or wire by forces at each end.

Air Resistance – air friction.

Big reason for bad fuel economy in cars.

Spring or Elastic Force – Force applied by stretched

or compressed spring

described by Hooke’s Law.

Magnetic Force – due to excess electron spin

Electrical – due to excess charge buildup.

• Weight Fg = mg • gravity interaction with mass.

Weight depends on gravity,Mass does not.

What is mass?

Amount of matter in object.

Question: What will be the mass of a 10-kg object on the moon?

• 1. Zero

• 2. Less than 10 – kg

• 3. 10 – kg.

• 4. Impossible to tell without more information.

Calculation of Weight Fg and mass.

• g = Fg/m

• Fg = mg.

• Fg = gravity on object = weight N.

• m = mass, kg.

• g = accl gravity m/s2.

• On Earth g = 9.81 m/s2.

1. A student weighs 850-N on Earth. What is his mass?

• Fg = mg.

• Fg = m

• g

• 850 N

• 9.81 m/s2.

• = 86.6 kg

• Find mass on Earth.

• m = Fg/g

• m = 650 N/ 9.81 m/s2.• m = 66 kg.

• g = Fg/m

• g = 12.6 N/ 66 kg.• g = 0.2 m/s2.

2. A 650-N Earth student weighs 12.6 N on planet Tessaz. What is the acceleration of gravity on Tessaz?

3. Little Jimmy weighs 490-N on Earth. If the acceleration of gravity on MonsterY is 50 m/s2, what is the force of gravity on Little Jimmy on MonsterY?

• m = Fg/g

• 490N/9.81 m/s2.

• m = 50 kg.

• Fg = mg

• (50kg)(50m/s2) = 2500N

3. Make a sketch graph of weight (Y) vs. mass (X). Write the equation for the graph. What is the slope of the line?

Hwk Sheet 1 - 15.

Normal Force Fn.

• Contact between surfaces.

• Always perpendicular to surface.

• If object rests on horizontalhorizontal surface, then:

• Fn = -W = mg.

Fn

How does Normal Force Arise?

How does a wall know to push back harder when I push with increasing force?

The object will be at rest only if the surface can exert an equal & opposite force to sustain it, otherwise the object crashes through the surface.

Friction Between Surfaces

Friction depends on the nature of the Friction depends on the nature of the materials in contact and the materials in contact and the smoothness of their surfaces.smoothness of their surfaces.

. Friction is independent of surface area.. Friction is independent of surface area.

. F. Fff is directly proportional the normal force. is directly proportional the normal force.

The ratio of The ratio of FFff, to , to FFnn, is, is

the the coefficient of frictioncoefficient of friction, , ..

=FFff

FFnn

hashasno units.no units.

Ff = Fn. Where is a constant.

File Cabinet Demo

• depends on two surfaces in contact

• the type of sliding motion – starting or moving.

• Does not depend on any other factor.

• It is a constant.

Types of Friction

• Static Friction use st – Present for objects at rest. Need to overcome to start objects moving.

• Kinetic Friction or sliding use k – Present for objects in motion. Need to overcome to keep objects in motion. Always less than static friction.

Reference Table

1. A 10-kg rubber box is at rest on a dry horizontal concrete surface.

Calculate:• a. the weight of the box.

• b. the normal force on the box.• c. the force needed to start the box moving.

• d. the force to keep the box moving a constant velocity.

98.1 N

98.1 N

Ff = stFn. = (0 .9 )98.1 N = 88.9 N

Ff = stFn. = (0 .68 )98.1 N = 66.7 N