Types of Forces • Contact Forces • Forces at a distance.
Dec 29, 2015
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
Sliding Friction – force acts at contact between surfaces. Direction of friction opposes velocity or attempted velocity.
Ff = Fn is constant
Which way is friction?
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?
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 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.
• 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.
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