Transcript Physics Chp4

Physics Chp4
Force - a push or pull
Contact Force – friction, tension, shoving
Noncontact Force – gravity, magnetism, electric
Measured in newtons N = kgm/s2
Free body diagrams
Show the forces acting on an object for easy
viewing and analyzing.
Ex. Lifting a text book
Lift Force
Flift
BOOK
Gravitational Force
If there is no acceleration how do the forces compare?
Fg
Inertia – an objects tendency to resist a change
in motion
Mass – the quantitative measure of mass
Newton’s First Law
An object continues it’s motion unless acted
upon by a net force
Net force is the vector sum of all forces acting
on the object
• If I lift my 1.5 kg text book causing it to
accelerate upwards with a = 2.3 m/s2, how
much force do I have to lift with?
• Think about the earlier free-body diagram.
• Fnet = ma
• Fnet = Flift – Fg
• ma = Flift – m(9.81m/s2)
• Flift = ma + m(9.81m/s2)
• Flift = (1.5kg)(2.3m/s2) + (1.5kg)(9.81m/s2)
• Flift =
• Newton’s Second Law
• F = ma
• Newton’s Third Law
• Every action has an equal and opposite
reaction
• If your car engine exerts a 4500 N force on it’s
1500 kg mass, how much acceleration will
your car experience at full throttle?
• F = ma
• 4500 N = (1500kg) a
• a= 3 m/s2
• Normal Force
• The force exerted by the object in contact
with another object perpendicular to the
surface usually due to gravity.
• Ex. Your book on the table, the gravitational
force is downward on the book but the normal
force is upward from the table against the
book.
• What is the sum of those forces?
• Frictional Forces
• Static vs Kinetic
Fs  s Fn
• Fs is not in the direction of Fn, they are
perpendicular or at an angle to eachother.
Kinectic Friction
Always less than static friction
Fk  k Fn
μs > μk
why?
Both frictional forces oppose the motion of
the object across the other object
• Ex. If my shoes have a static friction
coefficient of 0.6 with the floor and I have a
mass of 92 kg, what is the frictional force
resisting my movement?
Fs  s Fn
Fn = mg
Fs  0.6(92kg )(9.80m / s )
2
Fs  ?