Physics Chp4

Force - a push or pullContact 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

BOOK

Lift Force

Gravitational Force

Flift

Fg

If there is no acceleration how do the forces compare?

Inertia – an objects tendency to resist a change in motion

Mass – the quantitative measure of mass

Newton’s First LawAn 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 is not in the direction of Fn, they are perpendicular or at an angle to eachother.

s s nF F

k k nF F

Kinectic Friction

Always less than static friction

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?

s s nF F

20.6(92 )(9.80 / )

?

s

s

F kg m s

F

Fn = mg