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 Fg Gravitational Force 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 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 is not in the direction of Fn, they are perpendicular or at an angle to eachother.

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 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?

Fn = mg