# SPH3U Grade 11 Physics Forces Test

SPH3U University Physics 11 Forces Unit Test Study Notes

Force

–          acceleration requires force

–          push or pull

–          measured in Newtons (N)

–          causes objects to accelerate or decelerate

–          objects accelerate when there is a net force acting on it

–          objects moving at constant speeds do not have net force

–          objects at rest do not have net force

Fundamental Forces

–          4 Fundamental forces

–          Gravitational force

–          Electromagnetic force – electrons/protons attract/repel

–          Weak nuclear force

–          Strong nuclear force – holds nucleus of atoms together

–          Other everyday forces are one or more of these acting in a specific way

Everyday Forces

–          Force of friction – always parallel to a surface, resisting motion

–          Magnetic force

–          Electrostatic force

–          Normal force – stops things from going through a surface, acts perpendicular to a surface

–          Force of buoyancy – causes less dense objects to float in denser liquids

–          Force of tension

–          Force of gravity

–          Applied force

–          Majority of the forces discussed in this unit are contact forces, meaning the force is a result of two objects making contact with one another

Free-Body Diagrams (FBD)

–          diagram of forces acting on an object

–          Steps: 1) draw a diagram of the object isolated from surroundings

2) draw all forces acting on the object with arrows

3) forces are usually all draw through the same point in the object

Newton’s First Law of Motion

–          If net force on an object is 0, the object will either stay at rest or constant velocity

Newton’s Second Law

–          Acceleration is proportional to the net force

–          Acceleration is inversely proportional to mass

–          FNET = m*a (net force is equal to mass multiplied by acceleration)

Newton’s Third Law

–          Forces act BETWEEN two objects

–          Example: everything is attracted to the Earth, but everything also attracts the Earth, because all objects have a gravitational pull, however miniscule it may be

Gravity

–          Force of gravity is proportional to mass

–          Force of gravity is proportional to 1/(distance between objects)2

–          Therefore, Force of gravity is equal to (G x mass1 x mass2)/d2

–          G is the universal gravitational constant, which is equal to 6.67 x 10-11

–          However, the equation becomes much simpler when determining the force of gravity on an object near Earth’s surface

–          The much simpler equation for objects near Earth’s surface is Fg = 9.8 x mass

Friction

–          Acts between 2 surfaces

–          Always parallel to surfaces, and resists attempts by other forces to accelerate an object moving along that surface

–          Friction resists applied force on a horizontal surface

–          Friction resists the force of gravity on a vertical surface

–          Friction is proportional to the normal force acting on an object

–          Force of friction is equal to μ x FN

–          Ff = μ x FN

–          μ is called the co-efficient of friction and depends on the two materials that are in contact

–          Ice on steel means lower μ

–          Rubber on asphalt means higher μ

Static/Kinetic Friction

–          μ also depends on whether an object is moving or not

–          μS is the co-efficient for stationary objects (static)

–          μK is the co-efficient for moving objects (kinetic)

–          Therefore, FfS = μS x FN

–          FfK = μK x FN

–          For static friction, the force of friction only needs to be as high as it needs to be

–          In other words, for stationary objects, the force of friction cannot be greater than applied forces

–          Example: the applied force is 3 N [East], and the force of friction is equal to 3 N [West], however, the force of friction was calculated to be 10 N [West]

–          The friction cannot be greater than the force it is opposing for stationary objects

Forces and Motion Equations/Extras Needed for Test

FNET is net force

m is mass

a is acceleration

Fg is force of gravity

FN is normal force

V is velocity

av means average

Δ d is change in displacement/change in distance

Δ t is change in time

μK is the co-efficient for friction for moving objects

μS is the co-efficient for friction for stationary objects

FfK is the force of friction of moving objects

FfS is the force of friction of stationary objects

G = 6.67 x 10-11

———————————————————————————————-

FNET = m x a

Fg = (G x mass1 x mass2) / d2

Fg = m x 9.8  –  only for objects near or on Earth’s surface

FfK = μK x FN

FfS = μS x FN

a = ΔV / Δt

Vav = Δd / Δt

Δd = Vi Δt + ½ a Δt2

Δd = Vav Δt

Vf2 = Vi2 + 2aav Δd

Δd = Vf Δt – ½ a Δt2