KIN 311 I A - Test 2

Force is a vector quantity:

it has both magnitude and direction.

Friction force:

acts parallel to a contact surface and opposes relative sliding.

Friction: F = μR

Weight vs. mass:

mass is the amount of matter; weight is a force caused by gravity.

Weight: W = mg   (g ≈ 9.8 m/s²)

Free-body diagram (FBD):

identifies and shows the external forces acting on the object/system being analyzed.

External vs. internal forces:

external forces act on the system from outside; internal forces act within the system.

Colinear forces:

forces that act along the same line.

Concurrent forces:

forces whose lines of action meet at a common point.

Resultant/net force:

the vector sum of the forces acting on an object.

Be able to add forces in horizontal and vertical directions and determine a resultant force.

Resultant magnitude (perpendicular components): R = √(Fx² + Fy²)

Direction: use appropriate trigonometry (e.g., θ = tan⁻¹(Fy/Fx))

Be able to resolve an angled force into horizontal and vertical components.

Horizontal component (Fx) = F * cos (θ) (adjacent side)

Vertical component (Fx) = F * sin (θ) (opposite side)

Know that zero net force means no acceleration

(the object is either at rest or moving at constant velocity).

Distance vs. displacement:

distance is total path length; displacement is the change in position with direction.

Rectilinear motion:

motion along a straight line.

Curvilinear motion:

motion along a curved path.

Speed:

how fast an object moves; a scalar quantity.

Velocity:

speed with direction; a vector quantity.

Acceleration:

the rate of change of velocity.

Speed = distance ÷ time

Average velocity = displacement ÷ time

Average acceleration = (Vf − Vi) ÷ Δt

Acceleration can occur

when an object speeds up, slows down, or changes direction.

Be able to interpret the sign of acceleration relative to velocity:

same sign → speeding up; opposite signs → slowing down.

Be able to determine what an athlete is doing from a velocity-time graph.

Above the x-axis: the athlete is running forward

Below the x-axis: the athlete is running backward

When the line touches the x-axis (velocity = 0): the athlete is at rest

Upward sloping line: the athlete is speeding up (positive acceleration)

Downward sloping line: the athlete is slowing down (deceleration or negative acceleration)

Steeper slope: indicates a higher rate of acceleration or deceleration

Be able to calculate and compare average speeds from distance and time.

Average Speed = Total distance / time

Distance Units: Miles (mi), kilometers (km), meters (m), feet (ft)

Time Units: Hours (hr), minutes (min), or seconds (s)

Newton’s First Law (law of inertia):

without a net external force, an object at rest stays at rest and an object in motion continues at constant velocity in a straight line.

Newton’s Second Law (law of acceleration):

net external force causes acceleration.

ΣF = ma

Be able to calculate acceleration when net force and mass are known, and calculate net force when mass and acceleration are known.

For calculating acceleration, Acceleration = F/m, and F=mxa for calculating net force

Newton’s Third Law (action-reaction):

forces between two interacting objects are equal in magnitude and opposite in direction.

Conservation of momentum is associated with Newton’s First Law:

if ΣF = 0, total system momentum remains constant.

Momentum

describes an object's current state of motion and resistance to change.

Momentum: L = mv

Be able to identify when momentum is conserved and use initial momentum = final momentum for an isolated system.

Total momentum is conserved in an isolated system where the net external force acting on the objects is zero

Friction, gravity from outside, or air resistance must be zero or small enough to ignore

The forces acting on the object come only from each other, such as a collision or a push

Newton’s Third Law: Action and reaction forces between the objects are equal and opposite, so internal impulses cancel out.

Using Initial = Final Momentum

Momentum (p) equals mass (m) times velocity (v) Momentum is a vector, direction matters (use positive and negative signs)

Before an event: calculate the mass times initial velocity for each object and add them together

After an event: set that sum equal to the combined mass times final velocity (or individual final velocities if they bounce apart)

Elastic collision:

objects collide and separate; system momentum is conserved.

Inelastic collision:

objects collide and stay together; system momentum is still conserved.

Coefficient of restitution:

ratio of the velocity of separation to the velocity of approach.

Impulse:

the product of average force and the duration of force application.

Impulse: FΔt = ΔL = m(Vf − Vi)

Understand the force-time tradeoff:

the same change in momentum can result from a large force over a short time or a smaller force over a longer time.

Application:

increasing the time over which momentum is reduced during landing or catching can reduce the force experienced.