Motion and Forces

Motion Distance, d Measurement of length Meters, m Kilometer, km Feet, ft Miles, mi

Motion Speed, v Amount of distance traveled in a given amount of time (m/s) How fast something is moving Also called velocity

Motion Average velocity, v Total distance/ total time (Initial velocity + final velocity)/2

Example A car is driven 80 meters in 4 seconds, what was its average speed?

Example A car drives 20 km in an hour, then 25 km in an hour, and then 35 km in two hours. What is the average speed?

Motion Acceleration Change in velocity with respect to time

Example A car is at rest and accelerates up to 40m/s in 8s, what is the car’s acceleration?

Example A car traveling at 4m/s accelerates at 2m/s2 for 4s. What is the final speed?

Gravity Force that pulls objects together Causes an acceleration Earth’s gravitational acceleration is g=9.81m/s2 Often rounded to 10m/s2 Only works in the vertical direction Always pulls downward

Free Fall Situation in which an object is falling downward with an acceleration of g No air resistance

Example A stone is dropped from a bridge, and it takes 3s to hit the ground. How high is the bridge?

Upward Motion When an object is thrown upward into the air, the time it takes to reach its highest point is equal to the time it takes to drop back to its starting point. The speed at any point upward will be the same but opposite in direction with the same height on the way down

Parabola

Example A ball player jumps up to get a rebound. If they are in the air for 0.6s, how high did they jump?

Forces A push or pull that could cause a change in motion Measured in Newtons (N) kg∙m/s2 Many Types Tension Weight Friction Normal

Concurrent Forces Two concurrent forces acting on an object….. Three forces acting concurrently…… Means the forces are acting on the same object at the same time

Net Force Net Force = 6N to the Left F1 = 10N F2 = 4N Total force when all forces acting on an object are accounted for F1 = 10N F2 = 4N Net Force = 6N to the Left

Newton’s Laws of Motion 1st Law – Equilibrium (No acceleration) 2nd Law – Non Equilibrium (Acceleration) 3rd Law – Equal and Opposite

Newton’s 1st Law (N1L) An object at rest will stay at rest unless acted upon by an external force An object in motion will remain in motion unless acted upon by an external force.

Law of Equilibrium (N1L) The sum of all the forces acting on an object equals zero. All forces cancel out Object may be at rest or moving at constant velocity

Example Box at rest on a table

Example Force of table pushing up equals weight of box pushing down Force of table pushing up – weight of box = 0

Force Diagram Also called Free Body Diagram Shows all forces acting on an object Arrows originate on object and point in the direction that the force is acting Weight Table Pushing Up

Example ΣF = 0 T - w =0 T = w T = 50N A 50N chandelier hangs from ceiling by a single cord. Determine the tension in the cord. ΣF = 0 Tension, T T - w =0 T = w Weight, w T = 50N

Another Example ΣF = 0 T + T – w =0 2T = w 2T = 50N T = 25N A 50N object hangs from ceiling by two cords. Determine the tension in each cord. ΣF = 0 T T T + T – w =0 2T = w 2T = 50N w = 50N T = 25N

Newton’s 2nd Law (N2L) The acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to the mass of the object

N2L

N2L Example A 5kg box is being pulled by a 10N force. What is the acceleration of the box? 5kg F = 10N

N2L Example A 2kg box is being pulled by 2 forces as shown. What is the acceleration of the box? F1 = 10N F2 = 4N 2kg

Weight, w Force due to gravity, Fg

Friction, Ff Force that opposes motion. Always in the direction opposite to motion

Example A 75kg skydiver is falling at his terminal velocity. What is the force of the air resistance? Ff w = mg

Example Ff w = mg

Another Example A 2000kg car is accelerating at a rate of 3m/s2. If the engine is exerting a force of 8000N, what is the force of friction? Ff F=8000N a

Another Example Ff F=8000N a