Force Chapter 6

Force Any push or pull exerted on an object

System The object with the force applied

Environment The world surrounding the object

Contact Force A force that acts on an object by touching it

Contact Force A baseball bat striking a ball

Long-range Force A force that acts on an object w/o touching it

Long-range Force The force of gravity

Agent Whatever is causing the force

Inertia The resistance to change (in motion)

Equilibrium When the net forces acting on an object = zero

Force Vector Diagram A Diagram showing the vectors of all forces acting on an object.

Force Vector Diagram Weight on table Force of table on the ball

Draw Force Vector Diagrams of: 1)A book on a desk 2)A book being pushed across the desk 3)A book falling

Newton’s Laws of Motion

Newton’s 1 st Law An object will remain at rest or in constant straight-line motion if the net force acting on it is zero

Newton’s 1 st Law The velocity is constant and acceleration is zero when the net force on an object is zero

Newton’s 2 nd Law The acceleration of an object is directly proportioned to the net force applied to it

Newton’s 2 nd Law F net m a =a =a =a =

Newton’s 2 nd Law F net = ma

Newton’s 3 rd Law For every action, there is an equal & opposite reaction

Newton’s 3 rd Law F A on B = -F B on A

Drill: Write out Newton’s Laws of Motion

Two horizontal forces of 23.5 N & 16.5 N are acting in the same direction on a 2.0 kg object. Calculate: 1) net Force on the object 2) its acceleration

Two horizontal forces of 23.5 N & 16.5 N are acting in opposite directions on a 2.0 kg object. Calculate: 1) 1) net force on the object 2) its acceleration

Forces of 4.0 N west & 3.0 N north are acting on a 2.0 kg object. Calculate: 1) net Force on the object 2) its acceleration

Calculate the acceleration of a 1500 g object falling towards Earth when the F air friction is 11.7 N.

List Newton’s Laws of Motion

Types of Forces Friction Tension Normal Thrust SpringWeight

Friction (F f ) The contact force that acts to oppose sliding motion between surfaces Its direction is parallel & opposite the direction of sliding

Normal (F N ) The contact force exerted by a surface on an object Its direction is perpendicular & away from the surface

Spring (F sp ) A restoring force, or the push or pull a spring exerts on an object Its direction is opposite the displacement of an object at the end of a spring

Tension (F T ) The pull exerted by a string, rope, or cable when attached to a body & pulled taut Its direction away from the object & parallel to the string at the point of attachment

Thrust (F thrust ) A general term for the force that moves rockets, planes, etc Its direction is the same direction as the acceleration of the object barring any resistive forces

Weight (F g ) Force due the gravitational attraction between two objects like an object & the Earth Its direction is straight down towards the center of the Earth

Drill: Name & describe the 6 types of forces

Weight (F g ) Weight = F g = ma g = mg F g = W = mg

When an object is launched, the only forces acting upon it are the forces gravity & air friction.

No net force is required to keep an object in motion. Frictional forces oppose motion.

Inertia is not a force, but the resistance to the change in motion or momentum.

Air exerts huge & balanced frictional forces on an object. When in motion, the net F f of air is large.

Terminal Velocity The constant velocity that is reached when the force of air friction of a falling object equals its weight

Friction (F f ) Kinetic frictional force F f, kinetic Static frictional force F f, static

Draw Vector Force Diagrams of: 1) a skydiver gaining downward velocity 2) a skydiver at terminal velocity

Draw Vector Force Diagrams of: 3) a rope pulling a ball up at constant velocity 4) a rope accelerating a ball upwards

An object’s weight on Earth is 490 N. Calculate: 1) its mass 2) its weight in the moon where g moon = 1.60 m/s 2

An g object on an unknown planet has a weight of 250 N. Calculate the acceleration caused by the planet’s gravity.

Static F f The force exerted on one surface by another when there is no relative motion

Kinetic F f The force exerted on one surface by another when in relative motion

Drill: Forces of 5.0 N west, 9.0 N east, & 3.0 N north act upon a 15 kg object. Calculate its acceleration

Forces acting on an object: F N = -W F A > F f F applied F g or Weight FfFfFfFf FNFNFNFN

Net Force (F net ) Summation of all forces acting on an object Resultant vector of all the forces

Net Force (F net ) F net = ma

Net Force (F net ) F net = F A + F B + F C + etc

Static F f F f, static =  s F N

  is proportionality constant called the frictional coefficient

Kinetic F f F f, kinetic =  k F N

A 25 N force is required to pull a 50.0 N sled down the road at a constant speed. Calculate the sliding frictional coefficient between the sled & the road.

A person & a sled have a total weight of 490 N. The sliding frictional coefficient between the sled & the snow is Calculate the force required to pull the sled at constant speed.

Drill: Calculate the acceleration of the sled if the applied force pulling on the sled is 299 N. W = 490 N  = 0.10

Calculate the force required to pull a g block with an acceleration of 3.0 m/s 2.  = 0.50

Periodic Motion Repetitive or vibrational motion like that of a spring, swing or pendulum

Simple Harmonic Motion Periodic motion in which the restoring force is directly proportional to the displacement

Period (T) The time required to complete one full cycle of motion

Amplitude Maximum displacement from the zero point or equilibrium

Pendulum Motion Formula T = 2  ---- l agagagag

Calculate the period of a pendulum with a length of 49 cm:

Drill: Calculate the length of the pendulum of a grandfather clock whose period is equal 1.0 second: C HW

Fundamental Forces Gravitational Electromagnetic Strong Nuclear Weak Nuclear

Calculate the force required to pull a 150 g block at a constant velocity of 180 km/hr.  = 0.20

A 9.8 kN car went from 0 to 25 m/s in 5.0 s.   between car & road = Calculate the force applied by the engine of the car.

Drill: Calculate the force required to start a 2.0 kg block & its acceleration when moving.  s = 0.20,  k = 0.10

Calculate the force required to start a 2.0 kg block & calculate its acceleration when moving.  s = 0.20,  k = 0.10

A 6.0 kg ball is attached by a rope over a pulley to a 4.0 kg ball. 1) 1) Draw the problem. 2) 2) Calculate each ball’s acceleration

A 6.0 kg ball is attached by a longrope over a pulley to a 4.0 kg ball. 1) 1) Calculate air friction at max velocity

A 150 g baseball, was hit & came to rest in 4.0 s after going m. Calculate: v i, a, & F f on the ball.

A 50.0 kg box falls off a 0.49 km cliff. 1) 1) Calculate v i, v f, a, & t. 2) 2) Calculate F f at terminal velocity

A 10.0 kg box falls off a 0.49 km cliff & hits the ground in 20.0 s. 1) 1) Calculate v f & a. 2) 2) Calculate F f if air friction is included

Calculate the force required to pull a 250 g block at a constant velocity of 360 km/hr.  = 0.30

Drill: Calculate the force required to accelerate a 1500 g block along the floor at 3.0 m/s 2.  = 0.25

A 65 kg boy & a 35 kg girl are in a tug-of-war on ice. The girl’s acceleration is 13 cm/s 2. Calculate the boy’s acceleration.

Calculate the apparent weight of a 50.0 kg person on a scale on an elevator descending at 2.0 m/s 2.

Calculate the apparent weight of a 50.0 kg person on a scale on an elevator ascending at 2.0 m/s 2.

Drill: Calculate the period of the pendulum on Big Ben which is 4.9 m long.

Calculate the force required to accelerate a 10.0 kg block straight up at 25 cm/s 2.

Calculate the force required to accelerate a 50.0 kg block straight up over a pulley at 5.0 m/s 2.

Calculate the acceleration of a system of a 55.0 kg block tied to a 45.0 kg block hanging over a pulley.

Calculate the frictional coefficient of a kg block if a 150 N force causes it to accelerate at 50.0 cm/s 2.

Drill: Calculate the frictional coefficient of a 10.0 kg block if a 98 N force causes it to slide at 30.0 cm/s.

A 5.0 N force accelerates a g block at 45.0 cm/s 2. Calculate  K.

Calculate the acceleration of a system of a kg cart on a plane tied to a 50.0 kg block hanging over a pulley.