Unit 3: Forces and Newton's Laws

Equations: F3=Fg=mg EFy=F1Sinø-F3=0<<<(solve for F1) EFx=F2=F1Cosø=0<<<(solve for F2)

A 25 kg block is being supported in EQUILIBRIUM by three ropes. The system is in EQUILIBRIUM. Rope 1 is at an angle of 62˚. Find the tension in each rope. Assume ideal, massless rope. *diagram is given*

Diagram: balances 1 arrow up, 1 arrow down.

A 40.0 kg girl is suspended MOTIONLESS from a bar that hangs from the ceiling. Which force diagram is consistent with this motion?

0N because it is already in equilibrium.

A car is traveling at CONSTANT VELOCITY in a straight line. What is the amount of force required to bring the object to equilibrium?

Diagram: all 4 arrows balanced.

A force is applied to a 1.0 kg book to move it to the left across a horizontal desk at CONSTANT VELOCITY. Which force diagram is consistent with this motion?

Diagram: unbalanced 3 arrows, 1 up=Fa=1320N, 1 down=Fg=1250N, 1 right=Fa=375N, 1 angle arrow=N of E Equations: EFy=Fn-Fg EFx=Fa Fnet2= EFx2 + EFy2 Ø=tan-1(EFy/EFx)

A hot air balloon is floating in the air, pulled downward by a gravitational force of 1250 N. It is pushed upwards by its engine with a force of 1320 N. The wind pushes it to the right with a force of 375 N. a) draw a force diagram showing the individual forces exerted on the hot air balloon. b) find the magnitude and direction of the resultant force on the hot air balloon.

Diagram: unbalanced 3 arrows, 1 up=Fa=1625N, 1 down=Fg=1445N, 1 right=Fwind=471N, 1 angle=75º Equations: EFx=Fwind-FaCosø EFy=FaSinø-Fg Fnet2= EFx2 + EFy2 Ø=tan-1(EFy/EFx)

A hot air balloon is floating in the air. It is pulled by gravity with a force of 1445 N. Its "engine" pushes with a force if 1625 N at angle of 75.0˚ up from horizontal-left. The wind pushes it to the right with a force o 471 N. a) draw a diagram showing the individual forces exerted on the hot air balloon. b) find the magnitude and direction of the resultant force on the hot air balloon.

Diagram: all 4 arrows balanced, 1 up=Fn, 1 down=Fg=mg=255.1N, 1 left=Ff, 1 right=Fa=175N(at angle), 1 angle=33.5º(under Fa) Equations: EFy=Fn+FaSinø-Fg=0 (solve for Fn) EFx=FaCosø-Ff=0 (solve for Ff) Ff=µFn (solve for µ)

A person is pulling a 26.0 kg box ties to a string. The person pulls the box with a force of 175 N along an angle of 33.5˚ up from right. The box is sliding to the right on a flat, horizontal surface with a constant velocity. a) draw a free body diagram of the box. b) calculate the coefficient of kinetic friction.

Diagram: unbalanced 4 arrows, 1 up=Fn, 1 down=Fg=mg=313.9N, 1 left=Ff, 1 right=Fa=78(at angle), 1 angle=33º(under Fa) (i)(ii) Equations: EFy=Fn+FaSinø-Fg=o<<<(solve for Fn) Ff=µFn<<<(solve for Ff) *µ=0.126 EFx=FaCosø-Ff F=ma<<<(solve for a)

A person is pulling a 32.0 kg wagon with a force of 78.0 N directed along an angle of 33.0˚ up from right. The wagon is moving to the right along a flat horizontal surface. The coefficient of kinetic friction on the wagon is 0.126. a) draw a free body diagram of the wagon. b) calculate the magnitude and direction of (i) the net force on the wagon and (ii) the net force acceleration of the wagon.

Diagram/Equations: all 4 arrows balanced, 1 up=Fn=117.7N, 1 down=Fg=mg=117.7N, 1 right=Fa=56N, 1 left=Ff=µFn=32.1N (i)(ii) Equations: EFy=0 EFx=Fa-Ff Fnet2=EFx2 + EFy2 Ø=tan-1(EFy/EFx) Fnet=m(anet)<<<(solve for anet)

A person is pushing a 12.0 kg chair to the right across a horizontal surface. The person applies a force of 56.0 N to the right. The coefficient of kinetic friction between the chair and the ground is 0.273. a) draw a free body diagram of the chair and determine the magnitude of all the forces acting on the chair. b) calculate the magnitude and diagram of (i) the net force on the chair and (ii) the net acceleration of the chair.

Diagram: all 4 arrows balanced, 1 up=Fn=19.62N, 1 down=Fg=mg=19.62N, 1 right=Fa=35N, 1 left=Ff=µFn Equations: Fn=Fg Ff=µFn *used to find the arrows* EFy=0N EFx=0=Fa-Ff<<<(solve for Ff) Net force: 0N

A person is pushing a 2.0 kg block to the right across a horizontal surface with a steady force of 35 N. The block maintains CONSTANT VELOCITY as it slides across the surface. a) draw a free body diagram of the forces acting on the block. b) calculate the magnitude and direction of the normal force and kinetic force of friction. c) what is the net force acting on the block?

Force

A push or a pull on an object.

Diagram: unbalanced 1 arrow up, 1 arrow down, 1 arrow to the left.

A rightward moving sledder slows down as it coast to a stop. Which force diagram is consistent with this motion?

Diagram: unbalanced 1 arrow down, 1 smaller arrow up

A skydiver is falling and speeding up as she falls. Which diagram is consisten with this motion?

Translational equilibrium

A state of no change in an objects motion

Arrow 1 because Fn<Fg

A wagon is moving to the right at a CONSTANT VELOCITY. Friction acts on the wagon. A force (Fa) is applied to the wagon at 30˚ up from horizontal-right. (Arrow 3 is pointing down and is the longest. Arrow 2 is at a slight angle and medium length. Arrow 1 is pointing left and is the shortest.) Which arrow is the correct normal force?

A- magnitude B- direction

Action and reaction forces are equal in __(A)__ and opposite in __(B)__.

Reaction: road pushes against tires.

Action: the tires of your car push against the road.

Reaction: ball exerts force on bat.

Action: when a batter swings, the bat exerts a force on the ball.

Reaction: water pushes you.

Action: when swimming, you push the water backward.

Reaction: floor pushes against you.

Action: when you walk, you push against the floor.

Diagram: all 4 arrows balanced, 1 up=Fn=78.5N, 1 down=Fg=mg=78.5N, 1 right=Fa, 1 left=Ff Equations: Ff=µFn (solve for Ff) Fa=Ff Biggest horizontal force: 52.8N (Fa and Ff are same answer)

An 8.0 kg block is sitting on a horizontal surface. The coefficient of static friction between the block and the surface is 0.672. a) draw a free body diagram of the block. b) what is the biggest horizontal force with which someone can push the block without moving it?

Newton's 1st Law

An object at rest stays at rest, and objects in motion stay in motion unless acted on by an outside force. Law of Inertia.

At rest, or moving at a constant velocity

An object is in equilibrium if and only if it is under what two conditions?

Balanced, 0N

Apply the conditions of equilibrium to various situations and problems.

Kinetic = in motion Static = at rest

Define the two kinds of friction force.

It accelerates in same direction as net force.

Describe the motion of an object when a NON-ZERO (unbalanced) net force acts upon it.

In equilibrium, a=0m/s2, Fnet=0N

Describe the state of equilibrium in terms of acceleration and net force.

a=3.625m/s2 *half of 7.25m/s2*

During a lab experiment, the same net force of 5.0 N is applied to two objects of different masses. The first object has an acceleration of 7.25 m/s2. The second object has double the mass of the first object. What is the acceleration of the second object?

Newton's 2nd Law

F=ma

Newton's 3rd Law

For every action there is an equal and opposite reaction.

Pair

Force is always part of a ____ that involves another force.

Gravity= downward Normal, Applied= upward Applied, Friction, Tension= left or right

How are the forces drawn on a free body diagram?

Inversely

How are the mass and acceleration of an object related?

Acceleration and net force are directly proportional if mass is CONSTANT. Acceleration and mass are indirectly proportional if net force is CONSTANT. EX: if the mass remains CONSTANT and the net force acting on an object is doubled, the acceleration would be doubled. if the same force is applied to an object twice the mass, the acceleration would be divided in half.

How are the variables of force, acceleration, and mass related?

Applied Force (Fa)= applied to an object by a person or another object Normal Force (Fn)= upward force, acts on "sitting" object Tension Force (Ft)= transmitted through a string, rope, cable, or wire when it is pulled tight by forces acting from opposite ends Frictional Force (Ff)= resistive force between two touching surfaces Net Force (Fnet)= sum of the forces, resultant force Gravity (Fg)= attraction between 2 objects, downward, determines weight

How do the different types of forces act on objects?

µFn...... µ=M

How do you solve for friction force?

Fnet=ma

How do you solve for net acceleration?

Pythagorean Theorem (Fnet=EFx2 + EFy2)

How do you solve for net force?

Find the gravitational weight, or use (EFy=) equation

How do you solve for normal force?

mass x acceleration

How do you solve for weight?

Law of Inertia

How does inertia relate to Newton's 1st Law?

The more inertia an object has, the more mass

How does inertia relate to mass?

Hammer and Nail Earth and Moon Fish and Water

Identify action-reaction pairs.

False, because it could be accelerating to the left due to a net force to the left.

If a rightward moving object experiences an unbalanced force, then the object will have an acceleration that is directed to the right. True or False? Explain.

Zero force (0N) because force is already balanced.

If you were a spaceship and fired a cannonball into space, how much force would have to be exerted on the cannonball to keep it moving once it has left the spaceship?

Vector

Is force a vector or scalar?

1, 2, or both

Is force one or two dimensional?

Coefficient force

M (µ) < unwritten between 0 and 1. Describes relationship between contact surfaces.

"Law of Inertia" An object at rest stays at rest and an object in motion stays in motion unless acted upon by an unbalanced force.

Newton's 1st Law?

"Acceleration" An object's acceleration is equal to the net force on the object divided by the mass of the objects.

Newton's 2nd Law?

If object 1 exerts a force on object 2 then object 2 exerts a force on object 1 and the forces are equal in magnitude but opposite in direction. *for every actions there is a equal but opposite reaction.*

Newton's 3rd Law?

Translational equilibrium

State of no change in an objects motion EX: object in motion, stays in motion

Net force

Sum of all forces

Resultant force

Sum of all forces acting on an object

25N Newton's First Law

Suppose the force of friction on a sliding object is 25 N. What is the force needed to maintain constant velocity? Which law describes the motion of this object?

Inertia

Tendency for an object to remain in constant motion

A- directly B- same C- increase

The acceleration of an object is __(A)__ proportional to the net force acting on it. The direction of the acceleration is in the __(B)__ (same/opposite) direction as the net force. When the mass is held constant, as the net force acting on the object increases, the acceleration of the object __(C)__ (increases/decreases).

Coefficient of friction (M)

The ratio between frictional force and the normal force of two surfaces, describes relationship between contact surfaces.

One force is 3X's as large as the other.

Two shopping carts of equal mass are pushed by two different people. One cart accelerates three times as fast as the other cart. Describe the forces acting on each cart.

1st- hockey player hits a puck to the right across the ice. The system is frictionless. The puck will glide forever in a straight line unless an outside force acts on it. 2nd- if you use the same force to push a truck and push a car, the car will have more acceleration than the truck, because the car has less mass 3rd- when a hammer hits a nail it applies a force to the nail. If the hammer pushes the nail to the right, then the nail pushes the hammer to the left.

What are examples on Newton's three laws of motion?

Constant velocity or at rest

What are the conditions of equilibrium?

Contact -actual physical contact (applied force) Field- acts between objects with no physical contact or an "unseen" force

What are the differences between contact and field forces?

Mass -amount of matter an object consists of *never changes due to gravity* Weight -how hard gravity pulls on a mass (Fg)

What are the differences between mass and weight?

Mass is the measure of the amount of matter an object contains. Weight is the amount or magnitude of the gravitational force exerted on an object.

What are the differences between mass and weight?

Static- at rest Dynamic- in motion

What are two types of friction?

Horizontal- balanced (4 arrows) Two dimensional- balanced (2 arrows/up, down) Incline- balanced (2 arrows/down, angle to horizontal)

What does a free body diagram look like when an object's at rest horizontally, in two dimensions, and on inclines?

Horizontal- unbalanced (3 or 4 arrows) Two dimensional- unbalanced (1 or 2 arrows) Incline- unbalanced (2 or 3 arrows)

What does a free body diagram look like when an object's moving horizontally, in two dimensions, and on inclines?

Inertia influences an object's motion and is dependent on an object's mass.

What does inertia influence and depend on?

Push or pull on an object

What is a force, conceptually?

"Support" force from sitting on a surface. Acts perpendicular to contact surface.

What is the function of a normal force?

Kg

What is the variable and unit for mass?

Fnet, newton (N)

What is the variable and unit for the resultant force?

N

What is the variable and unit for weight?

a1=F/m1 a2=F/m2

When the same unbalanced forces are applied to objects of different masses, m1 and m2, how does the acceleration of each object vary?