Chapter 5Chapter 55.1 Forces & 5.3 5.1 Forces & 5.3

Forces & EquilibriumForces & Equilibrium

pp. 108-115 & pp. 124-129pp. 108-115 & pp. 124-129

FORCESFORCES

►Where do you see a force happening in Where do you see a force happening in the room right now?the room right now?

►Which object is exerting the force? Which object is exerting the force? Which object us receiving the force?Which object us receiving the force?

IMPORTANT DEFINITIONSIMPORTANT DEFINITIONS

►Force – Force – a push or a pulla push or a pull

All forces have both a size (a number) and a direction

5.1 The cause of forces5.1 The cause of forces►A A forceforce is a push or pull, or an is a push or pull, or an action action

that has the ability to change motionthat has the ability to change motion..► Forces can Forces can increase or decrease the speed increase or decrease the speed

of a moving objectof a moving object. . ► Forces can also Forces can also change the direction in change the direction in

which an object is movingwhich an object is moving..

5.1 How are forces created? 5.1 How are forces created? ►Forces are created in many ways. Forces are created in many ways. ►For example, your muscles create force For example, your muscles create force

when you swing a baseball bat.when you swing a baseball bat.

Four Four Elemental Elemental

ForcesForces► All forces in the All forces in the

universe come universe come from only four from only four basic forces.basic forces.

► Electromagnetic Electromagnetic forces are forces are important to important to technology.technology.

► Gravity is a Gravity is a universal force.universal force.

5.1 Units of force5.1 Units of force

►The The poundpound is a unit of force commonly is a unit of force commonly used in the United States.used in the United States.

►For smaller amounts, pounds are For smaller amounts, pounds are divided into ounces (oz.). divided into ounces (oz.).

►There are 16 ounces in 1 pound.There are 16 ounces in 1 pound.

5.1 Newtons5.1 Newtons

►Although we use pounds all the time in Although we use pounds all the time in our everyday life, scientists prefer to our everyday life, scientists prefer to measure forces in measure forces in newtonsnewtons..

►The The newtonnewton (N) is a metric unit of force. (N) is a metric unit of force.

5.1 Unit conversions5.1 Unit conversions

►The newton (N) is a smaller unit of force The newton (N) is a smaller unit of force than the pound (lb).than the pound (lb).

► If one pound of force equals If one pound of force equals 4.4484.448 newtons, then a 100 lb person weighs newtons, then a 100 lb person weighs 444.8 newtons.444.8 newtons.

CONTACT FORCESCONTACT FORCES

►A contact force acts as a result of A contact force acts as a result of direct contact. Contact forces include:direct contact. Contact forces include:

►TensionTension►FrictionFriction►Normal Force – the perpendicular force Normal Force – the perpendicular force

that a surface exerts on an object that that a surface exerts on an object that is pressing on itis pressing on it

►Air ResistanceAir Resistance

5.1 Contact forces from 5.1 Contact forces from ropes and springsropes and springs

► Ropes and springs are often used to Ropes and springs are often used to make and apply forces.make and apply forces.

► Ropes are used to transfer forces or Ropes are used to transfer forces or change their direction.change their direction.

► The pulling force carried by a rope is The pulling force carried by a rope is called called tensiontension..

► Tension always acts along the Tension always acts along the direction of the rope.direction of the rope.

5.1 Gravity5.1 Gravity► The force of gravity on an object The force of gravity on an object

is called is called weightweight. . ►At Earth’s surface, gravity exerts a At Earth’s surface, gravity exerts a

force of 9.8 N on every kilogram of force of 9.8 N on every kilogram of mass.mass.

5.1 Weight vs. mass5.1 Weight vs. mass►Weight and mass are not the sameWeight and mass are not the same..►Mass is a fundamental property of Mass is a fundamental property of

matter measured in kilograms (kg). matter measured in kilograms (kg). ►Weight is a Weight is a force force measured in measured in

newtons newtons (N(N).).►Weight depends on mass and Weight depends on mass and

gravity.gravity.

Weight depends on mass and gravity

A 10-kilogram rock has the same mass no matter where it is in the universe. On Earth, the10 kg. rock weighs 98 N.. On the moon, the same rock only weighs 16 N.

5.1 Calculating weight5.1 Calculating weight

► Calculate the weight of a 60-kilogram Calculate the weight of a 60-kilogram person (in newtons) on Earth and on person (in newtons) on Earth and on Mars.Mars.

1.1. Looking for: Looking for: ……weight of person in newtons on both planetsweight of person in newtons on both planets Given: Given: ……mass = 60 kg; mass = 60 kg; g g = 3.7 N/kg on Mars; = 3.7 N/kg on Mars; ……implied implied g g = 9.8 N/kg on Earth= 9.8 N/kg on Earth

2.2. Relationships: Relationships: W = m x W = m x g g

3.3. Solution: Solution: 60 kg x 9.8 N/kg = 588 N60 kg x 9.8 N/kg = 588 N 60 kg x 3.7 N/kg = 222 N60 kg x 3.7 N/kg = 222 N

Solving ProblemsSolving Problems

5.3 Forces and Equilibrium5.3 Forces and Equilibrium► The sum of all the forces on an object is The sum of all the forces on an object is

called the called the net forcenet force..► The word The word net net means total but also means total but also

means the direction of the forces has means the direction of the forces has been taken into account.been taken into account.

In what direction will this plane go?

5.3 Equilibrium5.3 Equilibrium

When several forces When several forces act on the same act on the same object:object:

1.1. The net force is The net force is zero, orzero, or

2.2. The net force is The net force is NOT zero.NOT zero.

5.3 Normal forces5.3 Normal forces► When the forces When the forces

are are balancedbalanced,, the the net force is zero.net force is zero.

► When the net force When the net force on an object is on an object is zero, we say the zero, we say the object is in object is in equilibriumequilibrium..

BALANCED FORCESBALANCED FORCES

►When the net force on an object is When the net force on an object is zero, the forces are zero, the forces are balancedbalanced..

►Balanced forces Balanced forces do notdo not cause a cause a nonmoving object to begin moving.nonmoving object to begin moving.

►Balanced forces Balanced forces will notwill not cause a cause a change in motion of a moving object.change in motion of a moving object.

UNBALANCED FORCESUNBALANCED FORCES

►When the net force in an object is When the net force in an object is not not zerozero, the forces on the object are , the forces on the object are unbalancedunbalanced..

UNBALANCED FORCESUNBALANCED FORCES

►Unbalanced forces produce Unbalanced forces produce a change a change in motion (acceleration)in motion (acceleration)

►Unbalanced forces are needed to Unbalanced forces are needed to cause a nonmoving object to begin to cause a nonmoving object to begin to movemove

►Unbalanced forces are also needed to Unbalanced forces are also needed to change the motion of moving objects.change the motion of moving objects.

5.3 Equilibrium and normal 5.3 Equilibrium and normal forcesforces

► A A normal forcenormal force is is created whenever an created whenever an object is in contact object is in contact with a surface. with a surface.

► The normal force has The normal force has equal strengthequal strength to the to the force pressing the force pressing the object into the surface, object into the surface, which is often the which is often the object’s weight.object’s weight.

The normal force is sometimes called the support force.

5.3 The free body diagram5.3 The free body diagram► How do you keep How do you keep

track of many track of many forces with different forces with different directions? directions?

► Draw a Draw a free-body free-body diagramdiagram that that contains the contains the objects, like a book objects, like a book on a table.on a table.

5.3 Solving equilibrium 5.3 Solving equilibrium problemsproblems

► For an object to be in equilibrium, For an object to be in equilibrium, all the forces acting on the objectall the forces acting on the object must add up to must add up to zerozero..

Is this object in equilibrium?

Solving ProblemsSolving Problems

Two chains are used to support a Two chains are used to support a small boat weighing 1,500 newtons. small boat weighing 1,500 newtons.

One chain has a tension of 600 One chain has a tension of 600 newtons. newtons.

What is the force exerted by the What is the force exerted by the other chain?other chain?

1.1. Looking for: Looking for: ……tension on chain 2 tension on chain 2

2.2. GivenGiven ……weightweightboatboat = 1,500N; tension = 1,500N; tension11 = 600 = 600

NN Implied: weight and tension are forcesImplied: weight and tension are forces

3.3. Relationships: Relationships: Net force on boat = zeroNet force on boat = zero

Solving ProblemsSolving Problems

4.4. Solution: Solution: Draw free body Draw free body

diagramdiagram

Solving ProblemsSolving Problems

Upward force of chains = weight of boat 600 N + tension2 = 1,500 N tension2 = 900 N