explaining motion

Dr Peter Hubber January 2008 EMS301 Week 1A

Explaining motion

Written by Dr Peter Hubber


Ideas about Forces

• Forces are pushes orpulls of one objectonto another.

• Forces are vectorquantities (magnitudeand direction) thatcan be representedas arrows.

• What forces arenecessary to twist offthe jam jar cap?


Forces are vector quantitiesrepresented as arrows

Push on front ofcap due to thumb

Push on back of capdue to fingers

Push on front ofbase due to thumb

Push on back ofbase due to fingers


Key ideas about forces

• Forces are pushes or pulls of one object onanother.

• Forces are vector quantities that have bothmagnitude (Newtons) and direction.

• Forces only occur in pairs in situations wherethere are two objects interacting.

• One of the force pair acts on one interactingobject, the other force pair acts on the otherinteracting object.

• The force pairs are equal in size but in oppositedirections.


Newton’s Third Law of motion inwritten and symbolic form

Object A Object B

FBAFAB

FAB = - FBANewton’s Third LawWhenever one object exerts a force on asecond object, the second object exerts anequal and opposite force on the first


Arnold Strongman and Suzie Small pull onopposite ends of a rope in a tug of war. Thegreater force exerted on the rope is by

1. Arnold.2. Suzie.3. Neither. The force is the same.


1. Arnold.2. Suzie.3. Neither. The force is the same.

Arnold Strongman and Suzie Small pull onopposite ends of a rope in a tug of war. Thegreater force exerted on the rope is by


Two identical rubber bandsconnect masses A and B toa string over a frictionlesspulley of negligible mass.The amount of stretch isgreater in the band thatconnects

1. A.2. B.3. Both the same.


1. A.2. B.3. Both the same.

Two identical rubber bandsconnect masses A and B toa string over a frictionlesspulley of negligible mass.The amount of stretch isgreater in the band thatconnects


Forces terminologyFor each of the following types of forces think of a context

in which the force is applied. For example, field force –force that is applied at a distance, like a gravitationalforce, or gravity.

• Contact force• Weight• Friction

– Static friction– Dynamic friction

• Tension• Compression• Shear force• Normal force


Weight and Mass• In physics weight is defined as the

gravitational force of attractionbetween two objects with mass. It ismeasured in Newtons (N)

• Mass is the quantity of matter in anobject. It is measured in kilograms(kg).

• If the apple falls to the groundbecause of the weight force due tothe Earth then why doesn’t theEarth fall to the apple? After all,isn’t the weight force on the appleequal to the weight force on theEarth (Newton’s Third Law ofmotion)?


Forces add vectorially to give theNet Force (or Resultant Force)

Forces in the same direction

Forces in opposite directions

Forces in different directions

Net Force, F

F =20 N E

F = 0 N

F =14 N NE

Push from each person = 10 N

North


Net Force (F) on an object

• If F = 0, then object is in mechanicalequilibrium. In this situation the object can be– at rest (stationary), or– moving at constant velocity.

• If F = non-zero, the object is accelerating. Inthis situation the object changes its velocity by– changing direction, and/or– changing the magnitude of velocity (speed).


Forces Simulation


Newton’s First Law of Motion:Inertia and Mechanical Equilibrium

• Inertia is the property of an object to resistchanges in motion.

• Newton’s First Law of Motion (Law ofInertia) – “every object will maintainmechanical equilibrium unless acted on by anon-zero net force.”– Symbolically, if F = 0 then a = 0– Conversely, if a = 0, then F = 0.


What is known about the forces actingon the child whilst she rests on the tablereading her book?

What is likely to happen to the child in ahead-on car collision if his seat belt isnot connected?Why?

What is known about the forces acting on the caras it moves along the street at a constant speed of50 km/hr?The car hits an icy section of the road, the driverslams on the brakes but the car continues toslide forward. Why?


Nellie Newton hangs motionless by one handfrom a clothesline as shown—which is on theverge of breaking. Which side of the line ismost likely to break?

1. Left side2. Right side3. 50/50 chance

of either sidebreaking


1. Left side2. Right side3. 50/50 chance

of either sidebreaking

Nellie Newton hangs motionless by one handfrom a clothesline as shown—which is on theverge of breaking. Which side of the line ismost likely to break?


1. A

2. B

3. C

When the pelletfired into the spiraltube emerges,which path will itfollow? (Neglectgravity.)


When the pelletfired into the spiraltube emerges,which path will itfollow? (Neglectgravity.)

1. A

2. B

3. C


Which path will the ball then follow?

1. 2. 3.

When the ball at the endof the string swings to itslowest point, the string iscut by a sharp razor.


When the ball at the endof the string swings to itslowest point, the string iscut by a sharp razor.

1. 2. 3.

Which path will the ball then follow?


If F ≠0 what happens?

• If the net force on an object is non-zerothen it will accelerate at a value that isinversely proportional to the object’s mass(Newton’s Second Law of Motion).

• Symbolically, F = ma– The direction of the net force (vector

quantity) is the direction of the acceleration(vector quantity).


1. The regular dart.2. The weighted dart.3. It’s a tie.

Two identical spring-loaded dart gunsare simultaneously fired straightdownward. One fires a regular dart;the other a weighted dart.

Which dart hits the ground first?


Two identical spring-loaded dart gunsare simultaneously fired straightdownward. One fires a regular dart;the other a weighted dart.

Which dart hits the ground first?

1. The regular dart.2. The weighted dart.3. It’s a tie.


1. 2.

In both cases an applied force of100 N accelerates the 100-N block.In which case is the accelerationgreater?


Friction• Friction is a force that opposes the

motion of an object whose surfaceslides or tends to slide overanother surface.

• Friction forces don’t just slowmoving objects.– A box resting on an incline is held in

place by friction force of the incline’ssurface onto the box’s surface.

– A person walking is able to do sobecause for friction between theground surface and the shoe surfaceholds the show in place for each step.

– A car is able to travel around a cornerbecause of the friction force of theroad surface on the wheel surface.


Static and Sliding Friction• To explain friction one can imagine on amicroscopic scale hills and valleys of onesurface sliding over another. However, thereare other processes at work, includingchemical bonding and electrostatic interactions.

• There are two forms of friction, staticand sliding (or kinetic) friction.

Static Frictionfriction applied on objects at rest

Sliding Frictionfriction applied when object is moving (sliding)

The maximum static friction is greater thanThe maximum sliding friction.


Coefficient of Friction• The level of friction that

different materials exhibit ismeasured by the coefficientof friction.

µ = F / Nwhere

µ is the coefficient offriction,F is the amount of forcethat resists motion,N is the normal force.

µ is independent of surfacearea of contact.

0.200.30Tyre onsnow

0.400.60Tyre onwet road

0.801.00Tyre onconcrete

0.400.94Glass onglass

0.040.04Teflon onteflon

0.570.74Steel onsteel

Sliding µStatic µSurfaces


She holds the bookstationary against thewall as shown. Frictionon the book by thewall acts

1. upward.2. downward.3. can’t say.


1. upward.2. downward.3. can’t say.

She holds the bookstationary against thewall as shown. Frictionon the book by thewall acts


Fluid Friction

• Fluids can be liquids (water) or gases (air).• Fluid friction terminology

– Air resistance– Drag

• Air resistance or Drag depends on– Mass– Surface area of contact


1. A fallingelephant, or

2. A fallingfeather?

Which encountersthe greater forceof air resistance—


Two smooth balls ofexactly the same size,one made of wood andthe other of iron, aredropped from a highbuilding to the groundbelow. The ball toencounter the greaterforce of air resistanceon the way down is1. the wooden ball.2. the iron ball.3. Neither. The force is

the same.


Two smooth balls ofexactly the same size,one made of wood andthe other of iron, aredropped from a highbuilding to the groundbelow. The ball toencounter the greaterforce of air resistanceon the way down is

1. the wooden ball.2. the iron ball.3. Neither. The force is

the same.


1. increases.2. decreases.3. remains

the same.

As she falls faster and fasterthrough the air, her acceleration


Newton’s Laws of Motion

These laws should be not be seen asindependent of each other.

• First Law – Law of Inertia– If F = 0 then a = 0

• Second Law – Law of Acceleration– If F ≠0 then a ≠0 and a F/m

• Third Law – Law of Action & Reaction.– F12 = F21

explaining motion

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