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Explaining Motion
Transcript of Explaining Motion
Dr Peter Hubber January 2008 EMS301 Week 1A
Explaining motion
Written by Dr Peter Hubber
Dr Peter Hubber January 2008 EMS301 Week 1A
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?
Dr Peter Hubber January 2008 EMS301 Week 1A
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
Dr Peter Hubber January 2008 EMS301 Week 1A
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.
Dr Peter Hubber January 2008 EMS301 Week 1A
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
Dr Peter Hubber January 2008 EMS301 Week 1A
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.
Dr Peter Hubber January 2008 EMS301 Week 1A
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
Dr Peter Hubber January 2008 EMS301 Week 1A
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.
Dr Peter Hubber January 2008 EMS301 Week 1A
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
Dr Peter Hubber January 2008 EMS301 Week 1A
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
Dr Peter Hubber January 2008 EMS301 Week 1A
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)?
Dr Peter Hubber January 2008 EMS301 Week 1A
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
Dr Peter Hubber January 2008 EMS301 Week 1A
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).
Dr Peter Hubber January 2008 EMS301 Week 1A
Forces Simulation
Dr Peter Hubber January 2008 EMS301 Week 1A
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.
Dr Peter Hubber January 2008 EMS301 Week 1A
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?
Dr Peter Hubber January 2008 EMS301 Week 1A
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
Dr Peter Hubber January 2008 EMS301 Week 1A
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?
Dr Peter Hubber January 2008 EMS301 Week 1A
1. A
2. B
3. C
When the pelletfired into the spiraltube emerges,which path will itfollow? (Neglectgravity.)
Dr Peter Hubber January 2008 EMS301 Week 1A
When the pelletfired into the spiraltube emerges,which path will itfollow? (Neglectgravity.)
1. A
2. B
3. C
Dr Peter Hubber January 2008 EMS301 Week 1A
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.
Dr Peter Hubber January 2008 EMS301 Week 1A
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?
Dr Peter Hubber January 2008 EMS301 Week 1A
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).
Dr Peter Hubber January 2008 EMS301 Week 1A
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?
Dr Peter Hubber January 2008 EMS301 Week 1A
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.
Dr Peter Hubber January 2008 EMS301 Week 1A
1. 2.
In both cases an applied force of100 N accelerates the 100-N block.In which case is the accelerationgreater?
Dr Peter Hubber January 2008 EMS301 Week 1A
1. 2.
In both cases an applied force of100 N accelerates the 100-N block.In which case is the accelerationgreater?
Dr Peter Hubber January 2008 EMS301 Week 1A
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.
Dr Peter Hubber January 2008 EMS301 Week 1A
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.
Dr Peter Hubber January 2008 EMS301 Week 1A
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
Dr Peter Hubber January 2008 EMS301 Week 1A
She holds the bookstationary against thewall as shown. Frictionon the book by thewall acts
1. upward.2. downward.3. can’t say.
Dr Peter Hubber January 2008 EMS301 Week 1A
1. upward.2. downward.3. can’t say.
She holds the bookstationary against thewall as shown. Frictionon the book by thewall acts
Dr Peter Hubber January 2008 EMS301 Week 1A
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
Dr Peter Hubber January 2008 EMS301 Week 1A
1. A fallingelephant, or
2. A fallingfeather?
Which encountersthe greater forceof air resistance—
Dr Peter Hubber January 2008 EMS301 Week 1A
1. A fallingelephant, or
2. A fallingfeather?
Which encountersthe greater forceof air resistance—
Dr Peter Hubber January 2008 EMS301 Week 1A
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.
Dr Peter Hubber January 2008 EMS301 Week 1A
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.
Dr Peter Hubber January 2008 EMS301 Week 1A
1. increases.2. decreases.3. remains
the same.
As she falls faster and fasterthrough the air, her acceleration
Dr Peter Hubber January 2008 EMS301 Week 1A
1. increases.2. decreases.3. remains
the same.
As she falls faster and fasterthrough the air, her acceleration
Dr Peter Hubber January 2008 EMS301 Week 1A
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