Explaining Motion

Dr Peter Hubber January 2008 EMS301 Week 1A

Explaining motion

Written by Dr Peter Hubber


Ideas about Forces

• Forces are pushes or pulls of one object onto another.

• Forces are vector quantities (magnitude and direction) that can be represented as arrows.

• What forces are necessary to twist off the jam jar cap?


Forces are vector quantities represented as arrows

Push on front of cap due to thumb

Push on back of cap due to fingers

Push on front ofbase due to thumb

Push on back of base due to fingers


Key ideas about forces

• Forces are pushes or pulls of one object on another.

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

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

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

• The force pairs are equal in size but in opposite directions.


Newton’s Third Law of motion in written and symbolic form

Object A Object B

FBAFAB

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


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


Two identical rubber bands connect masses A and B to a string over a frictionless pulley of negligible mass. The amount of stretch is greater in the band that connects

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


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

Two identical rubber bands connect masses A and B to a string over a frictionless pulley of negligible mass. The amount of stretch is greater in the band that connects


Forces terminology

For 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 gravitational force, 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 attraction between two objects with mass. It is measured in Newtons (N)

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

• If the apple falls to the ground because of the weight force due to the Earth then why doesn’t the Earth fall to the apple? After all, isn’t the weight force on the apple equal to the weight force on the Earth (Newton’s Third Law of motion)?


Forces add vectorially to give the Net 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 mechanical equilibrium. In this situation the object can be– at rest (stationary), or– moving at constant velocity.

• If F = non-zero, the object is accelerating. In this 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 resist

changes in motion.• Newton’s First Law of Motion (Law of

Inertia) – “every object will maintain mechanical equilibrium unless acted on by a non-zero net force.”– Symbolically, if F = 0 then a = 0– Conversely, if a = 0, then F = 0.


What is known about the forces acting on the child whilst she rests on the table reading her book?

What is likely to happen to the child in a head-on car collision if his seat belt is not connected? Why?

What is known about the forces acting on the car as it moves along the street at a constant speed of 50 km/hr?The car hits an icy section of the road, the driver slams on the brakes but the car continues to slide forward. Why?


Nellie Newton hangs motionless by one hand from a clothesline as shown—which is on the verge of breaking. Which side of the line is most likely to break?

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

of either side breaking


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

of either sidebreaking

Nellie Newton hangs motionless by one hand from a clothesline as shown—which is on the verge of breaking. Which side of the line is most likely to break?


1. A

2. B

3. C

When the pellet fired into the spiral tube emerges, which path will it follow? (Neglect gravity.)


When the pellet fired into the spiral tube emerges, which path will it follow? (Neglect gravity.)

1. A

2. B

3. C


Which path will the ball then follow?

1. 2. 3.

When the ball at the end of the string swings to its lowest point, the string is cut by a sharp razor.


When the ball at the end of the string swings to its lowest point, the string is cut 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-zero then it will accelerate at a value that is inversely 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. I t’s a tie.

Two identical spring-loaded dart guns are simultaneously fired straight downward. One fires a regular dart; the other a weighted dart.

Which dart hits the ground first?


Two identical spring-loaded dart guns are simultaneously fired straight downward. 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. I t’s a tie.


1. 2.

In both cases an applied force of 100 N accelerates the 100-N block.In which case is the acceleration greater?


Friction• Friction is a force that opposes the

motion of an object whose surface slides or tends to slide over another surface.

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

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

– A person walking is able to do so because for friction between the ground surface and the shoe surface holds the show in place for each step.

– A car is able to travel around a corner because of the friction force of the road surface on the wheel surface.


Static and Sliding Friction• To explain friction one can imagine on a microscopic scale hills and valleys of one surface sliding over another. However, thereare other processes at work, including chemical bonding and electrostatic interactions.

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

Static Friction friction applied on objects at restSliding Friction friction 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 is measured by the coefficient of friction.

µ = F / Nwhere

µ is the coefficient of friction, F is the amount of force that resists motion, N is the normal force.

µ is independent of surface area of contact.

Surfaces Static µ Sliding µ

Steel on steel

0.74 0.57

Teflon on teflon

0.04 0.04

Glass on glass

0.94 0.40

Tyre on concrete

1.00 0.80

Tyre on wet road

0.60 0.40

Tyre on snow

0.30 0.20


She holds the book stationary against the wall as shown. Friction on the book by the wall acts

1. upward.2. downward.3. can’t say.


1. upward.2. downward.3. can’t say.

She holds the book stationary against the wall as shown. Friction on the book by the wall 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 falling elephant, or

2. A falling feather?

Which encounters the greater force of air resistance—


Two smooth balls of exactly the same size, one made of wood and the other of iron, are dropped from a high building to the ground below. The ball to encounter the greater force of air resistance on 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 faster through the air, her acceleration


Newton’s Laws of Motion

These laws should be not be seen as independent 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

Documents

Transcript of Explaining Motion