Forces and Transfer of Energy Notes

Types of Forces and Transfer of Energy

Learning Outcomes

• Identify contact forces (e.g., friction) and non-contact forces (e.g., magnetic, gravitational).
• Measure force using the Newton (N) as the SI unit.
• Distinguish between mass and weight.
• Apply the relationship between weight, mass, and gravitational field strength: $W = mg$.

What is a Force?

• A force is a push or a pull that can:
  • Change the shape and/or size of an object.
  • Cause an object to start or stop moving.
  • Change the direction of a moving object.
  • Slow down or speed up a moving object.

Measuring Force

• The SI unit for force is the Newton (N).
• Force is measured using a spring balance or Newton meter.
• The Newton is named after Sir Isaac Newton (1643–1727), who formulated the laws of motion.

Representing Forces

• Forces are represented on diagrams using arrows.
• The direction of the arrow indicates the direction of the force.

Types of Forces

• Contact Forces: Act between objects that are physically touching.
  • Frictional force (friction). Air resistance and water resistance are types of friction.
  • Tension force (exerted through a rope/string).
  • Normal force (upwards support force from surfaces).
  • Applied force (such as a push).
  • Elastic force (spring force).
• Non-Contact Forces: Act between objects that are not physically touching.
  • Gravitational force (gravity).
  • Magnetic force.
  • Electrostatic force.

Frictional Force

• Acts in the opposite direction of motion.
• Occurs in gases (air resistance) and liquids (water resistance).
• Microscopic roughness and jaggedness of surfaces cause friction as they grind and drag against each other.

Uses and Drawbacks of Friction:

• Useful:
  • Brakes on a bicycle.
  • Parachutes (air resistance).
  • Grip on surfaces.
  • Rubbing hands together to generate thermal energy.
  • Creating sparks to light matches.
  • Sandpaper smoothing surfaces.
• Nuisance:
  • Reduces machine efficiency (energy lost as thermal energy).
  • Generates thermal energy during rocket re-entry.
  • Causes wear and tear on materials.

Ways to Reduce Friction:

• Using lubricants.
• Using smooth or polished surfaces.
• Using a layer of air (e.g., hovercrafts).
• Using wheels or rollers (e.g., conveyor belts).
• Streamlined shapes.

Normal Force

• The upward support force from surfaces, also known as normal contact force.
• Objects at rest on a surface do not fall through because the normal force balances their weight.

Gravitational Force

• The force an object experiences due to gravity; also known as weight.
• A gravitational field is the area where an object experiences gravitational force.
• Gravitational field strength (g) is approximately $10 N/kg$ on Earth and $1.6 N/kg$ on the Moon.

Mass vs Weight

• Weight is the gravitational force acting on an object, measured in Newtons (N).
• Weight is calculated using the formula: $W = mg$ where:
  • $W$ = weight (N)
  • $m$ = mass (kg)
  • $g$ = gravitational field strength (N/kg)
• Mass is constant, while weight varies depending on gravitational field strength.

Example Calculation:

• Man with a mass of 83.5 kg:
  • Earth (g = 10 N/kg):
    • Mass = 83.5 kg
    • Weight = $83.5 kg <br /> umber 10 N/kg = 835 N$
  • Mars (g = 4 N/kg):
    • Mass = 83.5 kg
    • Weight = $83.5 kg <br /> umber 4 N/kg = 334 N$

Weighing Scales

• Scales measure weight but display it as mass by dividing the weight by $g$ ($10 N/kg$ on Earth).
• This is inaccurate on the Moon because the gravitational field strength is different.

Balanced and Unbalanced Forces

Scenarios:

1. Stationary Object:
   • Balanced forces result in a resultant force of zero.
   • The object remains stationary.
2. Object Moving with Constant Speed:
   • An object moving with constant speed will continue moving with constant speed if there are no forces acting on it.
   • If forces are balanced, the object continues moving with constant speed (no resultant force).
   • Friction opposes motion and causes deceleration unless balanced by another force.

Newton’s First Law of Motion

• An object stays at rest or keeps moving at the same speed in a straight line unless a force acts on it.

Effects of Forces on Motion

Scenarios:

1. Unbalanced Forces on a Stationary Object:
   • A resultant force causes the object to move in the direction of the force with increasing speed (acceleration).
2. Continuous Force Applied:
   • The object moves in the direction of the resultant force with increasing speed.
   • Removing the applied force results in constant speed if forces are then balanced.
3. Friction Present:
   • The object moves with increasing speed while the force is applied.
   • Removing the applied force results in deceleration due to friction until the object stops.
4. Force Applied in the Opposite Direction:
   • The object initially moves with increasing speed.
   • Removing the force results in constant speed.
   • Applying a force in the opposite direction decreases speed, changes direction, and increases speed in the opposite direction.

Conclusion

• Balanced Forces:
  • No resultant force.
  • Stationary objects remain at rest, and moving objects maintain constant speed.
• Unbalanced Forces:
  • Resultant force applied.
  • The object starts or stops moving, increases or decreases speed, or changes direction.

Calculating Resultant Force

• The resultant/net force is the overall force after considering all forces acting on an object.
• A force has both magnitude and direction (vector quantity).
• Example: 50 N to the right.

Example Problems:

1. 10 N + 20 N to the right = 30 N to the right (object moves with increasing speed to the right).
2. 40 N to the right - 20 N to the left = 20 N to the right (object moves with increasing speed to the right).
3. 8 N to the right - 8 N to the left = 0 N (object remains stationary).

Turning Effect of Forces

• A force can produce a turning effect (moment) about a pivot.
  • Can be clockwise or anticlockwise.

Making Use of Turning Effects

• The greater the distance between the force and the pivot, the greater the turning effect.

Calculating Turning Effect

• Moment/Turning Effect M = F x d

  Where:

  • M = Moment/ Turning Effect
  • F = force applied
  • d = perpendicular distance from pivot
    • The magnitude of a turning effect is affected by the size of the force and the distance between the force and the pivot.

Example

• Pushing a door at the position furthest from the hinge produces the largest turning effect, thus it opens easily

Pressure

• Pressure is defined as force acting per unit area.
• Formula: $P = F/A$
  Where:
  * $P$ = pressure in $N/m^2$ or Pa
  * $F$ = force (N)
  * $A$ = area of contact ($m^2$)
• The SI unit for pressure is Pascal (Pa), where 1 Pa = 1 N/m2.
• Blaise Pascal (1623–1662) conducted experiments on fluid mechanics and atmospheric pressure.

Examples of Pressure

• Weight A sinks further than B. Same force, but smaller area results in higher pressure.
• Weight D sinks further than C. Same area, but greater weight (force) results in higher pressure.
• It is more painful to wear higher heels because the smaller area of contact results in higher pressure.

Applications of Pressure

• Thin, sharp knife blades create large pressure with small force.
• Sharp pins create large pressure to pierce materials.
• Water jets use small openings to create high water pressure.
• Snowshoes distribute weight over a large area to prevent sinking.

Pressure Due to Liquids

• Pressure of water increases with depth due to the greater weight of water above.

Pressure Due to Air

• Atmospheric pressure is the pressure exerted by air in the Earth's atmosphere.

Atmospheric Pressure

• Suction cups stick to walls due to atmospheric pressure.
• Liquids are drawn up a straw due to atmospheric pressure.

Work Done

• Work is done when there is a force acting on an object, the object moves through a distance, and the movement is in the direction of the force.

Conditions for Work to Be Done:

1. A force acts on the object.
2. The object moves through a distance.
3. The movement is in the direction of the force.

Scenarios Where Work Is Not Done:

• Pushing against a solid wall (no movement).
• Holding a heavy pile of books in a stationary position (no movement).
• Carrying a load while walking (motion is perpendicular to the force).

Scenarios Where Work Is Done:

• Pushing a cupboard that moves.

Calculating Work Done

• Formula: $W = F umber d$ Where:
  • $W$ = work (Joules, J)
  • $F$ = force (Newtons, N)
  • $d$ = distance (meters, m)
• 1 Joule (J) = 1 Newton-meter (Nm)
• Work results in a transfer of energy

Example Calculations:

1. Pulling a bucket of water:
   • $W = 4.08 kJ = 4080 J$
   • $F = 1.0 N + 23.0 N = 24 N$
   • $d = W/F = 4080 J / 24 N = 170 m$
2. Carrying a pail of water up stairs:
   • $d = 15 <br /> umber 23.0 cm = 345 cm = 3.45 m$
   • $F = mg = 8.50 kg <br /> umber 10 N/kg = 85.0 N$
   • $W = F <br /> umber d = 85.0 N <br /> umber 3.45 m = 293 J$
3. Marie carrying a bag:
   • 0 J of work done against gravity because the movement is sideways (perpendicular to the force).

Real-World Example:

• Mark Kirsch pulling a Boeing 767:
  (a) No work done initially because there was no movement.
  (b) More work would be done over 40 m because W = F x d
  (c) Time does not affect the amount of work done.

Energy

• Energy is the ability to do work.
• The SI unit for energy is the Joule (J), the same as for work done.
• Examples of forms of energy:
  • Kinetic
  • Potential
  • Light
  • Electrical
  • Sound
  • Heat

Relationship Between Energy and Work Done

• When work is done, energy is transferred.

Example

• Tennis ball GPE: W = F x d = $1.2 N <br /> umber 0.80 m = 0.96 J$

Energy Transfer Scenarios:

1. Archer pulling bowstring: Chemical potential energy to elastic potential energy
2. Arrow released: Elastic potential energy to kinetic energy, kinetic energy to thermal and sound energy.

Conservation of Energy

• The law of conservation of energy states that energy can only be converted from one form to another, but it cannot be created or destroyed.

Example

• Bowling ball cannot exceed height it was released from as energy cannot be created
• Bowling ball doesn't reach the same height due to energy loss as thermal energy
• Pushing the ball will add more energy to the ball, causing the ball to exceed the height and damage could occur

Skater Example (No friction)

• At the highest point, gravitational potential energy is at maximum, and equal to the total energy,
• At the lowest point, all potential energy has converted into Kinetic energy, where Kinetic energy is at Maximum, and equal to the total energy
• As the Skater rises and falls, Kinetic and Potential energy convert back and forth accordingly

Skater example (with friction)

• As the Skater rises and falls, Gravitational Potential Energy turns into Kinetic and Thermal energy
• Due to thermal energy, which is lost to the surroundings, the skater does not reach their original height
• Eventually, the skater comes to a halt and all energy has been lost to the surroundings from thermal energy