Module 3: C4 - Force, Mass, And Weight Flashcards
What are 6 types of Transfers of Energy
- Kinetic
- Thermal (Heating)
- Electrical
- Light
- Sound
- Forces
What are 6 types of Stores
- Elastic Potential
- Nuclear
- Chemical Potential
- Gravitational Potential
- Magnetic Potential
- Thermal Store
What are the 3 Non-Contact Forces
- Electrostatic Force (Push/Pull)
- Magnetic Force (Push/Pull)
- Gravitational/Weight (Pull)
What are 9 Contact Forces
- Air Resistance
- Water Resistance
- Friction
- Tension
- Upthrust
- Compression
- Torsion
- Thrust
- Lift
What are the 4 Fundamental Forces
- Gravitational Force (W=mg)
Acts on anything with mass (Protons, neutrons, electrons) - Electromagnetic/Electrostatic Force
Acts on anything with charge (Protons, electrons) - Weak Nuclear Force
- Strong Nuclear Force
What are the 4 Types of Motion
Something can be:
- Stationary
- Accelerating
- Decelerating
- At Constant Velocity
Examples of Contact Forces
- Frictional Force
- Normal Contact Force
- Tension Force
- Air Resistance
Examples of Non-Contact Forces
- Gravitational Force
- Electrostatic Force
- Magnetic Force
What is Newton’s 3rd Law of Motion
Newton’s Third law of motion:
For every action there is an equal and opposite reaction.
- Forces always act between two objects.
- Forces either push the objects apart, or pull
them closer together. - The force acts on each object with equal strength (it is the same force acting on them both!).
What is Resultant Force
A resultant force is the sum of all forces. (ΣF)
What is Weight
The gravitational force acting on an object through its centre of mass
What is Friction
The force that arises when two surfaces rub against each other.
What is Drag
The resistive force on an object travelling through a fluid (e.g air and water); the same as friction.
What is Tension
The force within a stretched cable or rope
What is Upthrust
An upward buoyancy force acting in an object when it is in fluid.
What is Normal Contact Force
A force arising when one object rests against another object
What can you represent forces
You can represent forces using a free-body diagram.
- Each force vector is represented by an arrow labelled with the force it represents.
- Each arrow is drawn to the same scale (the longer the arrow, the greater the force)
Worked Example: Down the slope
An 859g trolley is held at the top of a 1.2, long ramp. The ramp makes an angle of 15° to the horizontal. The trolley is released from rest. Calculate the acceleration, a, of the trolley as it travels down the ramp and the time, t, it takes to reach the bottom of the ramp.
Step 1: Identify the equations needed.
Force in the trolley down the ramp = mg sinΘ (F=ma)
Step 2: Substitute the values into the equation and calculate the answer.
Acceleration of trolley a = F/m = g sinΘ (note: acceleration is independent of the mass)
a = 9.81 x sin(15) = 2.54ms^-2
You can now use the equation of motion: s = ut+1/2at^2 to calculate the time, t.
1.2 = 1/2 x 2.54 x t^2 (u=0)
t = √2x1.2/2.54 = 0.97s (2sf)
What is the component of the weight (+ what is it responsible for) when an object is on a slope
The component of the weight down the slope is responsible for the acceleration of the object down the slope. There is no acceleration of the object perpendicular to the slope. Therefore, this component of the weight must be equal to the normal contact force M acting on the object, that is
Fy = N = mg cos Θ
What 2 components can weight be resolved to when an object is on a slope
Assuming that there is no friction - the only force acting on the object is it’s weight. This weight can be resolved into two components, parallel and perpendicular to the slope.
Force parallel to the slope = W sinΘ or Fx = mg sinΘ
Force perpendicular to the slope = W cosΘnor Fy = mg cosΘ
What is the Centre of Gravity
Weight is caused by the attraction if every atom inside an object to every atom inside the Earth. The sum of all these forces appear to act from a single point of any object, this is called the centre of gravity.
What is the Centre of Mass
The centre of mass is defined as “the point at which an object’s mass is centred on” or “The centre of mass of a body is the point through which a single force on the body has no turning effect.
What is a Moment
- A moment is the turning effect of a force.
- A moment will act around a pivot.
- A moment can act clockwise or anticlockwise.
Equation for Calculating a Moment
Moment (Nm) = Force (N) x Perpendicular Distance (m)
How can you increase the size of a moment (it’s turning effect)
You can increase the perpendicular distance from the pivot, or you can increase the force you are using.
How do you know if an object with turning forces acting on it is in equilibrium
If a body is in equilibrium, the sum of the clockwise moments is equal to the sum of the anti-clockwise moments
What is the Principle of Moments
For a body in rotational equilibrium, the sum of the anti-clockwise moments about any point is equal to the sum of the clockwise moments about the same point.
What is Stable Equilibrium
If a body is displaced from the equilibrium position then released, it will return to the equilibrium position
What is Unstable Equilibrium
If a body is displaced from the equilibrium position then released, it will not return to the equilibrium position
What is a Couple
A couple is defined as two equal and opposite forces acting on a body but on different lines of action
Equation for the Moment of a Couple
The moment of a couple = force x perpendicular distance between the line of action of the forces
How are Triangles of Forces Drawn?
- Arrows are drawn to represent each of the three forces end-to-end
- The triangle is closed because the net force is zero and so the object is in equilibrium
Example Question:
The tension in two springs supports a weight of 19N 35° apart. What are T1 and T2?
19N/2 = 9.5N
35°/2 = 17.5°
(SOHCAHTOA - CAH)
9.5 / cos(17.5) = 9.961N ~ 10.0N
Example Question:
The tension in two springs supports a weight of 37N 5° apart. What are T1 and T2?
37N/2 = 18.5N
5° / 2 = 2.5°
(SOHCAHTOA - CAH)
18.5 / cos(2.5) = 18.518N ~ 18.5N
What is Density?
Density is the amount of mass in a volume (mass per unit volume).
It tells us how tightly matter is packed together.
The equation for Density is:
ρ = m/V
What is the Equation for Density (+ it’s units)
Density = Mass/Volume
ρ = m/V
Unit: kgm^-3
Worked Example: Dense Osmium
A 50.0cm^3 sample of osmium has a mass of 1.13kg. Calculate its density
Step 1: Select the equation for density
ρ = m/V
Step 2: Substitute in the known values in SI units and calculate the density.
m = 1.13kg
V = 50x10^-6 m^3
ρ = m/V
1.13 / 50.0x10^-6 = 2.26x10^4 kgm^-3
Osmium is 22.6 times denser than water.
How to determine density of Solids and Liquids (both regular and irregular)
You need to know mass and volume to determine the density of a substance. The mass can be measured directly using a digital balance. For liquids, you can use a measuring cylinder to determine the volume. The volume of a regular-shaped solid can be calculated from measurements taken with a ruler, digital callipers, or a micrometer. The volume of irregular solids can be determined by displacement.
Definition of Pressure
Pressure is the normal force exerted per unit cross-sectional area. You can use the following equation to calculate pressure p.
p = F/A
Equation for Pressure (+ SI unit)
Pressure = Force / Cross-Sectional Area
p = F/A
SI Unit: Nm^-2
What is the Upthrust in an Object according to Archimedes’ Principle
According to Archimedes’ Principle, the upthrust on an object in a fluid is equal to the weight of the fluid displaced. So the volume of the object multiplied by the density of the fluid.
What is Upthrust (What is it equal to?)
Upthrust = Weight of Fluid Displaced
A fluid will exert a force upward on a body if it is partly or wholly submerged within it. This is because the deeper into a fluid you go, the greater the weight of it and so the greater the pressure. This difference in pressure between the top and the bottom of the object produces an upward force on it. This is called Upthrust.
What causes Upthrust
Upthrust is equal to the weight of the fluid displaced.
Equation to calculate the pressure exerted by a vertical column of any liquid
p = hρg
How is the equation p = hρg derived
With a cylindrical column of liquid with height h and base cross-sectional area A.
- W = mass of column x g
The mass of column is the density x the volume
- W = (pV) x g
The volume of the column is Ah.
- W = p x Ah x g
Finally, the pressure p is given by
- p = ρ x A x h x g / A = hρg
This equation shows that pressure does not depend on the cross-sectional area. It also clearly shows that p∝h, so water pressure increases with depth. The term ρ shows that denser liquids will exert greater pressure.
What is Archimedes’ Principle
The Upthrust exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces.
When will an object Float or Sink
An object will sink if the Upthrust is less than the weight of the object. For a floating object, such as a ship or a person in water, the Upthrust must equal the weight of the object. This in turn means that the weight of a floating object must be equal to the weight of the fluid it displaces.
Calculate the pressure exerted by a column of height 1.00mm.
The density of mercury is 1.36x10^4 kgm^-3
P = hρg
0.001 x 1.36x10^4 x 9.81 = 133.416
= 133Pa
Standard atmospheric pressure (the mean value of atmospheric pressure at sea level) is 101kPa. Calculate the height, in m, of a mercury column that corresponds to this pressure.
(The density of mercury is 1.36x10^4 kgm^-3)
P = hρg
h = 1.36x10^4 x 9.81 = 101000
h = 0.757m
= 757mm
Changes in atmospheric pressure can result in the height of the mercury column changing by ±15mm from its standard position. Calculate these changes in pressure in pascals
(The density of mercury is 1.36x10^4 kgm^-3)
±15mm
h = 15mm
h = 0.015mm
P = hρg
0.015 x 1.36x10^4 x 9.81 = 2000Pa
= ±2000 Pa
Changes in atmospheric pressure are often accompanied by changes in temperature. Describe how this may affect the accuracy of a barometer.
As the temperature increases the mercury would expand and its density would decrease. This would increase the height of the mercury column (for the same pressure) and so give pressure readings that are too high.
In a particular barometer, the mercury column has a diameter of 4.00mm. What mass of mercury would be present in a column 750mm high.
(The density of mercury is 1.36x10^4 kgm^-3)
V = πr^2h
= π x (0.002)^2 x 750x10^-3 = 9.42x10-6 m^3
M = Vρ
9.42x10^-6 x 13.6x10^3 = 1.28x10^-1 kg = 0.128kg
