3.2 Forces in Action Flashcards
Define what mass is
Mass, which is a physical property of an object, is the amount of matter an object contains
Describe what happens when a resultant force acts on an object
When a resultant (net) force is acting on an object, it will make the object accelerate in the direction of the net force
What is the equation linking the mass, acceleration, and net (resultant force)?
F = ma
- Where Net Force (F) is measured in Newtons (N)
- Mass (m) is measured in kg
- Acceleration is measured in ms⁻²
Using the formula F = ma, what happens if we give an object of 1kg mass an acceleration of 1ms⁻²?
Using F = ma, if we give an object of 1kg mass, an acceleration of 1ms⁻², then we say that a force of 1 Newton has been applied to it
Define what weight is
Weight is defined as the gravitational force acting upon an object
What is g?
g is the acceleration an object in free fall will have, which is 9.81ms⁻² on Earth, but can be different on other planets
What formula can we use to determine the weight of an object?
W = mg, where:
- Weight (W) is measured in (N) Newtons
- (M) Mass is measured in kg
- (g) is the acceleration of an object in free fall on Earth - 9.81ms⁻²
How do we derive the weight equation W = mg ?
We can derive the formula W = mg from F = ma, where:
- The Force is the Weight
- The acceleration is
Describe what Tension is
Tension is the force within a stretched cable or rope
Tip: Tension acts away from the mass
Describe what Normal Contact Force is
Normal contact force is the force arising when one object rests against another object. It acts at a 90° angle to the plane of contact.
What is friction
Friction is the force that arises when two surfaces rub against each other
What are free body diagrams used for?
Free body diagreams are used to model all of the forces that are acting on an object
What are the rules for drawing free body diagrams
→ Each force vector is represented by an arrow labelled with the force it represents
→ Each arrow is drawn to scale (the longer the arrow, the greater the force)
Do questions on free body diagrams on a slope (Page 52)
Tip: It is often easier to resolve the forces in to the component acting parallel to the slope, and the component acting perpendicular to the slope, rather than horizontally and vertically
What are fluids?
Fluids are substances that can flow - such as gases and liquids
What is drag?
Drag is the frictional force that opposes the motion of an object travelling through a fluid
Remember, air is also a fluid
In air, it is called air resistance
What are the factors that affect the magnitude of the drag force on an object?
The 2 most important factors that affect the magnitude of the drag force:
- The speed of the object
- The cross-sectional area of the object
The other factors that affect the magnitude of the drag force on an object:
- Speed of the object
- Shape of the object
- Roughness or texture of the object
- The density of the fluid through which the object travels
What happens to the drag force for an object with a large cross-sectional area
Objects with a large cross-sectional area experience a greater drag force
Describe the relationship between the speed of an object travelling through a fluid, and its drag force
For an object travelling through a fluid, including air, the drag force is directly proportional to the square of speed
Describe the motion of an object falling through air or any fluid
- ## During a vertical fall through air or another fluid, the weight of the object remains constant but the speed increases due to acceleration (
F = ma). This in turn results in an increase of the drag force - At the instant an object starts to fall, there is no drag force on the object. The total force is equal to the weight (
W = mg) - The acceleration of the object is g, the acceleration of free fall
- As the object falls, its speed increases and this in turn increases the magnitude of the opposing drag force
- The resultant (net) force on the object decreases and the instantaneuous acceleration of the object becomes less than g
- Eventually, the object reaches terminal velocity - when the drag force on the object is opposite and equal to its weight.
- At terminal velocity, the object has zero acceleration and zero net force, and its speed is constant. It has reached terminal velocity
- The object will no longer accelerate and will remain at this acceleration for the duration of the fall before reaching the ground
Tip: you can also think of g as the gravitational field strength being the force acting on the object
Terminal Velocity Diagram
Define the moment of a force
The moment of a force can be described as the __turning effect__ of a force about some axis or point
What is the formula to find a moment?
moment = force x perpendicular distance of the line of action of a force from the axis or point of rotation
moment = Fx
- Moment is in
N m - Force is in
N - Distance is in
m
Introduction to couples
Describe what a couple is
A couple is a pair of equal and opposite forces that act on the same plane in parallel with each other but along __different lines__ of the plane/body, in order to produce a rotational force only (no translational motion)
What is a torque?
A torque is known as the total moment of a couple
How do we work out the torque of a couple
torque of a couple = one of the forces x perpendicular SEPARATION between the forces
Describe what the centre of mass is
The centre of mass of is the imaginary point where the entire weight of an object appears to act
What is the centre of gravity?
The centre of gravity is the point where the resultant gravitational force of an object (W - its total weight) will act.
Describe a similiarty between the centre of mass and centre of gravity
The centre of gravity and centre of mass coincide at the same point
In other words they are in the same location
Describe what density is
The density of a substance is defined as it mass per unit volume
What is the formula for density
ρ = m/v
where m is mass in kg and V is volume in m⁻³
How do we determine the density of an object
In order to determine the density of an object, we need to work out its mass and volume:
- To measure the mass, a digital balance can be used
- To measure the volume:
1. For a liquid, a measuring cyclinder can be used to determine the volume
2. For a regular shaped solid, measurements can be taken with a ruler to work out the volume
3. For an irregular shaped solid, the volume can be determined by the displacement of a liquid and its change in volume when an object is fully submerged into it
What is pressure?
Pressure is the normal force exerted per unit cross-sectional area
Give the equation for pressure
P = F / Apressure = force / cross-sectional area
- Pressure is in Nm⁻² or Pa (Pascals)
- Force is in N
- Area is in m⁻²
How do fluids (gases and liquids) exert pressure?
Gases, such as air, exert pressure on surfaces because of the constant bombardment by their molecules. Liquids also exert pressure for the same reason.
Describe what happens to an object immersed in a column of fluid
An object immersed in a column of fluid experiences pressure due to the weight of the fluid
Why does pressure increase with depth in a fluid?
Pressure increases with depth in a fluid because of the force exerted by the increased weight of the fluid above
Describe the pressure around the atmosphere of the Earth
The pressure exerted by the atmosphere of the Earth varies with altitude.
The more depth there is, the more weight there is, so the pressure increases as the altitude goes down
How do we calculate the pressure p, exerted by a vertical column of any liquid
We can calculate the pressure p, exerted by a vertical column of any liquid from its weight and the cross-sectional area of the base, using the formula:
p = h ρ g
- Where h is the height of the vertical column
- ρ is the density of the liquid
- g is the acceleration of free fall (9.81ms⁻²)
——————————–
1. This is derived from the pressure at the base using the formula pressure = force / area
2. The only force is the weight, so you can change it to pressure = weight / area
3. The weight is equal to the mass of the column x g (W = mg), so p = mg / a
4. The mass of the column can be substituted with the density x the volume as ρ = m/v.
5. So p = ρ v g / a
6. The volume v can be substituted with volume = area x h. So p = ρ a h g / a
7. So p = h ρ g
Give an unusual feature about the pressure in a fluid
The pressure in a fluid at any particular depth is the same in all directions
Define what Upthrust is
Upthrust is an upward buoyancy force acting on an object when it is submerged in a fluid.
This happens because the pressure at the bottom surface of the object is greater than at its top surface.
Definition: buoyancy - the ability or tendency of something to float in water or other fluid.
Describe how upthrust works on this submerged wooden block
Upthrust happens because the pressure at the bottom surface of the object is greater than at its top surface
What does Archimedes’ principle state?
The upthrust exerted on a body immersed in a fluid, whether partially or fully submerged, is equal to the weight of the fluid that the body displaces
When does an object sink, and when does it float?
An object will sink if the upthrust is less than the weight of the object
For a floating object, the upthrust must equal to the weight of the object. This in turn means that the weight of the object must be equal to the weight of the fluid it displaces - due to Archimedes’s principle.
