Unit 1.3 Dynamics Flashcards
Define an isolated system.
It is one on which no external forces act and no particles enter or leave.
How may Newton’s three laws of motion be stated as?
N1, N2 and N3
State N1 (Newton’s first law).
A body’s velocity will be constant unless a resultant force acts upon it.
State N2 (Newton’s second law).
The rate of change of momentum of a body is directly proportional to the resultant force acting upon it.
State N3 (Newton’s third law).
If a body A exerts a force on body B, then B exerts an equal and opposite force on A.
State the principle of conservation of momentum.
The vector sum of the momenta of the bodies in a system is constant provided there is no resultant external force.
Give an alternative statement of conservation of momentum.
The vector sum of the momenta of the bodies in an isolated system is constant.
State the formula of momentum.
p = mv
Give the units of momentum, p.
kg ms⁻¹
What is “v” in p = mv?
“V” is the velocity of the body (ms⁻¹)
What is “m” in p = mv?
“M” i s the mass (kg)
Define mass.
It is a measure of the body’s inertia.
What is inertial mass assumed to be?
It is assumed to be independent of velocity.
What is used to indicate the direction of a single straight line?
+ or - is used to indicate the direction.
What type of quantity is mass?
Scalar quantity
What type of quantity is momentum?
Vector quantity
State 2 points concerning momentum.
- If asked to find the momentum of a body we should always specify the direction.
- We add or subtract momenta
Define ‘momenta’.
Plural of momentum
What are inelastic collisions?
They are collisions, in which the objects combine on impact.
Give a situation in which the conservation of momentum applies.
When one object ejects another, such as a nucleus emitting an α particles or a gun firing a bullet.
State the formula of ΣF.
ΣF = Δp/ Δt
Give the gradient of a momentum-time graph.
Resultant force
Give 3 forces which arise between objects due to molecular interactions.
- The normal
- Friction
- Air resistance
What does the word ‘normal’ mean?
At 90°
How do bodies experience a force at right angles to the surface?
If an object rests against a surface, the surface exerts a force on the object. This force arises because the molecules in the two bodies are placed in close contact. If molecules are close together, the electrons in the outer shells repel one another and so, in this case, the bodies experience a force at right angles to the surface.
Describe static and dynamic friction.
It opposes relative motion between surfaces. E.g. if a surface would slide to the right, the frictional force on it is to the left.
Give 2 other names for static friction.
- Fᵣ
- Grip
Give an example on how static friction works.
The box on the slope will remain stationary on the slope as long as the gradient is not too great. This can only be because a force acts on the box up the slope which counteracts the component of the weight, W sinθ, down the slope.
What is the limiting friction?
Maximum value of static friction, Fᵣ.
How does grip arise?
From bonds between molecules of the two surfaces in contact.
Define friction.
The force which opposes the relative motion when one surface slides over another.
How does friction arise?
From temporary bonds which form as molecules in the surfaces move past one another.
What happens to the bonds of a frictional force, if they stretch or break?
The stored energy in the bonds is converted to vibrational energy of the molecules, i.e the temperatures of the bodies increase.
What happens to the object if the value of dynamic friction < limiting value of grip?
Once an object starts to slide, it usually accelerates rather than just moving very slowly.
Give an example of air resistance.
Drag
Describe air resistance.
It opposes relative motion between the object and the fluid [= liquid or gas] through which the object is moving. It also exists when the fluid flows past a stationary object.
Describe the mechanism of drag (in terms of molecules).
The molecules of a fluid bounce off a moving object slightly faster than they hit it: they gain velocity in the direction that the object is moving. So there is a momentum transfer to the fluid- and momentum transfer means a force on the fluid in the direction of the object’s motion. By N3 the fluid exerts an equal opposite force on the body.
Give the formula for drag force, F𝒹.
F𝒹 = 1/2 * ρv²c𝒹A
What is “ρ” in drag force formula?
Density of the fluid
What is “c𝒹” in drag force formula?
Dimensionless quantity called the drag coefficient, which depends upon the shape of the object.
What is “v” in drag force formula?
Velocity
What is “A” in drag force formula?
Area
How can the drag force, F𝒹 increase?
F𝒹 increases with the area A if the object, the velocity “v” and the density “ρ” of the fluid.
Why is drawing a free-body diagram useful?
It is useful in identifying the forces on interacting objects.
State the formula for N2 force.
ΣF = ma
State the formula for N2 force (concerning weight).
W = mg
Give the SI units of weight, W.
N kg⁻¹
What is “g” referred to as in W = mg?
Gravitational field strength- it gives the force per unit mass on a body placed in the gravitational field.
What are the 2 significant forces for a falling object?
- The weight, W, which is a constant for objects close to the Earth.
- Air resistance, F𝒹, which varies according to F𝒹 = 1/2 ρv²c𝒹A
