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OCR A-Level Physics > C3-4. Mechanics I > Flashcards

C3-4. Mechanics I Flashcards

1
Q

What equation links velocity, displacement and time taken?

A

v=s/t

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2
Q

What does the gradient of a displacement-time graph show?

A

The velocity of the object.

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3
Q

What would a steeper gradient on an s/t graph represent?

A

A greater velocity (i.e. more distance travelled over a given time).

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4
Q

What is the difference between distance and displacement?

A

Distance is a vector quantity, whereas displacement is a vector quantity. This means that distance only considers magnitude, whilst displacement considers both magnitude and direction of travel.

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5
Q

What equation link acceleration, change in velocity and time?

A

a=v/t

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6
Q

How might we determine displacement from a velocity-time graph?

A

By taking the area underneath the graph. We can prove this by considering the units of the axes; we are multiplying velocity (ms-1) by time (s) to determine displacement (m).

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7
Q

What are the major SUVAT equations we need to know?

A
  • v=u+at
  • s=ut+1/2at2
  • s=1/2(u+v)t
  • v2=u2+2as
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8
Q

What are the two components of stopping distance, and how are they defined?

A
  1. Thinking distance: the time elapsed between the driver noticing a hazard, and then applying the brakes
  2. Braking distance: the time taken for the vehicle to reach 0ms-1 after the brakes are applied
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9
Q

How might we determine the following from a velocity-time graph?
- Average speed
- Instantaneous speed

A
  • Average speed is determined by dividing the total distance travelled by the total time taken to do so, over the spread of the entire graph.
  • Instantaneous speed is found by drawing a tangent to the graph at a given value of t, and finding the gradient y/x.
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10
Q

What is the value of acceleration of free fall (due to gravity) generally given as? When can we apply this as a value for acceleration in our calculations?

A

9.81ms-2 is the general value we use unless stated otherwise. We can use this value for acceleration if the object is not being acted on by an external force, such as thrust.

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11
Q

How does the ‘trapdoor’ method of determining g work?

A
  • An electromagnet holds a metal ball over a trapdoor at a know distance.
  • When the current switches off, the timer starts, then stops when the ball hits the trapdoor.
  • We use the drop distance and the time taken to calculate acceleration.
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12
Q

How does the ‘light gates’ method of measuring g work?

A
  • Two light gates are positioned a know distance apart.
  • The timer starts when the ball is dropped and passes through the first light gate; it stops when the second is reached.
  • We calculate acceleration from the distance fallen and time taken.
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13
Q

How does the ‘manual drop’ method of measuring g work?

A
  • A small metal ball is dropped from a known height at rest, and a timer is used to record the time taken for it to fall to the ground.
  • We then use the known fall distance and time taken to calculate acceleration.
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14
Q

What characterises projectile motion, in terms of trajectory and components of motion?

A
  • The projectile’s trajectory is curved
  • It’s vertical motion changes due to the acceleration of freefall, but horizontal motion remains constant and fully independent
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15
Q

Why are the horizontal and vertical components of projectile motion independent of one another?

A

In projectile motion, only one force affects the projectile: gravity, giving rise to the acceleration of freefall. The force of this acceleration only has a vertical component, so applies a horizontal force of 0N to the moving projectile. Therefore it remains in its constant state of motion horizontally.

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16
Q

What two trigonometric equations help us to resolve projectile forces?

A

V(h)=Vcos(theta)
V(v)=Vsin(theta)
We can then use these to calculate overall velocity using Pythagoras’ theorem, since the vertical and horizontal components are joined to form a right-angled triangle by the resultant.

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17
Q

What equation likes force, mass and acceleration?

A

F=ma, where force is measured in Newtons, mass in kilograms and acceleration in ms-2.

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18
Q

What factors affect an object’s weight? What equation can we form from this information?

A
  • The mass of the object
  • The object’s location (acceleration due to freefall differs around Earth)
    Therefore we can determine that w=mg.
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19
Q

What is the definition of centre of gravity?

A

The point through which the resultant gravitation force of an object due to its total weight is said to act. This coincides with the object’s centre of mass.

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20
Q

What we can say about the motion of an object upon which a force is applied along a line through its centre of mass?

A

The object will only undergo linear motion in the direction the force is applied; no rotational motion will occur.

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21
Q

What is the definition of the force of weight?

A

The gravitational force upon an object, acting through its centre of mass.

22
Q

What is the definition of the force of friction?

A

The force arising when two surfaces rub against one another.

23
Q

What is the definition of the force of drag?

A

The resistive force acting upon an object moving through a fluid, which is the same as friction. It acts in the opposite direction to thrust.

24
Q

What is the definition of the force of tension?

A

The force within a stretched cable or rope.

25
Q

What is the definition of the force of upthrust?

A

An upwards force of buoyancy acting upon an object either fully or partially submerged in a fluid such as water.

26
Q

What is the definition of the force of normal contact force?

A

A force arising when one object rests against another object. On a perfectly horizontal surface, this is equal and opposite to the downwards force of weight acting upon the surface the object rests upon.

27
Q

What two major factors affect the drag acting upon an object moving through a fluid, and how do they affect it?

A
  • Speed: the higher the speed, the greater the drag. Drag is proportional to speed squared for most objects.
  • Cross-sectional area: the greater the cross section, the greater the drag.
28
Q

What do we name the drag force experience by objects moving in air?

A

This variation of drag is called air resistance. We consider this when developing vehicles such as cars and planes, which require streamlined shapes to achieve efficiency and high speeds.

29
Q

What can we say about the forces acting upon an object when terminal velocity is reached during freefall?

A

The downwards force of weight due to gravity is equally opposed by the upwards force of air resistance/drag. This means that the object remains in its constant state of motion, so velocity remains constant from this point onwards.

30
Q

How do the forces acting upon an object change during freefall? How does this affect its changing velocity?

A
  • Initially, the object accelerates with acceleration due to gravity of 9.81ms-2. This is because negligible/no drag force takes place at this time, so the maximum resultant force (and acceleration) is achieved.
  • As the object increases in velocity, the magnitude of the drag force also increases. This causes the resultant force upon the object to decrease.p, so acceleration decreases.
  • Eventually these forces of drag and weight will equalise, reducing the resultant force to zero, so velocity remains constant.
31
Q

What equation do we use the calculate the moment of a force?

A

Moment=force*perpendicular distance from pivot

32
Q

What is the definition of a moment of a force?

A

The turning effect of a force applied about an axis or a point.

33
Q

What is the principle of moments?

A

This is the idea that when a body is in rotational equilibrium, the sum of the anticlockwise moments acting about any point is equal to the sum of clockwise moments about that same point. Therefore, the net force and net moment about that point is zero.

34
Q

What is the definition of a couple? What can we say about the motion induced by the application of a couple on a given object?

A

A couple occurs when two antiparallel forces along different lines across an object, in order to enable rotational motion without translational motion.

35
Q

What is the definition of the torque of a couple, and how might we calculate it?

A

The torque of a couple is the moment induced by a couple. We would calculate it by multiplying the magnitude of one of the forces by the perpendicular separation between the two ‘coupled’ forces.

36
Q

In order to resolve a resultant force, how should the force arrows be arranged in a vector diagram?

A

The forces should be arranged tip-to-tail. The resultant force can be determined by drawing an arrow from the start of the ‘chain’ to the end of the final arrow.

37
Q

What criteria must be met for a closed triangle of forces to be drawn?

A

The body concerned must be in equilibrium, so the resultant force must be zero.

38
Q

For an object to remain at rest, what can be said about the coplanar forces acting upon it, and how could we represent this diagrammatically?

A

The resultant of these coplanar forces applied to the body must be zero, so the object remains at rest. Where three forces are concerned, we can present these forces in tip-to-tail arrangement on a scale diagram. They should join up to form a closed triangle.

39
Q

For an object to remain in equilibrium, how what can we say about the horizontal and vertical components of the forces acting upon it?

A

The algebraic sum of the horizontal components must be zero, and so must the sum of the vertical components. We can use this to solve problems involving unknown magnitudes of forces, such as tension in a rope suspending another object.

40
Q

What is the definition of the density of an object?

A

Density is defined as an objects’s mass in kilograms per unit volume in metres cubed.

41
Q

How might we determine the density of a regularly shaped object?

A

We can use a measuring device like a calliper, ruler or micrometer to measure dimensions and therefore volume, whilst a digital balance can be used to determine mass. This can then be applied to the equation density=mass/volume.

42
Q

How might we determine the density of an irregularly shaped object?

A

We can use the displacement method. A measuring cylinder or similar can be filled with water, and the volume of water displaced by the addition of the object to the measuring cylinder will equal the volume of the object itself. We can then measure mass using a digital balance, and app,y our recording to the equation density=mass/volume.

43
Q

What equation links pressure, force and area?

A

Pressure=force/area, where pressure is measured in Nm-1 or Pascals, force is measured in Newtons and area is measured in metres squared.

44
Q

What is the definition of pressure?

A

Pressure is the normal force exerted per unit cross sectional area. Note that we are talking about normal force; this means that pressure is derived from the resolved perpendicular force upon a surface, not the overall force itself.

45
Q

Why do fluids exert a pressure on surfaces they come into contact with?

A

The surfaces they contact are under constant bombardment by fluid molecules. This exerts a force normal to the surface itself: the definition of pressure.

46
Q

What equation do we use to calculate pressure in a column of liquid?

A

Pressure in a column of fluid = height of the column * density of the fluid * acceleration of freefall

47
Q

What does the equation for pressure of a column of a fluid allow us to determine about the relationship between pressure and…
- depth of fluid
- density of fluid

A
  • Pressure is proportional to the depth of the fluid. As depth increases, pressure exerted increases,
  • As density of the fluid increases, the pressure exerted increases proportionally.
48
Q

How do we derive the equation for pressure in a fluid?

A
  1. W = m*g
  2. W = pV*g (since mass=pressure * volume)
  3. W = pAhg (since volume = area * height)
  4. pressure = (pAhg)/A = hpg (since pressure = force of weight / cross sectional area)
49
Q

How can we explain the buoyancy of objects in water using the concept of pressure in fluids?

A

The pressure on the top surface is less than that on the bottom surface of the object, since the bottom surface is submerged under a taller column height. This means that, since pressure is a force exerted normal to the object’s surface, there is a resultant upwards force - upthrust.

50
Q

Summarise Archimedes’ principle.

A

The upthrust on a body immersed in a fluid, regardless of it is partially- or fully-submerged, is equal to the weight of fluid that is displaced by the body.

51
Q

Describe how Archimedes’ principle is applied to floating objects.

A
  • For a floating object (at rest), we know that the upthrust acting on the object must be equal to the weight of the object.
  • By Archimedes’ principle, this upthrust is equal to the weight of fluid displaced.
  • From this, we can say that the weight of the floating object must therefore be equal to the weight of the fluid displaced.
  • We can use this to then work out the volume of the original object that is submerged.

OCR A-Level Physics (3 decks)

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