Flashcards in Mechanics and Materials Deck (53)

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1

## State two conditions for a rod supported by two wires to be in equilibrium (2)

###
Resultant force zero

Resultant moment about any point zero

2

## State three vector quantities (3)

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Velocity

Acceleration

Force

3

## State three scalar quantities (3)

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Speed

Distance

Mass

4

## State the three forces that will act on a helicopter that is moving horizontally (3)

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Lift

Air resistance

Weight

5

## How can force vectors be arranged to show that an object has constant velocity? (2)

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Closed triangle of vectors

Forces are in equilibrium

6

## What is the difference between a vector quantity and a scalar quantity? (1)

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Vector has a direction

Scalar does not

7

## What is meant by centre of gravity? (1)

### The point in a body where the weight of the object appears to act

8

## Define the moment of a force (1)

### Product of the force and the perpendicular distance from the line of action of the force to the point

9

## State the principle of moments (2)

### Sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments for a system in equilibrium

10

## State and explain a feature of a crane that helps prevent it from falling over when lifting heavy things (2)

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Counterweight

Provides a sufficiently large moment in the opposite direction

11

## Why would a thin card give a more accurate measurement of g compared to dropping a tennis ball? (2)

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Ball's acceleration will decrease

Air resistance affects cards less

12

## What feature of a speed against time graph would show a constant acceleration? (1)

### Constant gradient

13

## Why does an object first accelerate after being dropped then reach a terminal velocity? (4)

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Weight greater than air resistance

Therefore resultant force downwards

Air resistance increases with speed

Until resultant force is zero

14

## Why would an apple hit the ground first if dropped with a leaf? (2)

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Air resistance depends on surface area

Air resistance less on apple, therefore greater velocity

15

## How can using an inclined ramp rather than free fall produce valid data on investigating a falling object's motion? (2)

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Freefall is too fast

No accurate method to time freefall

16

## On a distance against time graph, how would speed at a certain time/distance be worked out? (1)

### Draw a tangent work out the gradient

17

## For impulse and force, state whether each one is a vector or a scalar (2)

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Impulse - vector

Force - vector

18

## The rate of change of momentum of a body in free fall under gravity is equal to its.. (1)

### Weight

19

## On a force against time graph, what is the area under the graph equal to? (1)

### The change in momentum of the body or impulse

20

## In an inelastic collision, which quantity is not conserved? (1)

### Kinetic energy

21

## Use Newton's laws of motion to explain how molecules of gas exert a force on the wall of a container (3)

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Force is exerted on molecule by wall

To change its momentum

Molecule must exert an equal but opposite force on wall

In accordance with Newton's third law

22

## Describe how a trolley moving down a sloped track can have its speed measured experimentally (3)

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Place piece of card on trolley of measured length

Card obscures light gate just before trolley strikes block

Calculate speed from length card and time the laser was obscured

23

## Define density of a material (1)

### Mass divided by volume

24

## State Hooke's law (2)

### The force needed to stretch a spring is directly proportional to the extension up to the limit of proportionality

25

## What is meant by the elastic limit of a wire? (2)

### The maximum amount that a material can be stretched and still return to its original length when the force is removed

26

## Define a material's Young modulus and state the units (2)

###
Ratio of tensile stress to tensile strain

unit: Pa

27

## Define tensile stress (1)

### Force per unit cross-sectional area

28

## Define tensile strain (1)

### Extension per unit length

29

## A wire is stretched beyond its elastic limit, explain why when the wire is unstretched, the line is parallel to original extension but does not return to zero (2)

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Plastic deformation has produced permanent extension

Gradient is the same because identical forces between bonds

30