Mechanics and Materials

Question 1

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

Answer 1

Resultant force zero

Resultant moment about any point zero

Question 2

State three vector quantities (3)

Answer 2

Velocity
Acceleration
Force

Question 3

State three scalar quantities (3)

Answer 3

Speed
Distance
Mass

Question 4

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

Answer 4

Lift
Air resistance
Weight

Question 5

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

Answer 5

Closed triangle of vectors

Forces are in equilibrium

Question 6

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

Answer 6

Vector has a direction

Scalar does not

Question 7

What is meant by centre of gravity? (1)

Answer 7

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

Question 8

Define the moment of a force (1)

Answer 8

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

Question 9

State the principle of moments (2)

Answer 9

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

Question 10

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

Answer 10

Counterweight

Provides a sufficiently large moment in the opposite direction

Question 11

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

Answer 11

Ball’s acceleration will decrease

Air resistance affects cards less

Question 12

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

Answer 12

Constant gradient

Question 13

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

Answer 13

Weight greater than air resistance
Therefore resultant force downwards
Air resistance increases with speed
Until resultant force is zero

Question 14

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

Answer 14

Air resistance depends on surface area

Air resistance less on apple, therefore greater velocity

Question 15

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

Answer 15

Freefall is too fast

No accurate method to time freefall

Question 16

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

Answer 16

Draw a tangent work out the gradient

Question 17

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

Answer 17

Impulse - vector

Force - vector

Question 18

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

Answer 18

Weight

Question 19

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

Answer 19

The change in momentum of the body or impulse

Question 20

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

Answer 20

Kinetic energy

Question 21

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

Answer 21

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

Question 22

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

Answer 22

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

Question 23

Define density of a material (1)

Answer 23

Mass divided by volume

Question 24

State Hooke’s law (2)

Answer 24

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

Question 25

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

Answer 25

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

Question 26

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

Answer 26

Ratio of tensile stress to tensile strain | unit: Pa

Question 27

Define tensile stress (1)

Answer 27

Force per unit cross-sectional area

Question 28

Define tensile strain (1)

Answer 28

Extension per unit length

Question 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)

Answer 29

Plastic deformation has produced permanent extension | Gradient is the same because identical forces between bonds

Question 30

A pellet hits a wooden block hung by a string and is embedded in it, the wooden block reaches a certain height. A steel block is used and the pellet rebounds. Comparing energy and momentum changes, which block will reach higher (4)

Answer 30

As pellet rebounds, change in momentum of pellet is greater so change in momentum of block is greater Initial speed of steel block is greater Initial kinetic energy of steel block is greater Therefore the height reached by the steel block is greater than with the wooden block

Question 31

Explain how and why the maximum range of an electric wheelchair on level ground is affected by: the mass of the user and the speed at which the wheelchair travels (4)

Answer 31

``` Increasing mass: Reduces range More energy used accelerating user to final speed Increasing speed: Reduces range Air resistance increases with speed ```

Question 32

Define a couple

Answer 32

a pair of equal and opposite coplanar forces

Question 33

On a velocity - time graph how can the acceleration be found?

Answer 33

Draw a target and work out the gradient

Question 34

On a velocity - time graph how can the displacement be found?

Answer 34

Area under the graph

Question 35

On an acceleration - time graph how can the velocity be found?

Answer 35

Area under the graph

Question 36

When can the equations of motion be used

Answer 36

With constant acceleration

Question 37

What is the initial rate of acceleration on earth? Why does it decrease?

Answer 37

9.8 | Air resistance increases reducing the resultant force

Question 38

Why does a kicked football move in a parabolic curve/projectile motion

Answer 38

Horizontal motion is constant speed | Vertical motion is constant acceleration downwards due to gravity

Question 39

Define termina velocity

Answer 39

The maximum velcoity an object will fall at when the force of weight is balanced by the force of air resistance

Question 40

State Newton's first law of motion

Answer 40

Is there is no resultant force acting on an object it will either stay at rest or move with a constant speed in the same direction

Question 41

State Newtons second law of motion

Answer 41

If there is a resultant force acting on an object it will accelerate in the direction of the larger force

Question 42

State Newtons third law

Answer 42

If object A exerts a force on object B, object B will exert and equal force on A but in the opposite direction

Question 43

State the units of momentum

Answer 43

kg m/s

Question 44

Using ideas of momentum explain how an air bag/ seat belt/crumple zone keeps you safe.

Answer 44

The increase the time to stop the person. For the same change in momentum as smaller force can be used Force = change in momentum/time

Question 45

How can the young modulus for a material be found from a stress strain graph?

Answer 45

The gradient - only up to the elastic limit

Question 46

Define the conservation of momentum

Answer 46

The principle of conservation of momentum states that, in any direction, in the absence of external forces the total momentum of a system remains constant Total momentum before = total momentum after

Question 47

Define an elastic collision

Answer 47

No momentum OR kinetic energy is lost during the collision. Both quantities are conserved (i.e. the same) before and after the collision

Question 48

Define an inelastic collision

Answer 48

Momentum is conserved | Total energy is conserved but kinetic energy is lost during the collision

Question 49

Define Power

Answer 49

Rate of change of energy, measured in watts (W)

Question 50

Define a W

Answer 50

1 joule of energy transferred per second

Question 51

Define efficiency

Answer 51

ratio of useful output energy to total input energy, often express as a percentage