Section 3 - Mechanics and Materials Exam Questions

Question 1

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

Answer 1

Resultant force zero

Resultant torque 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

A stationary car and trailer are attached by a towbar, F is the force exerted by the towbar on the trailer. What happens to the magnitude and direction of F when the car moves forward? (3)

Answer 11

Force must have a horizontal component
F increases in magnitude
F acts at an angle towards the car

Question 12

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

Answer 12

Ball’s acceleration will decrease

Air resistance affects cards less

Question 13

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

Answer 13

Constant gradient

Question 14

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

Answer 14

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

Question 15

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

Answer 15

Air resistance depends on shape

Air resistance less on apple, therefore greater velocity

Question 16

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

Answer 16

Freefall is too fast

No accurate method to time freefall

Question 17

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

Answer 17

Draw a tangent

Question 18

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

Answer 18

Impulse - vector

Force - vector

Question 19

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

Answer 19

Weight

Question 20

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

Answer 20

The change in momentum of the body

Question 21

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

Answer 21

Kinetic energy

Question 22

A person is kicking a football, discuss the radial force on the knee joint before impact and during the impact (2)

Answer 22

Before impact: radial pull on knee joint due to centripetal acceleration of boot
During impact: radial pull reduced

Question 23

State the relationship between the force acting on a body and the body’s momentum (1)

Answer 23

Force is equal to the rate of change of momentum

Question 24

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

Answer 24

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 25

A deep space probe ejects a capsule explosively, and immediately after the probe moves at a greater speed. Discuss how conservation of momentum and energy apply in this instance (6)

Answer 25

Momentum: Momentum conserved because there are no external forces acting on the overall system During the explosion there are equal and opposite forces acting between the probe and the capsule Because momentum is a vector, the capsule must move along the original line of movement Energy: Total energy is always conserved in any physical process Energy may be converted from one form to another In the explosion, some chemical energy is converted to kinetic energy

Question 26

A ball rolls towards a footballer and is then kicked, if the ball had a higher initial speed and received the same impulse, discuss how it may differ (3)

Answer 26

Final speed would be lower Initial momentum would be greater Change in momentum is the same

Question 27

A motor is used to lift a metal block vertically at constant speed. Describe an experiment to check whether the speed is constant (2)

Answer 27

Mark out equal distances along height being raised | Measure time taken to travel each of these distances

Question 28

Explain how the output power of the motor is calculated, state any measurements made (3)

Answer 28

Find work done by motor from gain in potential energy of metal block Divide work done by time to find power Measurements: mass of block, height block has risen and time taken

Question 29

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

Answer 29

Place piece of card on trolley of measured length Card obscures light gate just before trolley strikes block Calculate speed from length of time obscured

Question 30

State an advantage and disadvantage of wind power in comparison to fossil fuel (2)

Answer 30

Causes no air pollution | Wind is intermittent

Question 31

Define density of a material (1)

Answer 31

Mass divided by volume

Question 32

State Hooke's law (2)

Answer 32

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

Question 33

A student must measure the mass of a rock sample using a steel spring, standard masses, stand and a metre rule. Describe how they could measure the rock's mass and state the measurements that must be made (6)

Answer 33

Measurements: Use a metre rule to measure the length of the spring, when it supports a standard mass and when it supports the rock Repeat for different standard masses Accuracy - method to reduce parallax Measurement use: Plot a graph of mass against length Read off mass corresponding to length due to the sample

Question 34

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

Answer 34

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

Question 35

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

Answer 35

Ratio of tensile stress to tensile strain | unit: Pa

Question 36

Define tensile stress (1)

Answer 36

Force per unit cross-sectional area

Question 37

Define tensile strain (1)

Answer 37

Extension per unit length

Question 38

A graph of force against extension for a wire is shown, outline how the results and other measurements can be used to determine Young's modulus (3)

Answer 38

Measure original length and diameter Determine gradient of linear section to obtain F/Δl E=F/Δl x l/π(d/2)^2

Question 39

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 39

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

Question 40

If a wire is used to raise an object, in what direction will the wire's force be? (1)

Answer 40

Against object's weight

Question 41

Discuss the changes that could be made to a wire to lift a load of twice the weight at the same acceleration as before without exceeding a certain value of strain (3)

Answer 41

Lifting double the load at the same acceleration requires double the force Double cross sectional area Use a similar wire with double the Young's modulus

Question 42

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 42

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 43

A ball is released from a height with horizontal velocity, explain the curve of the path (2)

Answer 43

Vertical speed increasing | Horizontal speed constant

Question 44

An arrow is fired from a bow, state and explain the effect of air resistance on the arrow's motion (2)

Answer 44

Opposes motion of arrow | Range reduced

Question 45

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 45

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

Question 46

An aerial wire is hung between two masts, high winds produce large amplitudes of vibration on the wire. Explain why the wire may sag when the high wind stops (2)

Answer 46

Copper may be stretched beyond elastic limit | Does not return to original length

Question 47

A child bounces vertically on a trampoline. Highest point reached is H. Describe the energy changes involved as they bounce from H and back to H. Consider energy losses (3)

Answer 47

GPE to KE and elastic potential then back to GPE Energy lost due to work done on air Work done by child on trampoline makes up for energy losses