Section 3 - Mechanics and Materials Exam Questions Flashcards
Forces in equilibrium, On the move, Newton's laws of motion, Force and momentum, Work, energy and power, Materials.
State two conditions for a rod supported by two wires to be in equilibrium (2)
Resultant force zero
Resultant torque about any point zero
State three vector quantities (3)
Velocity
Acceleration
Force
State three scalar quantities (3)
Speed
Distance
Mass
State the three forces that will act on a helicopter that is moving horizontally (3)
Lift
Air resistance
Weight
How can force vectors be arranged to show that an object has constant velocity? (2)
Closed triangle of vectors
Forces are in equilibrium
What is the difference between a vector quantity and a scalar quantity? (1)
Vector has a direction
Scalar does not
What is meant by centre of gravity? (1)
The point in a body where the weight of the object appears to act
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
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
State and explain a feature of a crane that helps prevent it from falling over when lifting heavy things (2)
Counterweight
Provides a sufficiently large moment in the opposite direction
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)
Force must have a horizontal component
F increases in magnitude
F acts at an angle towards the car
Why would a thin card give a more accurate measurement of g compared to dropping a tennis ball? (2)
Ball’s acceleration will decrease
Air resistance affects cards less
What feature of a speed against time graph would show a constant acceleration? (1)
Constant gradient
Why does an object first accelerate after being dropped then reach a terminal velocity? (4)
Weight greater than air resistance
Therefore resultant force downwards
Air resistance increases with speed
Until resultant force is zero
Why would an apple hit the ground first if dropped with a leaf? (2)
Air resistance depends on shape
Air resistance less on apple, therefore greater velocity
How can using an inclined plane rather than free fall produce valid data on investigating a falling object’s motion? (2)
Freefall is too fast
No accurate method to time freefall
On a distance against time graph, how would speed at a certain time/distance be worked out? (1)
Draw a tangent
For impulse and force, determine whether each one is a vector or a scalar (2)
Impulse - vector
Force - vector
The rate of change of momentum of a body in free fall under gravity is equal to its.. (1)
Weight
On a force against time graph, what is the area under the graph equal to? (1)
The change in momentum of the body
In an inelastic collision, which quantity is not conserved? (1)
Kinetic energy
A person is kicking a football, discuss the radial force on the knee joint before impact and during the impact (2)
Before impact: radial pull on knee joint due to centripetal acceleration of boot
During impact: radial pull reduced
State the relationship between the force acting on a body and the body’s momentum (1)
Force is equal to the rate of change of momentum
Use Newton’s laws of motion to explain how molecules of gas exert a force on the wall of a container (3)
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
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)
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
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)
Final speed would be lower
Initial momentum would be greater
Change in momentum is the same
A motor is used to lift a metal block vertically at constant speed. Describe an experiment to check whether the speed is constant (2)
Mark out equal distances along height being raised
Measure time taken to travel each of these distances
Explain how the output power of the motor is calculated, state any measurements made (3)
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
Describe how a trolley moving down a sloped track can have its speed measured experimentally (3)
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
State an advantage and disadvantage of wind power in comparison to fossil fuel (2)
Causes no air pollution
Wind is intermittent
Define density of a material (1)
Mass divided by volume
State Hooke’s law (2)
The force needed to stretch a spring is directly proportional to the extension up to the limit of proportionality
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)
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
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
Define a material’s Young modulus and state the units (2)
Ratio of tensile stress to tensile strain
unit: Pa
Define tensile stress (1)
Force per unit cross-sectional area
Define tensile strain (1)
Extension per unit length
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)
Measure original length and diameter
Determine gradient of linear section to obtain F/Δl
E=F/Δl x l/π(d/2)^2
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)
Plastic deformation has produced permanent extension
Gradient is the same because identical forces between bonds
If a wire is used to raise an object, in what direction will the wire’s force be? (1)
Against object’s weight
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)
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
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)
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
A ball is released from a height with horizontal velocity, explain the curve of the path (2)
Vertical speed increasing
Horizontal speed constant
An arrow is fired from a bow, state and explain the effect of air resistance on the arrow’s motion (2)
Opposes motion of arrow
Range reduced
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)
Increasing mass: Reduces range More energy used accelerating user to final speed Increasing speed: Reduces range Air resistance increases with speed
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)
Copper may be stretched beyond elastic limit
Does not return to original length
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)
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
