3.4 Mechanics and materials Flashcards
What is the definition of a newton?
The force needed to give a 1kg mass an acceleration of 1ms^-1.
What is the cause of acceleration in terms of forces?
Unbalanced forces that produce a resultant force.
What is terminal velocity?
Maximum velocity reached by an object falling through a fluid where weight = drag. So, there’s no acceleration.
How do you calculate the distance if work done and force aren’t parallel?
scosx. This figures out displacement in the direction of work done.
What is the relationship between energy transferred and work done?
They are the same.
What is the area under a force- displacement graph equal to?
Work done
How do you calculate efficiency?
Useful output power/ input power
What happens when an object is lifted?
Work done is transferred to the gravitational potential store of the object.
The force required is equal to the weight of the object.
Why is work done not equal to energy transferred sometimes?
Some work has been done against frictional forces, energy is transferred to thermal energy in the surroundings.
What is the principle of conservation of energy?
Energy can’t be created or destroyed, only transferred.
So, in a closed system, energy remains constant.
What is power?
The rate at which energy is transferred.
What is the definition of momentum?
The product of mass and velocity.
What is Newton’s first law?
A body will remain at rest or continue to move at a constant velocity unless acted upon by an external force.
What is Newton’s second law?
The acceleration of an object is proportional to the resultant force acting on it and inversely proportional to the mass. (F=ma)
What is Newton’s third law?
If object A exerts a force on object B, then object B exerts an equal in magnitude and opposite in direction force on object A. (Forces come in Newton pairs)
What is the principle of conservation of momentum?
In a closed system, total momentum before and interaction = total momentum after interaction. (Interactions meaning collisions or explosions)
What is a perfectly elastic collision?
A collision in which kinetic energy is conserved.
What is an inelastic collision?
Some kinetic energy isn’t conserved. Some is lost as heat to the surroundings.
What is impulse?
The change in momentum. Impulse=FΔt. It can be calculated from the area under a force-time graph. So, FΔt =Δmv or FΔt=mΔv if mass is constant.
How do you figure out the resultant force of 2 perpendicular vectors?
Draw all the forces tip to tail and then calculate the magnitude and direction using trigonometry.
How do you calculate the horizontal and vertical components of a force?
For the side adjacent to the angle, do Force*cosA
What will the free body diagram for an object in equilibrium look like?
It will form a closed triangle.
How do you calculate the friction and reaction of the inclined plane?
Calculate the vertical and horizontal components of the weight using the angle between the horizontal and the plane.
What type of force is a moment?
It is a vector. The direction can be either clockwise or anticlockwise.
What is the definition of a moment?
force × perpendicular distance between the lines of action of the forces.
What is the principle of moments?
For an object in rotational equilibrium,
Total sum of clockwise moments= Total sum of anticlockwise moments
How do we calculate the moment of a force from a diagram where the force isn’t long enough to find the perpendicular to the pivot?
Moment= d(fcosx) This is basically finding the vertical component of the force. And then substituting it into the equation m=fd
What is a couple of forces?
A pair of equal and opposite coplanar forces separated by a perpendicular distance.
What is the moment of a couple?
force x perpendicular distance between the lines of action of the forces.
What is the centre of mass?
The point through which a single force on the body has no turning effect. The point at which the weight appears to act.
What is the definition of displacement?
Distance in a given direction.
What is the definition of velocity?
A change of displacement per unit time.
What is acceleration?
Change of velocity per unit time.
What is uniform acceleration?
The velocity of an object moving along a straight line changes at a constant rate.
What does the gradient tell us on a velocity-time graph?
The acceleration
Review acceleration-time, distance-time and velocity-time graphs for a bouncing ball.
How are we able to calculate the time taken for an object thrown upwards to come back down?
Horizontal and vertical motion are independent of each other, other than the fact that they both last for the same time. Horizontal motion has a constant velocity and vertical motion has the constant acceleration of 9.81.
How do you calculate the range of an object thrown at an angle?
- Split the launch velocity into its components.
• Then use SUVAT to calculate the time required to reach the highest point and double this value for total time of flight.
• Using the horizontal component of launch velocity and time, you can calculate the range.
What is the effect of air resistance on the projectile?
The range is shorter.
The entire trajectory is below the original.
The initial launch angle remains the same.
Explain why an inclined plane rather than freefall would produce more valid results in the freefall experiment?
Freefall is too quick and there is no accurate measuring method.
What is Hooke’s law?
A spring where extension is directly proportional to the force applied.
What is elastic potential energy?
Energy stored in an elastic material.
What happens to the spring constant when a force is applied to 2 identical springs in series?
The spring constant is halved.
What happens to the spring constant when a force is applied to 2 identical springs in parallel?
The spring constant is double that of a single spring.
What is stress?
Force per unit (cross-sectional) area
What is strain?
A measure of how much an object has extended per unit of length.
What is the equation used to calculate strain?
(Extension) / (original length) * 100
What is young modulus?
It a property of a material which is a measure of stiffness. The higher the young modulus, the more force is required to deform.
What is the elastic limit?
If you add more load to an object beyond this point, it will plastically deform.
What is elastic deformation?
The material returns to its original shape and size once the forces are removed.
What is plastic deformation?
The material doesn’t return to its original shape and size once the forces are removed. It is permanently deformed.
What is the spring constant?
A measure of stiffness.
What is the relationship between work done and elastic strain energy?
Before the elastic limit is reached. all this work done in stretching is stored as elastic strain energy in the material.
What is breaking stress?
The effect of stress is pulling the atoms apart. Breaking stress is when the atoms are completely separated and the material breaks.
What is ultimate tensile stress?
The maximum stress a material can withstand before fracturing.
What is the equation for elastic strain energy?
1/2 Fe
What is a ductile material?
It undergoes a lot of plastic deformation before fracture. E.g copper
What is a brittle material?
A material that undergoes little to no plastic deformation before fracture.e.g glass, ceramic
Rifle B is fired and the bullet emerges with a smaller horizontal velocity that the bullet from rifle A.
Explain why the horizontal distance travelled by bullet B will be less that bullet A.
Their vertical motion is independent of their horizontal motion.
Bullets A and B will be in the air for the same time.
(Horizontal acceleration is zero and thus horizontal) distance is proportional to horizontal speed
In practice, why is range shorter than calculated?
There is air resistance
Which causes horizontal deceleration
An apple and a leaf fall from a tree at the same height and time. Explain why the apple hits the ground first.
air resistance depends on shape
air resistance less significant (1) air resistance less, therefore greater velocity
[or average velocity greater or accelerates for longer]
What is the equation for the young modulus?
(Tensile stress)/ (tensile strain)
What is lift?
Lift is an upward force due to the collisions with air particles on the underside of the wing.
(The wing of a plane pushes air particles downward as it flies forward.As a result of this downward force on the air particles, there is an equal and opposite upward force on the wing.)
What increases the drag on an object?
Increased surface area
Less streamlined
Increase speed
What is the definition for equilibrium?
Net / resultant force on an object is zero.
Sum of the clockwise moments about a point equals the sum of the anticlockwise moments about a point.
What is the definition of a joule?
Energy transferred when 1 newton is applied over 1 metre.
What is the definition of a watt?
When 1 joule is transferred in 1 second.
What is the effect of speed on air resistance?
As speed increases, air resistance increases.
