Section 3 (mechanics and materials)

Question 1

What is Hooke’s law?

Answer 1

The extension of a stretched wire (🔺l) is proportional to the load or force (F).

F=k🔺l

A graph that obeys Hooke’s law will be a straight line through the origin.

Question 2

What is meant by the elastic limit of a material?

Answer 2

If you increase the force past the elastic limit the material will be permanently stretched.

Question 3

What is meant by plastic behaviour?

Answer 3

If a deformation is plastic the material is permanently stretched so when the load is removed the atoms don’t move back to their original position.

Question 4

What is tensile stress, tensile strain and breaking stress?

Answer 4

Tensile stress= F/A
Tensile strain= 🔺l/ l
Breaking stress- the stress that’s big enough to break the material.

Question 5

How do you work out the elastic strain energy stored in a stretched material?

Answer 5

Use the area under a force-extension graph.

Energy stored=1/2F🔺l

Question 6

How is energy conserved in elastic stretches?

Answer 6

Work done stretching the material, elastic strain energy, another energy
If it’s in a spring… Kinetic energy, gravitational potential energy

Question 7

How is energy conservation used in transport design?

Answer 7

Cars have crumple zones that deform plastically in a crash. The car’s kinetic energy is transferred into changing the shape of the vehicle’s body so less transfers to the people inside.

Question 8

What is the formula for the Young modulus?

Answer 8

E= resolve stress/ tensile strain= FL/A🔺L

Question 9

What is the yield point?

Answer 9

Where the material begins stretching and plastically deforming without any extra load.

Question 10

Why are brittle materials brittle?

Answer 10

They have a rigid structure so the atoms within them can’t move to prevent cracks.

Question 11

What is density?

Answer 11

Measure of the ‘compactness’ of a substance.

density (kg/m^3)= m (kg)/ v (m^3)

Question 12

What is a vector?

What is a scalar?

Answer 12

A scalar has no direction (mass, time).

A vector has magnitude and direction (velocity, displacement, acceleration).

Question 13

What conditions are needed for a body to be in equilibrium?

Answer 13

The forces must all be balanced so there is no resultant force. The object can be at rest or moving with a constant velocity.

Question 14

What equation do you use to calculate the moment of a force about a point?

Answer 14

M (in Nm) =Fd

Question 15

What is the principle of moments?

Answer 15

For a body to be in equilibrium, the sum of the clockwise moments about any point equals the sum of the anti-clockwise moments. If they are unbalanced there will be a resultant force and the object will turn.

Question 16

What is a couple?

Answer 16

A couple is a pair of forces of equal size that act parallel to each other but in opposite directions. They don’t cause a resultant linear force but produce a moment.

Question 17

What is meant by the centre of mass of an object?

Answer 17

It is where the mass of the object is thought to be concentrated.

Question 18

Where is the centre of mass in a uniform regular solid?

Answer 18

In the centre.

Question 19

What is meant by displacement, velocity and acceleration?

Answer 19

Displacement- how far an object has travelled from its starting point in a direction.
Velocity- speed in a given direction.
Acceleration- the rate of change of an object’s velocity.

Question 20

What are the equations for uniform acceleration?

Answer 20

v=u+at
s=(u+v)t
       2
s=ut+1/2at^2
v^2=u^2+2as

Question 21

Describe a displacement-time graph.

Answer 21

d|
  |        |
  |       /
  |     /
  |_/\_\_\_\_\_\_\_\_\_\_t
The gradient is the acceleration. The steeper the gradient, the bigger the acceleration.

Question 22

Describe a velocity-time graph.

Answer 22

v|
| /
| /
| /
|_/__________t
The gradient is the acceleration. The steeper the gradient, the greater the acceleration. The area underneath represents the displacement.
If the acceleration is non-uniform the gradient will be a curve.

Question 23

Describe an acceleration-time graph.

Answer 23

a|\                     acceleration 
  |  \      
  | \_\_\\_\_\_\_\_\_\_\_\_ t
  |              \
  |                \     deceleration 

The area is the velocity.

Question 24

What are Newton’s three laws of motion?

Answer 24

The velocity of an object will not change unless a resultant force acts on it.
The acceleration of an object is proportional to the resultant force acting on it.
Every action has an equal and opposite reaction.

Question 25

What is meant by freefall?

Answer 25

It is where there’s gravity acting Oman object and nothing else.
All objects in freefall will fall to the ground at the same acceleration.

Question 26

How does air resistance affect projectile motion?

Answer 26

Air resistance causes a drag force acts in the opposite direction to motion and affects the trajectory of a projectile. Drag reduces the horizontal and vertical distance the projectile travels.

Question 27

What 3 things do you need to know about frictional forces?

Answer 27

They always act in the opposite direction to the motion of the object.
They can never speed something or start something moving.
They convert kinetic energy into heat.

Question 28

What is lift?

Answer 28

An upwards force perpendicular to fluid flow.

Question 29

What is meant by terminal velocity?

Answer 29

It happens when the frictional forces equals the driving force.
Air resistance increases with speed- this affects the top speed of vehicles.
An object travelling through a gas or liquid will reach terminal velocity.

Question 30

What is the equation for momentum?

Answer 30

p=mv

Question 31

___ momentum is always conserved in a collision when no ____ ____ __.

Answer 31

Linear momentum is always conserved in a collision when no external forces act.

Question 32

When is kinetic energy conserved? In an elastic or inelastic collision?

Answer 32

Kinetic energy is conserved in elastic collisions but not in inelastic collisions.

Question 33

What is impulse?

Answer 33

It is equal to the change in momentum.

It is the area under a force-time graph.

Question 34

What are the equations for work done?

What is the rate of work done?

Answer 34

W=Fs
W=Fscos0
The rate of doing work is the rate of energy transfer from one form to another.
The area under a force-displacement graph represents work done.

Question 35

What is the principle for the conservation of energy?

Answer 35

Energy cannot be created or destroyed. Energy can be transferred from one form to another but the total amount of energy in a closed system will not change.