Section 4 - Mechanics

Question 1

How can you add vectors together?

Answer 1

1. Scale drawings - draw the arrows tip-to-tail, and draw the resultant vecotr from the tail of the first to the tip of the last and measure its length and angle
2. Pythagorus and trigonometry

Question 2

How can you resolve a vector?

Answer 2

Horizontal componenet Vx= vcosθ
Veritcal component Vy = vsinθ

Question 3

What should a free body diagram include?

Answer 3

All the forces that act on the body, but not the forces it exerts on the rest of the world
The forces are vector quantities so the arrow labels should show the size and direction of hte forces

Question 4

What is equilibrium in forces?

Answer 4

No resultant forces
A body can be at rest or moving with a constant velocity

Question 5

What is the moment of a force?

Answer 5

The turning effect of a force, it depends on the size of the force and the distance the force is applied form the turning point

Question 6

What is the principle of moments?

Answer 6

For a body to be in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about the same point

Question 7

What is a couple?

Answer 7

A pair of coplanar forces - parallel to each other and in opposite directions, they don’t cause any resultant linear force, but they do produce a turning effect

Question 8

What is the centre of mass?

Answer 8

The single point that you can consider its whole weight to act through (whatever its orientation), the object will always balance around this point, although in some cases the centre of mass will fall outside the object

Question 9

How can you find the centre of mass by symmetry?

Answer 9

1. The centre of mass of any regular shape is at its centre where the lines of symmetry will cross
2. The centre of mass is halfway through the thickness of the object at the point where the lines meet

Question 10

How to find the centre of mass by experiement

Answer 10

1. Hang the object freely from a point
2. Draw a vertical line downwards from the point of suspension - use a plumb blob to get your line exactly vertical
3. Hang the object from a different point
4. Draw another line vertically down
5. The centre of mass is where the 2 lines cross

Question 11

How does an obect’s stability depend on its centre of mass?

Answer 11

1. An object will topple over if a vertical line drawn downwards from its centre of mass falls outside its base area - this becomes a resultant moment which provides a turning force
2. An object will be stable if it has a low centre of mass and a wide base area
3. The higher the centre of mass and the smaller the base area, the less stable the object is

Question 12

What is instantaneous velocity?

Answer 12

The speed/velocity of an object at any given point in time - to find on a graph, draw a tangent to the curve at that point and find its gradient

Question 13

What information can a displacement-time graph give?

Answer 13

Velocity - gradient (average is just the total distance/time)

Question 14

What information can a velocity-time graph give?

Answer 14

Acceleration - gradient
Distance travelled - area under

Question 15

What information can an acceleration-time graph give?

Answer 15

Acceleration - height
Change in velocity - area
Constant velocity - a=0 (crosses x axis)

Question 16

How can you draw displacement-time or velocity-time graphs using ICT?

Answer 16

By using an ultrasound position detector

Question 17

What are the advantages of using data loggers?

Answer 17

1. The data is more acurate - don’t have to allow for human reaction times
2. Automatic systems have a much higher sampling rate than humans - can take a reading 10 times every second
3. Can see the data displayed in real time

Question 18

What is free fall?

Answer 18

• The motion of an object undergoing an acceleration of g
• The only force acting on an object in free fall is its weight
• Objects can have an initial velocity in any direction and still undergo free fall as long as the force providing the initial velocity is no longer acting

Question 19

How was it proven that objects in free fall accelerate at the same rate?

Answer 19

1. Galileo set up systematic and rigorous experimetns that could be repeated and the results described mathematically and compared
2. He rolled a ball down a slope to slow the balls fall and reduce the effect of air resistance - by rolling the ball down different fractions of the slope, he found that the distance travelled was proportional to the square of time taken; the ball was accelerating at a constant rate
3. Newton managed to explain why all free falling objects have the same acceleration - mathematically showed that all objects are attracted towards the earth due to gravity

Question 20

How can you demonstrate free fall in an experiment?

Answer 20

https://www.savemyexams.com/a-level/physics/aqa/17/revision-notes/4-mechanics–materials/4-3-equations-of-motion/4-3-7-required-practical-determination-of-g/
1. Set up the apparatus by attaching the electromagnet to the top of a tall clamp stand. Do not switch on the current till everything is set up
2.Place the glass tube directly underneath the electromagnet, leaving space for the ball-bearing. 3. Make sure it faces directly downwards and not at an angle
4. Attach both light gates around the glass tube at a starting distance of around 10 cm
5. Measure this distance between the two light gates as the height, h with a metre ruler
6. Place the cushion directly underneath the end of the glass tube to catch the ball-bearing when it falls through
7 Switch the current on the electromagnet and place the ball-bearing directly underneath so it is attracted to it
8. Turn the current to the electromagnet off. The ball should drop
9.When the ball drops through the first light gate, the timer starts
10. When the ball drops through the second light gate, the timer stops
11. Read the time on the timer and record this as time, t
Increase h (eg. by 5 cm) and repeat the experiment. At least 5 – 10 values for h should be used
12.Repeat this method at least 3 times for each value of h and calculate an average t for each

Question 21

How can you reduce errors in the determination of g experiment?

Answer 21

• Use a small and heavy ball bearing - assume air resistance is so small it can be ignored
• Having a computer automatically release and time the ball bearing’s fall can measure times with a smaller uncertainty than if you used a stopwatch
• The biggest error is in the measurment of h where there’ll be an uncertainty of about ±1mm

Question 22

How to process the data in the determination of g experiment

Answer 22

1. Plot a graph of height against time squared, then draw a line of best fit
2. The gradient of the line is equal to g/2
3. If you have to draw a displacement-time graph, the gradient is the acceleration and therefore g

Question 23

How to deal with projectile motion

Answer 23

1. Consider the vertical motion first, using the constant acceleration equaions
2. Do the horizontal motion after using s=d/t

Question 24

How to deal with projectile motion if it starts off at an angle

Answer 24

1. Resolve the initial velocity into horizontal and vertical componenets
2. Use the vertical component to work out how long it’s in the air and/or how high it goes
3. Use the horizontal component to work out how far it goes horizontally whilst it’s in the air

Question 25

What is Newton’s first law of motion?

Answer 25

The velocity of am object will not change unless a resultant force acts on it - a body will stay still or move in a straight line at a constant speed, unless there’s a resultant force acting on it

Question 26

What is Newton’s second law of motion?

Answer 26

F=ma
1. The more force you have acting on a certain mass, the more acceleration there is
2. For a given force, the more mass you have, the less acceleration there is

Question 27

What is Newton’s third law of motion?

Answer 27

If body A exerts a force on body B, then body B will exert a force on body A of equal magnitude but in the opposite direction

Question 28

What is fluid friciton/drag?

Answer 28

1. Fluid - a liquid or a gas that flows
2. The force depends on the thickness or viscosity of the fluid
3. It increases as the speed increases
4. It also depends on the shape of the object moving through it - the larger the area pushing agaisnt the fluid, the greater the resistance force
5. A projectile is slowed down by air resistance, your answer will be too loud if you don’t consider air resistance

Question 29

What should you always remember about frictional forces?

Answer 29

1. They always act in te opposite direction to the motion of the object
2. They can neber speed things up or start something moving
3. They convert kinetic energy into heat and sound

Question 30

What is lift?

Answer 30

An upwards force on an object moving through a fluid - happens whne the shape of an object causes the fluid flowing over it to change direction and the force acts perpendicular to the direction the fluid flows in

Question 31

How do you reach terminal speed?

Answer 31

1. The object accelerates from rest using a constant driving force
2. As the speed increases, the resistive forces increase, this reduces the resultant force on the object and hence reduces the acceleration
3. Eventually, the object reaches a speed where the resistive forces are equal to the driving force - no resultant force, no acceleration adn the object continues at the same velocity

Question 32

What fators affect a vehicle’s maximum speed?

Answer 32

1. Increasing the driving force
2. Reducing the frictional forces

Question 33

What does a v-t graph of a skydiver look like?

Answer 33

https://www.savemyexams.com/a-level/physics/ocr/17/revision-notes/3-forces–motion/3-3-dynamics/3-3-6-terminal-velocity/

Question 34

What is the conservation of momentum?

Answer 34

1. Assuming no external forces act, momentum is always conserved
2. Total momentum of 2 objects before they collide = total momentum after the collision

Question 35

What is an elastic and inelastic collision?

Answer 35

Elastic collision - momentum and kinetic energy is conserved
Inelastic collision - momentum is conserved but some kinetic energy is converted into other forms during the collision