mechanics L6 Spec Points

Question 1

Newtons first law of motion

Answer 1

Every object will remain at rest or constant speed in a straight line unless a force acts on it

Question 2

Newtons second law of motion

Answer 2

F=ma
A larger net force acting on an object causes a larger acceleration and objects with larger mass require more force to accelerate

Question 3

Core practical 1: determine the acceleration of a free-falling object. EM method
Variables and apparatus

Answer 3

Independent variable: height
Dependent variable: time
Control variables: mass of ball, same EM and trap door switching system
Apparatus: metre rule, ball bearing, EM, electronic timer, trapdoor, plumb line

Question 4

Core practical 1: determine the acceleration of a free-falling object. EM method
Method

Answer 4

• use plumb line to find vertical drop and position trap door switch directly underneath the EM
• check ball bearing triggers both the trap door switch and the timer when it is released
• ball drops and timer starts when current to magnet switches off
• timer stops when ball hits trapdoor
• measure distance from bottom of ball bearing to trap door switch with metre ruler
• increase height and repeat experiment
• at least 5-10 values for h should be used
• repeat this experiment at least three times for each value of h and calculate an average t for each

Question 5

Core practical 1: determine the acceleration of a free-falling object. EM method
Analysis of results

Answer 5

• acceleration is found by using the SUVAT equation: s=ut+1/2at^2
• plot straight line graph s=1/2at^2
• gradient - 1/2g

Question 6

Core practical 1: determine the acceleration of a free-falling object. EM method
Errors

Answer 6

• residue magnetism after EM is switched off may cause t to be recorded as longer than it should be
• large uncertainty in h from using metre rule
• parallax error from reading h
• ball may not fall accurately down centre of trap door

Question 7

Core practical 1: determine the acceleration of a free-falling object. EM method
Safety

Answer 7

• EM requires current
  —> no water near
  —> only switch on current to EM once everything is set up
• tall clamp stand needs to be attached to a surface with a G clamp to keep it stable

Question 8

Core practical 1: determine the acceleration of a free-falling object. Light gate method
Variables and apparatus

Answer 8

Independent: height
Dependent: final velocity
Control: length of dowel
Apparatus: metre ruler, clear tube with large enough diameter for dowel to fall cleanly through it, dowe, light gate, data logger, plumb line

Question 9

Core practical 1: determine the acceleration of a free-falling object. Light gate method
Method

Answer 9

• clamp the clear tube vertically using the plumb line as a guide
• attach the light gate about 20cm above the bench
• clamp metre ruler vertically next to the tube so that the vertical distance from the top of the tube to the light gate can be accurately measured
• record distance between the light gate and the top of the tube as height, h
• enter length of dowel into data logger
• release dowel from top of tube
• data logger records velocity
• repeat this measurement from same height two more times
• move light gate up by 5cm, record new height, h and drop dowel three more times
• repeat for five more values of height

Question 10

Core practical 1: determine the acceleration of a free-falling object. Light gate method
Analysis of results

Answer 10

Use v^2 = u^2 + 2αs
v^2 = 2as (because u is 0)

v^2 on y-axis
2h on x axis
gradient = a (g)

Question 11

Core practical 1: determine the acceleration of a free-falling object. Light gate method
Errors

Answer 11

• metre tube needs to be fixed vertically
• all height measurements are taken at eye level to avoid parallax errors
• large uncertainty in h from using a metre rule
• ## dowel may touch sides of tube, slowing it down

Question 12

Newtons third law of motion

Answer 12

If object A exerts a force on object B, then object B exerts an equal and opposite force of the same type on object A

Question 13

Linear momentum relating to newtons laws of motion

Answer 13

Newtons third law states that for every action there is an equal and opposite reaction. This law applies to linear momentum because when two objects collide, the total momentum of the system is conserved. The momentum of one object will be transferred to the other object in an equal and opposite direction.

Question 14

What is the principle of conservation of linear momentum

Answer 14

States that if two objects collide, then the total momentum before and after the collision will be the same if there is no external force acting on the colliding objects.

Momentum is always conserved in any interaction where no external forces act

Question 15

Relating the principle of conservation of linear momentum to newton’s third law

Answer 15

Using newton’s third law, you can see that the principle of conservation of linear momentum can also be expressed by saying that the sum of the change in momentum of the objects is zero. N3L states for each force experienced by an object, the object exerts an equal and opposite force. So for two objects in a collision, the force exerted by one onto two is equal and opposite to the force exerted by two onto one
F1 = -F2

Question 16

Principle of moments

Answer 16

For an object in equilibrium, the sum of anticlockwise moments about a pivot is equal to the sum of clockwise moments.

Question 17

Centre of gravity

Answer 17

Point at which gravity appears to act

Question 18

Centre of gravity if object is uniform

Answer 18

Exactly at its centre

Question 19

How to find centre of gravity if object is irregular

Answer 19

1. Attach the object and a plumbline (piece of string with a weight attached to it) to a clamp stand from the same point, so that the plumbline overlaps the object.
2. Wait until the object is stable, then draw a line on the object where the plumbline lies.
3. Repeat the above step at least two more times. The intersection point between all three lines is the centre of gravity.

Question 20

Explain how terminal velocity is reached

Answer 20

E.g. a skydiver
As they leave the plane, they accelerate because their weight is greater than the air resistance acting on them.
As the skydiver’s speed increases, the magnitude of air resistance also increases. This continues until the force of weight and air resistance become equal, where terminal velocity is reached.
No resultant force so no acceleration, so object travels at a constant velocity.

Question 21

Define work done

Answer 21

the force causing a motion multiplied by the distance travelled in the direction of the motion

Question 22

Why doesn’t the initial KE given to a ball not equal the maximum GPE when the ball has stopped mid-air.

Answer 22

Work is being done by the ball to work against resistive forces

Question 23

Define power

Answer 23

Rate of energy transfer

Question 24

The rate of energy transfer is equal to

Answer 24

The rate of doing work