P8

Question 1

What is the difference between a vector and scalar quantity ?

Answer 1

vectors have magnitude AND direction, whilst scalars only have magnitude

Question 2

Give 3 examples of both vector and scalar quantities

Answer 2

vector
- displacement
- velocity
- force

scalar
- time
- speed
- distance

Question 3

What is Newton’s third law ?

Answer 3

When two objects interact with one another, they exert equal and opposite forces on one another

Question 4

What effect can forces have on objects ?

Answer 4

• change its shape (deform it)
• change its motion
• change its state of rest

Question 5

What is resultant force ?

Answer 5

a single force that has the same effect as all of the forces acting on an object together

Question 6

What happens to an object if the resultant force is:
(a) zero
(b) more than zero

Answer 6

zero
- object stays at the same speed and direction or stays at rest

more
- object changes direction and/or speed

Question 7

What is Newton’s first law of motion ?

Answer 7

if the forces acting on an object are balanced:
- (if moving) continues in the same direction and speed
- (if at rest) stays at rest

Question 8

What is a moment and how can it be increased ?

Answer 8

The turning effect of a force centred around a pivot
- more force = bigger turning effect
- using a lever = bigger turning effect (acts as a “force multiplier”, meaning that less effort must be applied for a bigger force to act on the pivot)

Question 9

Give the equation for a moment of a force

Answer 9

moment (Nm) = force (N) x distance perpendicular from the line of action of the force to the pivot (m)

Question 10

What do levers and gears do ?

Answer 10

• act as force multipliers
  [] less effort for more turning effect
• transmit the turning effect of a force between two points

Question 11

How do gears work to transmit the turning effect of a car’s engine to its wheels (in general) ?

Answer 11

• gear A is connected to the engine, driven by it via an axle
  [] interlocks with gear B and drives it in turn
• gear B connected to the wheels via another axle
• gear B turns in the opposite direction to gear A and transmits the turning effect of the engine to the wheels
• the scale factor of the radius of gear B in relation to gear A determines what the turning effect of the engine is multiplied by
  [] eg. if gear B had a radius twice that of gear A, the turning effect from the engine would be doubled when reaching the wheels; in this case, gear A would also have to rotate fully twice to match one rotation of gear B, so both gears do the same amount of work

Question 12

How do the gears of a car work to transmit the engine’s turning effect to the wheels when a LOW GEAR is chosen ?

Answer 12

• small gear A drives larger gear B
• gear B rotates slowly BUT multiplies the turning effect of the engine
• low gear gives a lot of turning effect, but low speed

Question 13

How do the gears of a car work to transmit the engine’s turning effect to the wheels when a HIGH GEAR is chosen ?

Answer 13

• large gear A rotates small gear B
• gear B rotates faster than gear A BUT the turning effect is diminished
• high gear gives low turning effect and high speed

Question 14

What is the centre of mass of an object ?

Answer 14

the point at which the object’s mass can be thought of as concentrated

Question 15

What is suspended equilibrium ?

Answer 15

when a swinging object eventually always comes to rest with its centre of mass directly below the point of suspension

Question 16

Where is the centre of mass on a flat, symmetrical object ?

Answer 16

where its axes of symmetry meet (or in the middle of its axis of symmetry if there is only one)

Question 17

Describe how to find the centre of mass on an irregularly shaped piece of card

Answer 17

• put a hole in one corner of the card and suspend it on a clamp stand
• put a plumbline in front of it and mark the vertical line on the card created by the plumbline
• suspend the card from a different corner and repeat the process
• where the two straight lines meet, the centre of mass is

Question 18

What is a balanced moment ?

Answer 18

When anti-clockwise and clockwise moments (moments acting in opposite directions) are balanced and equal to one another
- eg. two people sitting on a level seesaw (not one up/one down)

Question 19

Why do objects topple when tilted ?

Answer 19

• objects weight acts straight downwards from its centre of mass
• as it is tilted, there comes a point where the force’s line of action from the object is partially outside of the base of the object, causing it to topple

Question 20

What is the principle of moments ?

Answer 20

if an object does not turn, the sum of all clockwise moments about a point = sum of all anti-clockwise moments about the same point

all the objects that DON’T run through this point may turn the object

Question 21

Draw a parallelogram of forces to determine the maximum tension when a tow rope is attached to a car at two points, 0.8m apart, at a 30 degree angle to one another, and the tug on each should not exceed 3000N.

Answer 21

• basically calc. the resultant force using the parallelogram of forces
• diagram should look like a parallelogram obvs.
• scale needed; eg. 1cm:500N (in this case, each section of rope would be 6cm long)
• angle between first two lines branching outwards should be 30 degrees
• resultant force is from the left and right corners of the parallelogram
  [] should be 5800N

Question 22

A car of weight 10kN is parked on an inclined plane at an angle of 10 degrees. Determine the horizontal and vertical components of this force

Answer 22

• draw slope at specified angle
• determine an appropriate scale
  [] 1cm:1000N
• draw a box on the slope representative of the car
• draw downwards force of 10 kN to scale using a ruler
• join the downwards force to the slope so it meets at a right angle (use a protractor)
• measure and calculate the length/magnitude of the force parallel to the slope