5.6 Forces and Motion

Question 1

does a distance quantity require a specific direction? i.e. is it a scalar or vector quantity?

Answer 1

no specific direction is required so it is a scalar quantity.

Question 2

if an object moves 3 metres to the left and then 3 metres back to its initial position, what is the object’s total displacement?

Answer 2

• the object has zero displacement
• displacement is a vector quantity so it also involves direction
• the object starts and ends at the same point

Question 3

how can the distance travelled by an object be calculated from a velocity-time graph?

Answer 3

it is equal to the area under the graph.

Question 3

state the typical value for the speed of sound.

Answer 3

330 m/s

Question 3

what is a typical value for human walking speed?

Answer 3

1.5 m/s

Question 3

what is a typical value for human running speed?

Answer 3

3 m/s

Question 3

state the equation linking distance, speed and time.

Answer 3

distance = speed x time
distance (m), speed (m/s), time (s)

Question 3

what is a typical value for human cycling speed?

Answer 3

6 m/s

Question 3

how can speed be calculateed from a distance-time graph?

Answer 3

the speed is equal to the gradient of the graph.

Question 4

why can an object travelling at a constant speed in a circle not have a constant velocity?

Answer 4

• speed is a scalar quanity
• velocity is a vector quantity which means it can only be constant if the direction is constant
• in circular motion, the direction is continuously changing

Question 5

what must be done to calculate speed at a given time from a distance-time graph for an accelerating object?

Answer 5

• drawing a tangent to the curve at the required time
• calculating the gradient of the tangent

Question 5

what is inertia?

Answer 5

the tendency of an object to continue in its state of rest or uniform motion.

Question 6

state the equation for the average acceleration of an object.

Answer 6

acceleration = (charge in velocity)/(time taken)
acceleration (m/s^2), velocity (m/s), time (s)

Question 6

what can be said about the resultant force acting on an object when it is falling at terminal velocity?

Answer 6

• the resultant force is zero
  -when at terminal velocity, the object is moving at a constant speed and so ins’t accelerating

Question 7

what is inertial mass?

Answer 7

• a measure of how diffidualt it is to change a given object’s velocity
• the ratio of force over acceleration

Question 7

give an approximate value for the acceleration of an object in free fall under gravity near the earth’s surface.

Answer 7

9.8 m/s^2

Question 8

if an object changes direction but remains at a constant speed, is there a resultant force?

Answer 8

since there is a change in direction, there is a change in velocity and so there must be a resultant force.

Question 8

state newton’s first law for a stationary object.

Answer 8

if the resultant force on a stationary object is zero, the object will remain at rest.

Question 9

state newton’s first law for a moving object.

Answer 9

if the resultant force on a moving object is zero, the object will remain at constant velocity (same speed in same direction).

Question 9

state newton’s second law in words.

Answer 9

an object’s acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass.

Question 9

statethe defining equation for newton’s second law.

Answer 9

resultant force = mass x acceleration
F = ma

Question 9

what can be said about the braking forces and driving forces when a car is travelling at constant velocity?

Answer 9

the braking forces are equal to the driving forces.

Question 10

what is the symbol used to represent an approximate value?

Answer 10

∼

Question 11

what is the stopping distance of a vehicle equal to?

Answer 11

the sum of thinking distance and braking distance.

Question 11

state newton’s third law.

Answer 11

whenever two objects interact, the forces that they exert on each other are always equal and opposite.

Question 12

for a given braking distance, if the vehicle’s speed is increased, what can be said about its stopping distance?

Answer 12

the stopping distance is increased with an increase in speed.

Question 12

give a typical range of values for human reaction time.

Answer 12

0.2 seconds - 0.9 seconds

Question 12

give two factors which may affect braking distancel.

Answer 12

1. adverse (wet/icy) road conditions
2. poor tyre/brake conditions

Question 13

describe the energy transfers that take place when a car applied its brakes.

Answer 13

• work is done by the friction force between the brakes and wheel
• kinetic energy of the wheel is converted to heat and is dissipated to the surroundings through the brake discs

Question 13

give three factors which can affect a driver’s reaction time.

Answer 13

1. tiredness
2. drugs
3. alcohol

Question 14

to stop a car in a given distance, if its velocity is increased, what must happen to the braking force applied?

Answer 14

the braking force must also be increased.

Question 15

state two consequences of a vehicle undergoing very large decelerations.

Answer 15

1. kinetic energy converted to heat is very high causing brakes to overheat
2. loss of control of the vehicle