motion and forces

Question 1

what is the difference between a scalar and vector quantity

Answer 1

vector quantities have a direction and magnitude

scalar quantities only have a magnitude

Question 2

give examples of each quantity

Answer 2

vector: force, velocity, displacement, weight, acceleration, momentum
scalar: time, speed, distance, mass, energy, temperature

Question 3

give the equation relating speed distance and time

Answer 3

d = sxt

Question 4

typical speeds for walking, running and a car in a built up area

Answer 4

w = 1.4m/s 
r = 3m/s
c = 13m/s

Question 5

define acceleration in terms of velocity and time

Answer 5

acceleration is the change in velocity in a certain amount of time

Question 6

give the equation for average acceleration

Answer 6

a = (v-u) / t

Question 7

what does the gradient represent for a distance/time graph

Answer 7

gradient at any point gives the speed of the object

Question 8

what does the gradient represent for a velocity/time graph

Answer 8

gradient = acceleration

Question 9

how would you find the distance travelled by an object from it’s velocity/time graph

Answer 9

area under any section of the graph = distance travelled in that time interval

Question 10

state newton’s first and second laws of motion

Answer 10

first law: a resultant force is needed to make something start moving, speed up or slow down
second law: resultant force = mass x acceleration
( acceleration is inversely proportional to
mass)

Question 11

explain why cars have safety features to reduce decelerations experienced by passengers

Answer 11

• large decelerations can cause serious injuries
• this is because large deceleration requires large force
• the force can be lowered by slowing the object down over a longer time
• safety features are designed to increase collision times which reduce the force and reduce risk of injury
• eg. seat belts stretch slightly, air bags slow you down and crumple zones allow the front and back of a vehicle to crumple up easily in a collison, increasing the time taken to stop

Question 12

what is the formula for calculating the weight of an object

Answer 12

weight = mass x gravitational field strength

Question 13

what is the force acting to produce circular motion

Answer 13

centripetal force

Question 14

explain why there must be a force acting to produce circular motion

Answer 14

• velocity is both the speed and direction of an object
• if an object is travelling in a circle ( constant speed) it is constantly changing direction therefore constantly changing velocity
• this means it’s accelerating
• this means there must be a resultant force acting on it

Question 15

describe an experiment to investigate newton’s second law of motion

Answer 15

• via the motion of a trolley on a ramp
• measure the mass of the trolley, the unit masses and the hanging hook - also measure the length of a piece of card which will interrupt the light gate beams
• adjust the height of the ramp until the trolley just starts to move
• mark a line on the ramp just before the first light gate to ensure the trolley travels at the same distance every time
• attach trolley to hanging masses , hold still at the start line and then let go
• each light gate will record the times
• acceleration can then be found via (v-u)/t

Question 16

what is inertia

Answer 16

the tendency for motion to remain unchanged

Question 17

what is newton’s third law of motion

Answer 17

when two objects interact, the forces they exert on each other are equal and opposite
eg. if you push a trolley, the trolley will push back against you just as hard

Question 18

state the formula used to calculate an object’s momentum

Answer 18

momentum = mass x velocity

Question 19

explain the link between newton’s third law and conservation of momentum

Answer 19

• for example snooker balls
• before collision the white ball has a momentum of 0.6kgm/s
• red ball has a momentum of zero
• total momentum is 0.6 kgm/s
• when they collide, the white ball exerts a force on the red ball causing it to begin moving.
• due to newton’s third law, the red ball also exerts an equal but opposite force on the white ball
• this force causes the white ball to slow down
• the collision lasts 0.1 s
• after the collision, the white ball continues moving at 1 m/s, the red begins at 3 m/s
• momentum has been conserved

Question 20

what is meant by a person’s reaction time

Answer 20

the time between noticing the hazard and applying the brakes / action

Question 21

describe an experiment to measure reaction time

Answer 21

• the ruler drop experiment
• sit with your arm resting on the edge of the table
• get someone else to hold a ruler to hang between your thumb and forefinger lined up with zero
• without any warning, the person drops the ruler
• close thumb and finger to catch as quickly as possible
• measurement is at the point it was caught
• longer the distance, longer the reaction time

Question 22

state two factors that affect the thinking difference for a stopping car

Answer 22

• reaction time

- speed

Question 23

state four things that can affect braking distance of a vehicle

Answer 23

• speed
• car mass
• brake condition
• friction between tyres and road