Forces - More on speed, Newton Laws and Momentum

Question 1

what is terminal velocity

Answer 1

when an objects velocity remains constant and they are no longer accelerating or decelerating

Question 2

what factors can affect how many collisions there are between a person and air particles

Answer 2

1. Surface area - the larger the surface area, the larger the area over which collisions can take place
2. Velocity - the faster the person is moving, the more particles they can collide with

Question 3

what happens when a person falls off an airplane (with parachute)

Answer 3

1. The skydiver accelerates as they begin to fall due to the force of gravity acting on it
2. As the skydiver falls, he experiences an upward force of friction with air particles called air resistance
3. At terminal velocity, the air resistance force and weight are equal so speed is constant
4. The parachute opens which increases the air resistance and causes diver to slow down
5. The skydiver continues to slow down until the new air resistance force and weight are equal again

Question 4

what does the size of air resistance depend on when a person is falling

Answer 4

how many collisions there are between the person and air particles

Question 5

what is air resistance due to

Answer 5

the collisions between the person and all the tiny air particles that make up the air

Question 6

why would a diver accelerate downwards immediately after leaving an aircraft

Answer 6

because there is a resultant force acting downwards

Question 7

what happens as a skydiver gains speed

Answer 7

their weight stays the same but the air resistance increases

Question 8

what is Newton’s first law

Answer 8

• an object will stay stationary or continue moving at the same speed or direction, unless a resultant force is applied

Question 9

what happens if the resultant force of a stationary object is 0

Answer 9

it remains stationary

Question 10

what happens if the resultant force on a moving object is 0

Answer 10

it will carry on moving at the same velocity

Question 11

what is Newton’s second law of motion

Answer 11

• the size of the resultant force is proportional to the acceleration it causes and inversely proportional to the mass of the object

Question 12

what are the 5 things that could happen to an object if a resultant force is applied

Answer 12

• it may start moving
• it might speed up
• it might slow down
• it might stop moving
• it might change directions

Question 13

why does a change in direction cause a change in acceleration

Answer 13

because acceleration is defined as the change in velocity/change in time, and since velocity is determined by speed and direction, any change in direction changes the velocity

Question 14

why do we say the moon is accelerating around the earth

Answer 14

although speed of the moon remains constant, its direction is always changing, as the Earths mass exerts a gravitational pull on the moon, which acts perpendicular to the moons motion

Question 15

what would happen to the acceleration if we doubled the resultant force

Answer 15

the acceleration will double

Question 16

what is Newton’s second law as an equation

Answer 16

F=ma - mass x acceleration

Question 17

what is inertia

Answer 17

the tendency for an object to remain unchanged (basically just newtons first law)

Question 18

what is an objects inertial mass

Answer 18

how difficult it is to change an objects velocity

Question 19

how to find an objects inertial mass

Answer 19

by dividing force by acceleration (newton second law)

Question 20

what is Newton’s third law

Answer 20

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

Question 21

what do we call the force that an object is exerting

Answer 21

the normal contact force

Question 22

in order for an object to be accelerated and moved, what needs to happen?

Answer 22

you need either a high force or smaller mass

Question 23

what is stopping distance

Answer 23

the minimum distance required to stop a vehicle in an emergency

Question 24

stopping distance equation

Answer 24

stopping distance = thinking distance + braking distance

Question 25

what is thinking distance

Answer 25

how far the car travels during the drivers reaction time

Question 26

what things affect thinking distance

Answer 26

- speed of vehicle - reaction time - anything that decreases alertness (being tired, drunk, high, or just distracted)

Question 27

what is braking distance

Answer 27

the distance taken to stop under the braking force

Question 28

what affects braking distance

Answer 28

- the speed of vehicle - the mass of vehicle - conditions and quality of the brakes - conditions of the road (eg whether or not the road is wet, icy) as they reduce friction betweeen the tyres and the road - conditions of tires, bald tires unable to grip road as well

Question 29

why might the quality of a cars brakes affect braking distance

Answer 29

if they are worn or faulty, they wont be able to apply as much pressure against the wheel, meaning they wont slow the car down as quickly

Question 30

why does the speed an d the mass of a vehicle affect braking distance

Answer 30

because they both affect kinetic energy, which will have to be reduced to 0 in order for the vehicle to stop

Question 31

what is momentum equation

Answer 31

mass x velocity

Question 32

what is the conservation of momentum

Answer 32

in a closed system, the total momentum before an event = the total momentum after an event

Question 33

when do objects have no momentum

Answer 33

when it is stationary

Question 34

change in momentum equation

Answer 34

force = change in momentum (kgm/s)/ time

Question 35

what would happen in a car crash in terms of momentum

Answer 35

- momentum will decrease from a large amount to 0 almost instantly - this massive change in monentum means that the people inside the car will experience a huge force which may cause injury like head trauma

Question 36

how to reduce the chance of damage in a car crash

Answer 36

spread the change in momentum over a longer period of time, as this will reduce the force they experience - in order to help them acheive this, cars have to have safety features such as crumple zones, seatbelts, air bags

Question 37

what do seat belts do

Answer 37

- stop you flying out of the car - slightly stretchy so you slow down a bit more slowly

Question 38

what are forces like friction called

Answer 38

resistive forces

Question 39

Forces acting on a skydiver

Answer 39

- as soon as the skydiver jumps out, the only force acting is weight. this is due to gravity - because of weight, the skydiver experiences a resultant force acting downward, so they accelerate towards the ground - as he falls, he experiences friction with air molecules, air resistance increases - weight is still greater than air resistance, so the skydiver continues to accelerate towards the ground - as the skydivers velocity increases, air resistance also increases. at a certain point the air resistance balances the weight - this means there is no resultant force and velocity stays constant. the skydiver reaches terminal velocity

Question 40

what happens when a skydiver opens their parachute

Answer 40

- surface area now increases, causing air resistance to massively increase - at this point, air resistance is greater than the weight, so there’s a resultant force acting upwards - this causes the river to decelerate (velocity decreases) - because velocity has decreased, air resistance also decreases - at some point, the air resistance will balance the weight and the resultant force will be 0, so velocity will stay constant - skydiver falls at a lower terminal velocity, safe for them to hit the ground

Question 41

what is reaction time

Answer 41

the time taken for the driver to spot the obstruction, make a decision, then move their foot to the brake

Question 42

the greater the speed of the vehicle…

Answer 42

the greater the stopping distance (assuming that the same braking force is applied)

Question 43

typical ranges of reaction times

Answer 43

0.2 to 0.9

Question 44

what happens in terms of energy stores, when a car brakes

Answer 44

- during braking, the brake presses against the wheel - the force of friction now acts between the brake and the wheel - the kinetic energy of the car is now converted into thermal energy in the brakes - this causes the temperature of the brakes to increase - at the same time, the car slows down as it loses kinetic energy

Question 45

if the car is travelling at a high speed, what can we say about the size of the braking force they have to apply to stop

Answer 45

it has to be large

Question 46

how can brakes overheat

Answer 46

- a large braking force will cause a car to decelerate very quickly - at the same time, a large amount of kinetic energy is transferred to the thermal energy in the brakes - this can cause the brakes to overheat and can also cause the driver to lose control of the vehicle

Question 47

momentum in a canon example

Answer 47

- before a canon fires, both the cannon and the cannonball are not moving, so the total momentum before firing equals 0 - when a cannon fires, the cannonball is moving at a very large velocity, so the cannonball has momentum in the forward direction - at the same time, the cannon recoils (moves backwards), meaning the canon has momentum acting in the backwards direction - the backwards momentum of the canon equals the forward momentum of the cannonball, meaning the total momentum after firing is 0

Question 48

what are crash tests used for

Answer 48

to design safety features such as seat belts

Question 49

examples of safety devices that slow down momentum change

Answer 49

- airbags - seatbelts - crash mat - bike helmet - cushioned surface