P10 Forces and motion

Question 1

Newtons first law of motion

Answer 1

Newton’s first law states that an object at rest or moving at a constant velocity will continue to do so unless an external force is applied

Question 2

Newtons 2nd law of motion

Answer 2

The 2nd law links together Resultant Force (F), Acceleration (a) and Mass (m). It states that the acceleration on an object is:​
- Directly proportional to the resultant force acting on the object,​
- Inversely proportional to the object’s mass​
- I.e. Force = mass x acceleration (F = ma)

Question 3

Newtons 3rd law of motion

Answer 3

Newton’s 3rd law of motion states that for every force there is an equal and opposite reaction force

Question 4

inertial mass

Answer 4

Inertia is a measure of how much a mass is reluctant to change its speed.​
i.e. it is the tendency of a mass to stay at rest or stay moving at a constant speed. MATTER IS LAZY!​
The inertial mass is given by:​
inertial mass = force/acceleration

Question 5

what is weight

Answer 5

The Weight of an object is the force acting on it due to gravity. Weight is measured in newtons (N)

Question 6

what is mass

Answer 6

Mass is a measure of how much matter is contained in an object. Mass is measured in kilograms (kg).

Question 7

how are mass and weight linked

Answer 7

If we know the mass of an object in kg we can use the equation that links weight (w) and mass (m) to find its weight in N, i.e.:​
Weight (N) = Mass (kg) x gravitational field strength (N/kg)​
In symbols: W = mg

Question 8

what is terminal velocity

Answer 8

The maximum acceleration on the object is at the very start that it enters the fluid. ​
As the object speed increases, the resisting frictional force acting on the object increases which acts to lower the size of the resultant force. As a = F/m If the resultant force decreases, so does the acceleration of the object.​
The weight (downwards force) is constant, but as the speed increases, then so does the frictional drag force. Eventually a speed is reached where the forces of weight and frictional drag force are balanced and a = zero. When this happens the size of the resultant force is zero and the object now moves with a constant velocity. The velocity is called the terminal velocity.

(the velocity object eventually reaches when it is falling)

Question 9

what is momentum

Answer 9

Momentum (symbol p) is defined as the product of an objects mass m and its velocity V.​
​
p = M x V​

As mass has units kg and velocity m/s, the units of momentum are kgm/s​
Momentum has both magnitude (size) and direction Therefore, momentum is a VECTOR quantity.

Question 10

what is the law of conservation of momentum

Answer 10

In a closed system (i.e. with no external resultant forces acting) the total momentum before an event is equal to the total momentum after the event. ​
In other words, momentum is always conserved

Question 11

how do we know different parts of an exploding body move in different directions

Answer 11

Before an explosion the total momentum is zero.​
As momentum is conserved, the total momentum afterwards must also be zero.​
This means that the different parts of the exploding body must move off in different directions.

Question 12

what is the resultant force acting on an object

Answer 12

F = m a

Question 13

what is the resultant force of an object moving at terminal velocity

Answer 13

zero

Question 14

what opposes the driving force of a vehicle

Answer 14

friction and air resistance

Question 15

what does the stopping distance of an object depend on

Answer 15

thinking distance and breaking distance

Question 16

what increases the braking distance

Answer 16

• high speed
• poor weather conditions
• poor vehicle maintenance

Question 17

what increases the thinking distance

Answer 17

• poor reaction time (our to tiredness, drugs, alcohol, or use of mobile phone)
• high speed

Question 18

what is the momentum of a moving object

Answer 18

p = m v

Question 19

what is momentum defined as

Answer 19

mass x velocity

Question 20

what is the total momentum of 2 object pushing away from each other

Answer 20

zero - they move with different speeds if they have different masses and with opposite momentum

Question 21

equation for when 2 objects recoil

Answer 21

m(A) v(A) + m(B) v(B) = 0

Question 22

what does the force of the impact depend on when two vehicles collide

Answer 22

• mass
• change in velocity
• length of impact time

Question 23

what does a longer impact time mean

Answer 23

the more the impact force is reduced

Question 24

how do we prove the shorter the impact time, the greater the impact force

Answer 24

impact force = change in momentum / impact time

Question 25

what do cycle helmets / cushioned surfaces do

Answer 25

they reduce impact forces by increasing the impact time

Question 26

what do seat belts and air bags do

Answer 26

they spread the force across the chest and increase the impact time

Question 27

at do side impact bars and crumple zones do

Answer 27

they give way in an impact, so increase the impact time

Question 28

when is an object called elastic

Answer 28

if it returns to its original shape after removing the force deforming it

Question 29

what is the extension

Answer 29

the difference between the length of an object and its original length

Question 30

what is the extension of a spring proportional to

Answer 30

the extension of a spring is directly proportional to the force applies to it, as long as the limit of proportionality is not exceeded

Question 31

what happens beyond the limit of proportionality

Answer 31

the extension of a spring is no longer proportional to the applied force so the relationship becomes non linear