3.5 Newton's Laws of Motion & Momentum

Question 1

what is Newton’s first law of motion?

Answer 1

a body will remain at rest or will continue to be at a constant velocity in a straight line unless an external force acts on it

Question 2

what is Newton’s second law of motion?

Answer 2

the resultant force acting on an object is proportional to the rate of change of momentum of the object and the momentum change takes place in the direction of the force
f = ma when the force is constant, otherwise
resultant force = Δp / Δt

Question 3

what is Newton’s third law of motion?

Answer 3

if object A exerts a force on object B, then object B will exert an equal and opposite force on A (every action has an equal and opposite reaction)

Question 4

what are the requirements for Newton’s third law of motion?

Answer 4

• the forces involved must be of the same type

- the forces involved must be acting in opposite directions on different objects

Question 5

what is the formula for linear momentum?

Answer 5

p = mv
where m is the mass of the body
and v is the velocity of the body

Question 6

is momentum vector or scalar?

Answer 6

vector

Question 7

what is the principle of conservation of momentum?

Answer 7

the principle of conservation of momentum is the total momentum before a collision is always equal to the total momentum after the collision, provided that no external forces are involved
m1v1 = m2v2

Question 8

what is the formula for resultant force generally?

Answer 8

F = Δp / Δt (f = ma is a special case of this equation)

Question 9

what is impulse of a force? and what is the formula?

Answer 9

• impulse is the change of momentum

- impulse (Δp) = force x Δt

Question 10

how do you get to f = ma from F = Δp / Δt ?

Answer 10

F = Δp / Δt
= Δmv / Δt
= mΔv / Δt
= ma

Question 11

what is the area under a force-time graph equal to?

Answer 11

impulse

Question 12

how do crumple zones act as a safety feature in cars?

Answer 12

• crumple zones are areas at the front and rear of a car that are designed to crumple on impact
• they increase the time taken for the car to slow down (Δt increases which reduces the force acting on passengers)
• absorb some of the car’s kinetic energy when they deform that would have otherwise been transferred to the passengers

Question 13

how do airbags act as a safety feature in cars?

Answer 13

• airbags are cushions in the dashboard that inflate very quickly on collision
• they protect passengers by making them slow down more gradually (Δt increases which reduces the force acting on passengers)
• they stop the passengers from hitting the wheel or dashboard etc. during a crash

Question 14

how do seat belts act as a safety feature in cars?

Answer 14

• seat belts are designed to stretch slightly
• they protect passengers by stopping them from being thrown from their seats
• absorbing some of the wearer’s kinetic energy by stretching
• increasing the time that the wearer comes to a stop over (Δt increases which reduces the force acting on passengers) again, by stretching

Question 15

what are the units of impulse?

Answer 15

Ns (think of the formula)

Question 16

when answering momentum equations using calculations what it important to note?

Answer 16

the direction of movement, assign one direction as positive and one as negative to not get confused

Question 17

what is an elastic collision?

Answer 17

an elastic collision is one in which the momentum AND the kinetic energy are conserved - no energy is transferred to other forms such as heat or sound
(the object leaves with the exact same velocity just in the other direction as it collides)

Question 18

what is an inelastic collision?

Answer 18

an inelastic collision is one in which the momentum is conserved but not the kinetic energy, some of the kinetic energy is transferred to other forms during the collision (like heat, sound etc.)

Question 19

can you get truly elastic collisions in nature?

Answer 19

no - because some energy is always transferred to other forms (however collisions between gas atoms are usually taken to be elastic, think ideal gas laws)

Question 20

what rule are you using when answering momentum equations?

Answer 20

the principle of conservation of momentum

Question 21

what is the defintion of momentum?

Answer 21

the quantity of motion of a moving body, measured as a product of its mass and velocity

Question 22

how are the forces acting on a book on a table not an example of Newton’s third law?

Answer 22

although the reaction force of the table pushing upwards on the book is equal to the weight force on the book, they do not satisfy the conditions for Newton’s third law as the two forces both act on the book and secondly they are not of the same type, one is gravitational and the other is reaction, this is actually an example of Newton’s first law as opposed to his third (see fig. 7 on page 139 for reference of AS textbook)

Question 23

what are the pairs of forces as a result of Newtons’s third law called?

Answer 23

interaction pairs