Unit 1.3)Dynamics

Question 1

what is Newtons 3rd law

Answer 1

Newtons third law states that if a body A exerts a force of magnitude F on a body B, the body B will exert a force of equal magnitude but opposite direction(-F) on body A.

Question 2

What is newtons second law

Answer 2

F = ma (the resultant force on a body in a direction is equal to the mass of the body multiplied by the acceleration of the body in that direction.)

Question 3

When can you apply newtons second law.

Answer 3

When the mass of the body is constant.

Question 4

Both force and acceleration are ____Quantities

Answer 4

Vector

Question 5

What does NL2 mean

Answer 5

NL2 means to apply newtons second law

Question 6

Why would you use nl2 when working on a example

Answer 6

NL2 help you to to specify which body to apply it on(this means you don’t get confused between bodies in large systems). Then you specify which direction you apply it in. So that you find the resultant force in the right direction and the acceleration in that same direction.

Question 7

What is the equation for linear momentum

Answer 7

p = mv

Question 8

What is newtons first law

Answer 8

an object will remain at rest or in uniform motion unless acted upon by an external resultant force

Question 9

Vector sum of momenta of the bodies in an isolated system is always___

Answer 9

zero

Question 10

A rugby player runs with a velocity of 5ms^-1 and has a mass of 97kg. Calculate the momentum(direction towards opponent try line is positive)

Answer 10

p = mv x 5 = +485 kg ms^-1

Question 11

5kg particle moves to the right with a speed of 1m/s after the collison, find the final velocity of the 2kg particle.
Total momentum before the collision to the right = 5(3) - 2(4.5) = 6 kgm/s

Answer 11

Total momentum before the collision to the right = 5(3) - 2(4.5) = 6 kgm/s
Total momentum after the collision = 5(1) + 2(v) = 5 + 2v kgm/s
Since total momentum must be conserved we can simplifies these two expressions as
6 = 5 +2v
2v = 1
v = 0.5 m/s
therefore, the velocity of the 2kg particle is 0.5 m/s to the right

Question 12

Inelastic Collision

Answer 12

A collision in which Kinetic energy is lost.

Question 13

Elastic Collision

Answer 13

A collision in which no Kinetic energy is lost
(total Kinetic energy before the collision = total Kinetic energy after the collision)

Question 14

Have a look at Pmt educations Dynamics page for a little refresher on the momentum equations

Question 15

Newton’s 3rd law pair rules(N3 pairs)

Answer 15

-The pair of forces must act on different bodies.
-Must be equal in magnitude but opposite direction.
-Are of the same type of force.
e.g Weight of skydiver acting downwards and the gravitational force f the skydiver pulling the earth upwards.

Question 16

What is the principle of conservation of momentum?

Answer 16

The principle of conservation of momentum states that the vector sum of the momenta of bodies in a system stays constant even if forces act between the bodies, provided there is no external resultant force.

This means that the total momentum before an interaction, e.g. a collision, is equal to the total momentum afterwards, as long as there is no external resultant force.

Question 17

Inelastic collision?

Answer 17

A collision in which kinetic energy is lost.

Question 18

Elastic collision?

Answer 18

Kinetic energy is conserved kinetic energy before is equal to kinetic energy after.

Question 19

What does newtons second law state?

Answer 19

the rate of change of momentum of an object is proportional to the resultant force acting on it and takes place in the direction of that force.

Question 20

Show how F=ma arises from F=rate of change of momentum

Answer 20

∑F=
Δt
Δp
​
=
Δt
Δ(mv)
​

As a=
Δt
Δv
​
, if the mass is constant this can be simplified to:

∑f=m
Δt
Δv
​
=ma