Module 3 Newton's Laws and Momentum

Question 1

What is Newton’s first law?

Answer 1

a body will remain at rest or at constant velocity unless acted on by a resultant force

Question 2

What is Newton’s second law?

Answer 2

the rate of change of momentum is equal to the resultant force in the same direction

Question 3

What are the two equations for resultant force?

Answer 3

F = Δp/Δt
F = ma

Question 4

When is F = ma used over F = Δp/Δt?

Answer 4

When object is at constant mass

Question 5

Derive F = ma from F = Δp/Δt

Answer 5

F = Δp/Δt

= (mv-mu)/t

= m(v-u)/t

= ma

Question 6

What is Netwon’s third law?

Answer 6

if body A exerts a force onto body B, body B exerts an equal and opposite force onto body A of the same type

Question 7

What are the four conditions which need to be satisfied for Newton’s third law?

Answer 7

• Equal magnitude
• Opposite direction
• Act on two different bodies
• Same type of force

Question 8

What are the two pairs of Netwon’s third law forces for an object resting on a surface?

Answer 8

• Earth exerts a gravitational force on object, object exerts an equal and opposite gravitational force on the Earth
• Surface exerts a contact force on the object upwards - object exerts an equal but opposite contact force on the surface downwards

Question 9

Explain how a rocket actually takes off using Newton’s third law (note, use idea of hot gases being emitted)

Answer 9

• Rocket exerts a force on the hot gases being expelled downwards
• Hot gases exert an equal but opposite force on the rocket upwards

Question 10

What happens to the change in momentum for a rebound calculation?

Answer 10

• for Δp in F = Δp/Δt
• Δp = mv-(-mu) as direction of velocity has changed thus the sign of the velocity changes
• Δp = mv+mu

Question 12

Define impulse

Answer 12

Impulse is force multiplied by time for which the force acts

Question 13

When using rebounds, which direction is easier to take as the +ve?

Answer 13

Final direction is +ve so we get mv+mu when calculating impulse

Question 14

Symbol used for impulse?

Answer 14

Δp

Question 15

What does the area under a force time graph show?

Answer 15

impulse = Area under a force time graph in Ns

Question 16

When two masses collide, what can be said about the impulse on each object?

Answer 16

NIII force on each object is each and opposite and of the same type

Impulse = force x time and time is the same for both objects

impulse on each object is of equal magnitude but opposite direction

Question 17

Definition for the principle of conservation of linear momentum

Answer 17

For any collision, the total initial momentum = the total final momentum for a closed system (if there are no external forces)

Question 18

What is an elastic collision?

Answer 18

a collision where total kinetic energy is conserved

total momentum always conserved

Question 19

What is an inelastic collision?

Answer 19

where total kinetic energy is not conserved

total momentum always conserved

Question 20

What can be said about energy transfer for any given collision?

Answer 20

total energy and total momentum are conserved

Question 21

What is linear momentum?

Answer 21

momentum = mass times velocity

p = m v

Question 22

What can be said about collisions between molecules?

Answer 22

they are always elastic as they never truly touch - one of the assumptions of kinetic theory

Question 23

When calculating collisions and rebounds, what must be stated with values of velocity?

Answer 23

Choose a direction as positive. Each velocity will be positive or negative.

Question 24

In an explosion, what can be said about the conservation of momentum?

Answer 24

total momentum is conserved, Total final momentum = total initial total momentum,
Momentum of one mass will be equal and opposite to the other mass

so change in momentum = 0,

momentum is vector