Module 3 Newton's Laws and Momentum Flashcards
What is Newton’s first law?
a body will remain at rest or at constant velocity unless acted on by a resultant force
What is Newton’s second law?
the rate of change of momentum is equal to the resultant force in the same direction
What are the two equations for resultant force?
F = Δp/Δt
F = ma
When is F = ma used over F = Δp/Δt?
When object is at constant mass
Derive F = ma from F = Δp/Δt
F = Δp/Δt
= (mv-mu)/t
= m(v-u)/t
= ma
What is Netwon’s third law?
if body A exerts a force onto body B, body B exerts an equal and opposite force onto body A of the same type
What are the four conditions which need to be satisfied for Newton’s third law?
- Equal magnitude
- Opposite direction
- Act on two different bodies
- Same type of force
What are the two pairs of Netwon’s third law forces for an object resting on a surface?
- 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
Explain how a rocket actually takes off using Newton’s third law (note, use idea of hot gases being emitted)
- Rocket exerts a force on the hot gases being expelled downwards
- Hot gases exert an equal but opposite force on the rocket upwards
What happens to the change in momentum for a rebound calculation?
- 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
Define impulse
Impulse is force multiplied by time for which the force acts
When using rebounds, which direction is easier to take as the +ve?
Final direction is +ve so we get mv+mu when calculating impulse
Symbol used for impulse?
Δp
What does the area under a force time graph show?
impulse = Area under a force time graph in Ns
When two masses collide, what can be said about the impulse on each object?
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
Definition for the principle of conservation of linear momentum
For any collision, the total initial momentum = the total final momentum for a closed system (if there are no external forces)
What is an elastic collision?
a collision where total kinetic energy is conserved
total momentum always conserved
What is an inelastic collision?
where total kinetic energy is not conserved
total momentum always conserved
What can be said about energy transfer for any given collision?
total energy and total momentum are conserved
What is linear momentum?
momentum = mass times velocity
p = m v
What can be said about collisions between molecules?
they are always elastic as they never truly touch - one of the assumptions of kinetic theory
When calculating collisions and rebounds, what must be stated with values of velocity?
Choose a direction as positive. Each velocity will be positive or negative.
In an explosion, what can be said about the conservation of momentum?
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
