Momentum Noadswood Science, 2012. Momentum To understand momentum Wednesday, May 20, 2015.
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Transcript of Momentum Noadswood Science, 2012. Momentum To understand momentum Wednesday, May 20, 2015.
Stopping Distance
Which takes the longest time to stop – the oil tanker travelling at 25mph or the motorbike travelling at 70mph?
Why do you predict this? What variables would you need to control to prove that
mass affects the stopping time?
Stopping Distance
The oil tanker takes much longer to stop because it is so much more massive (it has much more mass)!
To test that mass it the cause for this difference an experiment must be set up where only the mass varies between two or more otherwise identical experiments
Rugby
The same science is applied to rugby – a player with much more bulk is harder to stop than a smaller player (which is where the technique becomes vital)…
George Gregan and George Smith (Australia) struggle to stop Paul
Sackey
Momentum
A moving object has momentum – this is the tendency of the object to keep moving in the same direction
It is difficult to change the direction of movement of an object with a lot of momentum
Momentum (kg m/s) = Mass (kg) × Velocity (m/s)
Momentum has both a magnitude (an amount dependent on the object’s mass) and direction (dependent on the velocity of the object)
Momentum Questions
1. What is the momentum of a 5kg object travelling at 2m/s?
2. What is the momentum of a 40kg person running at 6m/s?
Momentum Questions
1. What is the momentum of a 5kg object travelling at 2m/s?
Momentum (kg m/s) = mass (kg) x velocity (m/s)
Momentum = 5 x 2 = 10kg m/s
2. What is the momentum of a 40kg person running at 6m/s?
Momentum (kg m/s) = mass (kg) x velocity (m/s)
Momentum = 40 x 6 = 240kg m/s
Conservation Of Momentum
As no external forces are acting on the objects involved, the total momentum stays the same in explosions and collisions – momentum is conserved
As momentum is conserved the mass, velocity or momentum of an object in an explosion or collision can be worked out
Conservation Of Momentum
Watch the demo of a vehicle colliding into another – as they collide the momentum of each object changes
One vehicle is pushed into the other, with the velocity being measured before the collision and then the velocity of both vehicles after the collision: -
Conservation Of Momentum
For two vehicles of the same mass – the velocity of vehicle A is halved by the impact, but the combined mass after the collision is twice he moving mass before the collision, meaning the momentum remains
For a single vehicle colliding into two vehicles the velocity of vehicle A is reduced to one third, but the combined mass after the collision is three times the initial mass, meaning the momentum remains
Conservation Of Momentum
If a vehicle crashes into the back of a line of cars, each car is ‘shunted’ into the car in front of it – momentum is being transferred along the line of cars to the one at the front
Conservation Of Momentum
Conservation of momentum: -
(mass A x velocity A) + (mass B x velocity B) = 0
∴ (mass A x velocity A) = -(mass B x velocity B)
Explanation
Why does a stationary boat recoil when someone jumps off it?
As someone jumps off a boat they and the boat move away with equal and opposite amounts of momentum - hence the boat moves away from them
Recoil
When a shell is fired from an artillery gun the gun barrel recoils backwards - this recoil is slowed by a spring, lessening the backwards momentum
Question 1
A 0.5kg trolley (A) is pushed at a velocity of 1.2m/s into a stationary trolley (B) of mass 1.5kg – the two trolleys stick together after the impact
Calculate: -
a) The momentum of the 0.5kg trolley before the collision
b) The velocity of the two trolleys immediately after the impact
A - 0.5 kg B - 1.5 kg
1.2 m/s 0 m/s
Answer 1
a) The momentum of the 0.5kg trolley before the collision.
Momentum = mass x velocity = 0.5 x 1.2 = 0.6kg m/s
b) The velocity of the two trolleys immediately after the impact
Momentum = mass x velocity
Momentum doesn’t change!
Momentum after impact = 0.6kg m/s
Mass = 1.5kg + 0.5kg = 2kg
Velocity = 0.6kg ÷ 2kg = 0.3m/s
Question 2
2. An artillery gun of mass 2000kg fires a shell of mass 20kg at a velocity of 120m/s - calculate the recoil velocity of the gun
(mass gun x recoil velocity) = -(mass shell x velocity shell)
(2000kg x recoil velocity) = -(20kg x 120m/s)
recoil = -(20kg x 120m/s)
2000kg
recoil = -1.2m/s
Answer 2
2. A 600kg cannon recoils at a speed of 0.5m/s when a 12kg cannon ball is fired from it - calculate the velocity of the cannon ball when it leaves the cannon
(mass cannon x recoil velocity) = -(mass ball x velocity ball)
(600kg x 0.5m/s) = -(12kg x velocity)
velocity ball = (600kg x 0.5m/s)
12kg
velocity ball = 25m/s
Question 3 & 4
3. A 30kg skater and a 40kg skater standing in the middle of an ice rink push apart - complete the following sentences using force; momentum; and velocity
a) They move apart with equal and opposite...
b) The 30kg skater move away with a bigger... than the 40kg skater
c) They push each other with equal and opposite...
4. The 30kg skater moves away at 2m/s
a) What is her momentum?
b) What is the velocity of the other skater?
Answer 3
3.
a) They move apart with equal and opposite momentum
b) The 30kg skater move away with a bigger velocity than the 40kg skater
c) They push each other with equal and opposite force
4.
a) Momentum (kg m/s) = mass (kg) x velocity (m/s)
= 30kg x 2m/s
= 60kg m/s