FORCES IN MOTION Flashcards
Define Newtons 1st Law.
Objects will remain stationary or travelling at uniform velocity unless acted on by a resultant force.
Define Newtons 2nd Law.
The resultant force on an object is directly proportional to the rate of change of momentum and takes place in the same direction as the force.
F = ma
Define Newtons 3rd Law.
If object A exerts a force on object B, then object B will exert an equal and opposite force on object A.
Describe the motion of a freefall diver in terms of the forces acting on him.
At the start only force acting is weight. Accelerating at a constant rate due to g.
As person speeds up the drag force increases. This means the resultant force is smaller so person continues to accelerate but at a decreasing rate.
At this point drag force is equal to the weight so there’s no resultant force, so no acceleration. They have reached terminal velocity.
Opening the parachute creates large surface area so large drag force. This results in a large resultant force in the opposite direction decelerating the person at a decreasing rate.
Drag force again equals the weight so at a terminal velocity which is much lower and therefore safe.
Define momentum.
The product of mass and velocity.
P = mv
State the conservation of linear momentum.
“The total momentum before a collsion equals the total momentum after a collsion providing no external forces are acting.”
Describe a practical to show conservation of momentum.
First have a inclined plane having two light gates further down. Then a trolley with a card of known width on it. And another of the same trolley inbetween the light gates.
The inclined used to counteract the effect of friction.
Measure the mass of trolley A and B with a top pan balance.
Measure time taken for card of known width to pass the light gate.
Total momentum before a collsion can be compared to total momentum after collsion.
Describe a Rebounding Ball with mass m and velocity v hitting the wall and rebounding with the same speed.
Change of momentum is -2mv.
This is as the change of momentum = final momentum - intial momentum.
Momentum is a vector. Change in momentum therefore = (-mv)-(mv) = -2mv.
Explain why a gun recoils when fired in terms of momentum change.
Before gun fired the total momentum is zero.
When gun is fired, bullet gains momentum.
From conservation of momentum there must be an equal and opposite momentum change of gun.
So the gun recoils in the opposite direction of the bullet.
Define Impulse.
The product of force and time.
F^t = ^(mv)
How does Impulse relate to Transport Safety?
Average Force = ^(mv) / ^t.
When vehicle comes to sudden stop in crash, very large change of momentum. If occurs in very short period of time, force acting is large.
If the length of time over which momentum change occurs can be increased then the average force on the vehicle and passengers I’d decreased.
How does Impulse help with Air Bags?
When airbags are deployed the passengers hit the bag and it deflates due to loads of small holes in the bag.
Deflation lengthens the distance and therefore the time over which deceleration and change of momentum occurs.
By F = ^(mv) / ^t with ^t being increased average force is decreased significantly.
Describe how Impulse works with Crumple Zones.
Most modern cars are designed to crumple in the event of a crash.
Kinetic energy of car absorbed by deformation of structures within engine compartment.
Extends time and distance over which car decelerates and changes momentum.
Decreases average force applied to passengers.
Define Work Done.
Transfer of energy due to movement of a force.
Defined as force multiplied by the distance moved in the direction of the force.
W =Fscos@
State relationship between gpe energy and kinetic energy.
Loss of gpe = gain Ek + work done against friction (if air friction is present.)
