Force and Motion Flashcards
distance-graph
velocity-graph
acceleration and distance in velocity time graph
distance: area under the line of the graph
acceleration: gradient (steepness of line equals to acceleration
Resultant force =
change in momentum / Time
Momentum units
kg m/s
Mass
Kg
Velocity
m/s
A person of mass 50kg is moving at a velocity of 24m/s in a car. The car is involved in an accident and the person is thrown forwards and hits the dashboard. They rapidly stop in a time of 0.1s. Calculate the force exerted on the person.
F= (50x0) - (50x24) ÷ 0.1
F= 0 - 1200 ÷ 0.1
F= -1200 ÷ 0.1
F= -12000N
The ……. it takes to stop the …….the impact force a person will experience.
1) longer
2) smaller
Spreading the change in momentum over a …… time ……. the forces required to act. This in turn ……. the injury.
1) longer
2) reduces
3) reduces
The initial momentum of the mass was 480kgm/s. Calculate its final momentum and hence its final velocity.
Change in momentum = Final momentum- 480
Final momentum = 1000kgm/s
Final Momentum = mass x Final Velocity
1000= 40 x Final Velocity
Final Velocity= 25m/s
A force of 20N is applied to a mass of 40kg for a time of 26s. Calculate the change in momentum.
20N= Change in momentum ÷ 26
20x26= Change in momentum
Change in momentum= 520kgm/s
As long as no external forces act on the system (such as friction) during an interaction:
(total momentum before and after)
Total Momentum Before = Total Momentum After
Momentum in explosions
-Momentum before the explosion is zero
-Momentum after the explosion is also zero because momentum is conserved.
The trolleys traveling in opposite directions have positive and negative momentum. Therefore when you add the total momentum after it will equal zero, so the net momentum is zero.
Newton´s 3rd Law
objects exert equal and opposite forces on each other
Pull of the Earth on the Moon - Pull of the moon on the Earth
Push of the ground on the stone- Push of the stone on the ground
Let the mass of the cannon be 500kg and the mass of the cannon ball be 40kg
If the recoil speed of the cannon is 6m/s calculate the velocity of the cannon ball. Assume to the right is positive.
Momentum Before = Momentum After
0 = (500 x -6) + (40 x v)
0 = -3000 + 40v
40v = 3000
V = 75m/s
A car of mass 1200kg is travelling at 30m/s. It slows down to 12m/s during a time of 8s. Calculate the resultant force applied to the car.
-2700N
A ball of mass 0.5kg is at rest. A force of 25N is applied for 0.4s. Calculate the final Momentum of the ball.
20 kg m/s
A rocket of mass 26,000kg is stationary on the launchpad. The rockets fire and produce a force of 800,000N. What is the final speed of the rocket after 2 minutes?
3700 m/s
A train of mass 50,000kg is travelling at 4m/s. It strikes and connects to a stationary carriage of mass 15,000kg. They move off together. What is their velocity after the collision?
3 m/s
Lever:
An object rotating or trying to rotate
Pivot:
The point around which the lever is rotating or trying to rotate
moment =
force × perpendicular distance from the pivot
Moment unit
Nm
A force of 2.5N is applied to a door handle to push a door open. The hinge (pivot) is 0.8m from the handle. Calculate the moment which is applied to the door.
moment= 2.5x0.8
moment= 2 Nm
How to find center of gravity of an irregular object?
-cut out an irregular shape
-Make a hole on the edge of a shape
-stick a pen through it and tie a plum bob on the edge of the pen
-wait until everything has stopped moving, it is balanced
-make a line where the string passes on the shape
-then on a 90 degrees angle from the line you just made make a whole at the edge
-stick the pen through it and tie the plum bob on the edge of the pen
-wait until everything has stopped moving, it is balanced
-make another line where the string is where the line intercepts is the center of gravity
-put your finger were the lines intercept and the shape should balance on your finger
How to help stability? (where is centre of gravity placed)
If the centre of gravity is within the base of an object it will be stable and if it tips it will likely right itself.
PRINCIPLE OF MOMENTS
When balanced:
Clockwise Moment = Anticlockwise Moment
A large stone has a weight of 6000N. A lever and a pivot are used to lift it up. The distance from rock to pivot is 5 cm. Distance from pivot to the other end of lever is 80 cm. Calculate the force F that needs to be applied to lift up the stone.
6000x5 = Fx80
F= 367N
2N weight is placed 60cm from the pivot . The newtonmeter is placed 10 cm from the pivot.
1) Calculate reading on the newtonmeter
2) the ruler is replaced by an iron bar. The iron bar is 1m long and has a weight of 10N. The newtonmeter and weight of 2N stay in their original positions. Explain how this change effects the reading on the newtonmeter
1) F x 10 = 2 x 60
F= 12N
2) -The force on the newton meter will increase
-The iron bar will produce an extra clockwise moment
-To maintain balance the anticlockwise moment must increase
distance unit
meters
The ……. the hanging masses are to a Newton meter the …… the force it will measure.
If the load is ….. distances from the Newton meters they will read …… value.
1) closer
2) greater
1) equal
2) the same
Weight
force of gravity, pulls and abject to the ground. A pen falling on the floor
Lift
force produced by a wing.
Airplane wings
Electrostatic
force between charges.
force between proton and electron.
Rub the balloon in your hair
Tension
force when you stretch or twist something. Tug of war
rubberband
Friction
force between 2 solids, force that tries to stop or restrict motion, even with the object stationary.
Hands rubbing together or the tire of a car
Thrust
force produced by an engine.
Car engine
Magnetic
force between 2 magnets.
compass fridge magnet
Drag
Force which restrict motion only when a object tries to move through a liquid or gas, but when the object is stationary there is no drag.
Boat in the sea and airplane flying through the air
Contact
force when 2 solids are touching.
Sitting on a chair
Up thrust
upward force produced by a liquid or gas. Hot air ballon or boat floating in the water
- If resultant force is in the same direction as the motion object will ….
- If resultant force is in opposite direction of motion object will …..
-If resultant force is zero, object will be ….. or moving at …… speed
1) accelerate
2) decelerate
3) stationary, constant
Scalar quatity
A Scalar quantity just has size (sometimes called magnitude).
speed
distance
mass
time
temperature
charge
density
Vector
A Vector quantity has size AND DIRECTION
velocity
displacement
(weight)
acceleration
force
a: An instant before the drop
Why is the drag zero?
b: 2 seconds after the drop
Compare the acceleration in part a & b. Why are they different?
c:18 seconds after the drop
What has happened to the speed by part c? Explain why.
d: An instant after the parachute opens
What happens to the speed in part d? Explain why.
A) Because it is not moving. The drag is 0 because you can’t have drag when something is stationary. You only have drag when moving. the speed is 0.
B) the acceleration in part a is greater than b. This is because the resultant force in part b is less than the resultant force in part a.
C) here the speed is constant because it has reached terminal velocity. This is because the resultant force is equal to 0 and if the force equals 0 the acceleration is 0 because of F=MA.
D) The speed is decreased because the resultant force is going up and the direction of motion. the direction of motion and the resultant force are opposite than you are slowing down. Drag is acting here are slowing you down because the surface area increased,
Braking Distance
The distance travelled between applying the brakes and the car stopping.
Thinking Distance
The distance travelled between realising there is a problem to the point where the brakes are applied.
stopping distance =
thinking distance + breaking distance
Safety features in cars that increase the time to stop and hence reduce the size of the force acting:
-Seat belt
-Crumple zone
-Air bag
Seat belt
The belt stretches and hence it increases the time to come to a stop
Crumple zone
Parts of the car are designed to collapse(crumple) so the car takes longer to stop moving during and accident
Air bag
Striking the bag increases the time it takes to come to a stop.
Factors affecting thinking distance
Alcohol or other drugs: The reaction time is longer and hence the car travels further before the brakes are applied
Speed: Travels further during the reaction time
factors affecting breaking distance
Mass of car: The brakes produce a smaller deceleration hence the car will travel further before it stops
Speed: Takes a longer time to be stationary from a greater speed and the car will travel further in this time
Road conditions (ice or water on the roads): Less friction and so smaller deceleration. The car will travel further before it stops
f the force … the acceleration …..
If the mass …. the acceleration …..
if the force doubles the acceleration doubles.
If the mass doubles the acceleration halves.
If the force extension graph is a straight line through zero we say that the object is obeying ……
Hooke´s law.
If the spring returns to original shape after the force is removed it is said to be acting …….
elastically
Metal wire
The metal wire obeys hooke’s law only for small forces, we know this because the line of best fit is initially a straight line through zero
Rubber band
The rubber band does not obey hooke’s law
-Any rubber or plastic will have this same shape
-Rubber band is elastic because it goes back to its original shape
Helical spring
-will obey hooke’s law
-straight line through zero
