motion and forces Flashcards
What does a scalar quantity have?
Magnitude but no direction
What is a vector quantity?
Has both magnitude and direction
What are examples of vector quantities?
Force, velocity, displacement, weight, acceleration and momentum
What are examples of scalar quantities?
Speed, distance, mass, energy, temperature and time
What is speed?
How fast you’re going
What is velocity?
Speed given in the direction
What is the equation for speed?
Distance divided by time
What is the equation for distance?
Speed times time
What are distance time graphs used for?
How far something travelled
What is the equation for acceleration?
(v-u)/time
V = final velocity
U = initial velocity
What are the measurements for acceleration and equation?
Acceleration = M/s squared
Velocity = M/s
Time = seconds
How do you work out uniform acceleration?
v squared - u squared = 2 x a x X
X = distance
a = acceleration
What is the speed of walking?
1.4m/s (5km/h)
What is the speed of running?
3m/s (11km/h)
What is the speed of cycling?
5.5m/s (20km/h)
What is the speed of cars in a built up area?
13m/s (47km/h)
What is the speed of cars on a motorway?
31m/s (112km/h)
What is the speed of trains?
55m/s (200km/h)
What is the speed of aeroplanes?
250m/s (900km/h)
What is the speed of ferries?
15m/s (54km/h)
What is the speed of wind?
5 - 20m/s
What is the speed of sound in air?
340m/s
What is Newton’s first law?
- if the resultant force on a stationary object is zero, the object will remain stationary
- If a resultant force on a moving object is zero, it will carry on moving at the same speed and direction
What is the question for Newton’s second law?
Resultant force (F) = mass (M) x acceleration (A)
What is weight?
The force acting on an object due to gravity
What is weight measured in?
Newtons
How is weight measured?
Using a calibrated spring balance or a newton meter
What is the equation for weight?
Weight (newtons) = mass (kg x gravitational field strength (N/kg)
What is gravitational field strength?
It varies with location
It’s stronger the closer you are to the mass causing the field
This means that the weight of an object changes with location
Core practical : How to set the trolley on a ramp up?
1) Measure of the trolley, the unit masses and the hanging hook. Measure the length of the piece of card which will interrupt the light gate beams. Then set up your apparatus but don’t attach the string to the trolley.
2) Adjust the height of the ramp the trolley just starts to move.
3) Mark line on the ramp just before the first light gate - this is to make sure the trolley travels at the same distance every time. The light gate will record the initial speed of the trolley as it begins to move.
4) Attach the trolley to the hanging mass by the string. Hold the trolley still at the start line, and then let go of it so that it starts to roll down the slope.
5) Each light gate will record the time when the trolley passes through it and the speed of the trolley at that time. The acceleration is then found.
Core practical : How to work out the acceleration of the trolley?
Acceleration = change in speed/time
Initial mass of the trolley as it passes through the first light gate
Final speed of the trolley, which equals the speed of the trolley through the second light gate
The time it takes to travel between the two light gates
Core practical : How do we investigate the effect of the trolley’s mass?
Add masses one at a time to the trolley. Keep the mass on the hook constant (so the accelerating force is constant - where the forces equal to the mass on hook x acceleration due to gravity).
Core practical : How do we investigate the effect of the accelerating force?
Start with all the masses loaded onto the trolley and transfer the masses to the hook one at a time.
Core practical : As the mass of the trolley increases, what decreases?
Acceleration
Core practical : What different equipment can we use to measure distance and time?
- rolling tape measure = mark out and measure distances
- videos = if you know how many frames per second the camera records, you can find the distance travelled by the object in a given number of frames and the time it takes to do so.
An object moving in a circular orbit at a constant speed has a changing what?
velocity
What is a centripetal force?
Force that acts towards the centre of the circle
What is the inertial mass?
A measure of how difficult it is to change the velocity of an object
Inertial mass is ratio of force over what?
Acceleration
What did Newton’s third law say?
When two objects interact, the forces they exert on each other are equal and opposite.
What is the equation for acceleration regarding force?
Force/mass
What is an example of Newton’s third law?
Skater A pushes on skater B, she feels an equal and opposite force from skater B’s hand. Both skaters feel the same sized force, in opposite direction, and so accelerate away from each other. However, skater A will be accelerated more than skater B because she has a smaller mass.
What is an example of Newton’s third law regarding an object in equilibrium?
The weight of the book pulls it down, and the normal reaction force from the table pushes it up. These forces are equal to each other - the book is in equilibrium and doesn’t move.
The pairs of forces due to Newton’s third law in this case are :
- The book is pulled down by its weight due to gravity from earth and the book also pulls back up on the Earth
- The normal contact frost from the table pushing up on the book and the normal contact force from the book pushing down on the table
What is momentum?
A property that all moving objects have
What is the equation for momentum?
p = m x v
p = momentum (kg m/s)
m = mass (kg)
v = velocity (m/s)
How can we use momentum in collisions? (red and white balls)
The red wall is stationary, so it has zero momentum. The white ball is moving with a velocity, so has a momentum of p = m x v
The white ball hits the red ball causing it to move . The red ball now has momentum. The whiteboard continues moving but at a much more smaller velocity and momentum. The combined momentum of the red and white balls is equal to the original momentum of the white ball, m x v
What is the equation for Newton’s second law?
Force = change in momentum / t
What is a typical reaction time?
0.2-0.6s
What are ways of measuring reaction times?
Ruler drop test and a computer based test
What is stopping distance (equation)
The sum of the thinking distance and the breaking distance
What are some factors that affect the stopping distance of a vehicle?
- Mass of the vehicle
- Speed the vehicle
- Drivers reaction time
- State of the vehicles brakes
- State of the road
- The amount of friction between the tire and the road surface
How can drugs and distractions affect a drivers reaction time?
It can disrupt their thinking distance
What is the stopping distance?
The distance covered between the driver first spotting hazard and the vehicle coming to a complete stop
What is the thinking distance?
The distance the car travels in the drivers reaction time between noticing the hazard and applying the brakes
What is the breaking distance?
Distance taken to stop once the brakes have been applied
Why is it better to use a light gate instead of a stopwatch to measure short time intervals?
It removes human error for timings
A car moves at a constant velocity along the road, so that it is in equilibrium. Give an example of a pair of forces that demonstrate newtons third law in this situation.
The tyres push the road backwards and the road pushes the tyres forward
A 10 kg object is travelling at 6 m/s. It hits a stationary 20 kg object and the two objects join together and keep moving in the same direction. Calculate the velocity of the combined object, assuming the momentum is conserved.
Momentum before :
(10 x 6) + (20 x 0)
= 60kg m/s
Momentum after :
(10 + 20) x v = 30v
60 = 30v 60/30 = 2
Velocity = 2m/s
Describe how momentum is conserved by a gun recoiling (moving backwards) as it shoots a bullet
Before the gun fires the bullet, the total momentum is zero. When the bullet leaves the gun, it has momentum in one direction. The gun moves backwards, and has an equal but opposite momentum to the bullet. This means that the total momentum after the bullet has been fired is still zero. Momentum has been conserved.
Calculate the force a tennis racket needs to apply to a 58 g tennis ball to accelerate it from rest to 34 m/s in 11.6ms
58g = 0.058kg
11.6ms = 0.0116s
(0.058 x 34) - (0.058 x 0) / 0.0116
Force = 170N
Drivers on long journeys should take regular breaks. Explain why, in terms of stopping distance.
If you’re tired, your reaction time is likely to be longer. This increases the thinking distance and the stopping distance. This would make the accident more likely if you needed to break suddenly.
