P8,P9, P10 Flashcards
Displacement
Distance without a change of direction
Vector quantities
Physical quantities with a size and direction
Examples of vectors
Acceleration, force, momentum, weight, gravitational field strength
Scalar quantities
Physical quantities with size, but no specific direction
Examples of scalar quantities
Speed, distance, time, mass, energy, power
Force
A push or pull that acts on an object because of its interaction with anotjer object
Contact forces
Forces if two objects must touch each other to interact e.g. friction, air resistance, stretching forces(tension) and normal contact forces
Non-contact forces
Magnetic force, electrostatic force and the force of gravity
Newton’s third law of motion
When two objects interact with each other, they exert equal and opposite forces on each other
Resultant force
A single force that has the same effect as all the forces acting on the object
Newton’s first law of motion
If the forces acting on an object are balanced, the resultant force on the object is zero, and:
If the object is at rest, it stays stationary
If the object is moving, it keeps moving with the same speed and in the same direction
What is the resultant force if two forces act in the same direction on an object along a straight line?
The resultant force is equal to the sum of the two forces
What does a free-body force diagram show?
The forces acting on an object without any other objects or forces shown
Centre of mass
The point in an object where its mass is thought of as being concentrated
Freely suspended object
When an object returns to its equilibrium position as the turning force is taken away, with the centre of mass directly underneath the point of suspension
How to find the centre of mass in an irregular-shaped card
1) Put a hole in one corner of the card and suspend the card from a rod.
2) Use a plumb line to draw a vertical line on the card from the rod.
3) Repeat the procedure, hanging the card from a different corner.
4) The point where the two lines meet is the centre of mass
Velocity
Speed in a given direction
Is the velocity of an object moving round in a circle constant?
An object moving round in a circle has a direction of motion changes continuously as it goes round, so its velocity’s not constant.
Acceleration of an object
Change in velocity per second
In a velocity time graph, what does the section with a horizontal line show?
Constant velocity
1km/h
0.278m/s
Equation linking constant acceleration and for calculations where time taken isn’t given
v2-u2= 2as
Required practical(investigating force and acceleration)
1) Use the newton-meter to pull the trolley along a known distance with a constant force.
2) A motion sensor and computer can be used to record the velocity of the trolley as it accelerates.
3) Predict what will happen to the acceleration of the trolley is the force is increased/decreased or the mass of the trolley is increased. You can double or treble the total moving mass of the trolley using double-deck and triple-deck trolley.
Safety: Protect your bench and feet from falling trolleys
Resultant force =
Mass x acceleration
Newton’s second law of motion
The acceleration of an object is:
Proportional to the resultant force on the object
Inversely proportional to the mass of the object
Inertia
The tendency of an object to stay at rest or continue in uniform motion(e.g. moving at constant gelocity’
How does the resultant force affect the velocity of an object?
Acceleration- when the resultant force is in the same direction as the velocity
Deceleration - if the resultant force is in the opposite direction to its velocity
Weight
The force acting on an object due to gravity
Mass
The quantity of matter on an object
Gravitational field strength
The gravitational force on a 1kg object at a place where the object is
Earth’s gravitational field strength
At its surfaces, about 9.8N/kg
Weight =
Mass x gravitational field strength
Terminal velocity
Constant velocity a falling object reacheswhere the frictional force on the object is equal and opposite to its weight
What does the braking force needed to stop a vehicle in a given distance depend on
The speed of the vehicle when the brqkes are first applied, the greater the speed, the greater the deceleration needed to stop a venicle withon a given distance.
The greater the mass, the greater the braking force needed for a given deceleration
Thinking distance
The distance travelled by the vehicle in the time it takes the driver to react
Thinking distance=
Speed x reaction time
Braking distance
The distance travelled by the vehicle during the time the braking force acts
Stopping distance=
Thinking distance+ braking distance
Factors affecting the thinking distance
Tiredness
Alcohol
Drugs
Distractions like using a mobile phone
What affects the braking distance
High speed
Poor weather conditions
Poor vehicle maintenance
What forces oppose the driving force of a vehicle?
Friction and air resistance
Momentum of a moving object(kg m/s)=
Mass(kg) x velocity(m/s)
The law of conservation of momentum
In a closed system, the total momentum before an event is equal to the total momentum after an event
Elastic object
An object is elastic if it returns to its original shape when the forces deforming it are removed
Examples of elastic objects
Rubber band
Required practical(stretch tests)
1) The spring to be tested is clamped at its upper end. An empty weight hanger is attached to the spring to keep it straight.
2) The length of the spring is measured using a metre ruler(original length).
3) The weight hung from the spring is increased by adding weights one at a time. The spring stretches each time more weight’s hung from it.
4) The length of the spring is measured each time a weight’s added. The spring should be measured from the same points each time to ensure accurate results. The total weight added and the total length of the spring are recorded in a table.
Safety(stretch tests- required practical)
Clamp the stand to the bench.
Take care with falling weights
Wear eye protection
Hooke’s law
The extension of a spring is directly proportional to the force applied, as long as its limit of proportionality isn’t exceeded
Force applied(N)=
Spring constant(N/m) x extension(m)
Forces on skydivers
1) When hanging from the leg of a helicopter, the weight of the skydiver is equal to the tension force of hanging from a helicopter.
2) When the person falls from the plane, the weight doesn’t change, but the air resistance increases, so the person accelerates.
3) The air resistance and weight become balanced forces, with no acceleration.
4) After pulling the shoot, the larger surface area for parachute increases the air resistance, with unbalanced forces(air resistance and weight) so there’s a big deceleration.
5) After pulling the shoot, you fall at a constant, terminal velocity, with weight = air resistance as air resistance slows down
Conservation of momentum(when a moving object hits a stationary object)
The mass of the moving object has increased, but its momentum is equal to the momentum before the collision. So an increase in mass causes a decrease in velocity
Sensible estimates for stopping distances
Travelling at 20mph:
Total stopping distance = 12m
Travelling at 40mph:
Total stopping distance = 36m
Travelling at 70mph:
Total stopping distance = 96m
Travelling at 30mph:
Total stopping distance = 25m
What unit is the joule equivalent to=
N/m
Momentum
The product of mass and velocity
Why large decelerations may be dangerous
Large decelerations cause a lot of force on the passengers, which can cause injuries
People’s typical reaction time
0.4s
How momentum is conserved when a moving object hits a stationary object
1) The moving object has momentum, the stationary object has zero momentum.
2) The bus hits the car, causing it to move, so the car now has momentum.
3)The bus continues to move, but with a smaller velocity(and momentum).
4) The combined momentum of the bus and car is equal to the original momentum of the bus(moving object).
If the force on the south decreases, what will happen to the resultant force on the scooter?
The direction changes to North, the resultant force will decrease
Typical speed of a person walking
1.5m/s
Typical speed of a car
25m/s
Typical speed of a person running
3m/s
Typical speed of a train
30m/s
Typical speed of a person cycling
6m/s
Typical speed of a plane
250m/s
Acceleration(required practical)- how the student can ensure the mass holder doesn’t hit the ground before the card passes through the second light gate
Move the second light gate closer to the first
Shorten the string length
Why would a newtonmeter have the spring with the greatest spring constant?
It needs the greatest force to extend the spring the same amount
Why the temperature of the brakes increases when used?
Decrease on kinetic energy of the car, causing the thermal energy store of the brakes to increase
What properties should be the same for each spring?
Spring constant
Original length
Diameter
What happens to a stationary spring stretched beyond its limit of proportionality?
The upward force on the spring is equal to the downward force
The spring is inelastically deformed
Why bowling ball decelerates?
Friction acts on ball - resultant force in opposite direction to velocity
Advantage of a reduced speed limit
The car will have less kinetic energy, so less likely to cause injury in the event of a collision
Why the empty van has a shorter stopping distance than the full van when driven at the same speed
1) Same maximum force applied by the brakes, because mass is less there’s a greater deceleration, so braking distance is less.
