Dynamics Flashcards
Describe how to measure average speed
- Mark a start line and finish line
- Measure the distance between the start and finish line with a metre stick
- Start a stopwatch when the object crosses the start line and stop it when it crosses the finish line
- Calculate the average speed = distance between start and finish line / time on stopwatch
Describe how to measure average speed using the equipment below
- Two light gates are connected to a timer
- The car rolls down the slope
- When the mask breaks the fist lightgate beam it starts the timer
- When the mask breaks the the second lightgate beam it stops the timer
- Measure the distance between the two lightgates using a metre stick
- Calculate the speed = distance between lightgates / time on timer
Describe how to use the equipment below to calculate the instantaneous speed of the car
- Measure the length of the mask using a ruler
- Roll the car down the slope
- When the mask breaks the beam it starts the timer connected to the lightgate
- When the mask has passed through, the beam is restored and the timer stops
- Speed = length of mask / time on timer
Explain the difference between average and instantaneous speed
Average is over a long period of time. Instantaneous speed is the speed over a very short period of time (an instant)
What is the difference between a vector and a scalar quantity?
A scalar needs size/magnitude to be described correctly. A vector quantity needs size/magnitude and direction to be described correctly.
Name 3 vector quantities
- Displacement
- Velocity
- Acceleration
- Force
- Weight
Name 3 scalar quantities
- Distance
- Time
- Speed
- Mass
- Energy
What is the difference between distance and displacement?
Distance is the length of the path you actually take and is a scalar. Displacement is the straight line distance between the start point and finish point and includes a direction as it is a vector.
What is the difference between speed and velocity?
Speed is a scalar and is equal to distance divided by time. Velocity is a vector and is displacement divided by time, with a direction.
What is speed?
Distance travelled per second
What is velocity?
Displacement travelled per second
What is acceleration?
Acceleration is the rate of change of velocity or the change in velocity per second
What is meant by an acceleration of 15 ms-2?
The velocity increases by 15 ms-1 every second
Explain how to measure the acceleration of the car using the equipment below and a stop watch
- The car starts from rest so u = 0 ms-1
- When the car is released start the stop watch and stop the stopwatch when it reaches the lightgate
- This is t, the time for the change in velocity
- When the mask passes through the lightgate, the electronic timer records the time
- Measure the length of the card using a ruler
- The final velocity, v = length of mask / time on electronic timer
- Calculate acceleration, a = (v-u)/t
Explain how to measure acceleration using the equipment below and a stop watch
- When the mask passes through the 1st light gate the electronic timer records this time
- Measure the length of the mask using a ruler
- The initial velocity, u = length of card / time on timer
- As the car rolls down the ramp, start the stop watch when it reaches the 1st light gate and when it reaches the 2nd light gate stop the stop watch
- This is t, the time for the change in velocity
- When the mask passes through the 2nd light gate, the electronic timer records the time
- The final velocity, v = length of mask / time on timer
- Calculate acceleration, a = (v-u)/t
What does this velocity-time graph show?
Constant velocity
What does this velocity-time graph show?
Constant acceleration
What does this velocity-time graph show?
Constant deceleration
What happens to the object at point X?
It changes direction
What does the area under the velocity-time graph give?
The distance travelled
Describe how to calculate acceleration from a velocity-time graph
- Pick 2 points on the slope
- u = initial speed, read it off the graph
- v = final speed, read it off the graph
- t = time to go from the initial speed to the final speed
- Calculate acceleration using a = (v-u)/t
What are the 3 effects a force can have?
- Change the speed of an object
- Change the direction of travel of an object
- Change the shape of an object
What is a balanced force?
Equal forces acting in opposite directions
Newtons 1st Law is about balanced forces. What 2 things can happen if a balanced force acts on an object?
- Remains at rest (stationary)
- Travels at a constant speed in a straight line
An object travels at a constant speed of 10 ms-1. What can be said about the forces acting on it?
The forces are balanced
What causes friction?
Two surfaces rubbing together
What is friction?
A force that opposes the motion of an object
How can you increase friction?
- Increase the area of the rubbing surfaces
- Make the surfaces rubbing together rougher
- Push the surfaces together more
- Increase the speed that the surfaces are rubbing together
How can you decrease friction?
- Decrease the area of the rubbing surfaces
- Make the surfaces rubbing together smoother
- Push the surfaces together less
- Decrease the speed that the surfaces are rubbing together
- Lubricate/oil/wax the surfaces
- Streamline the object
What is meant by streamlining?
Changing the shape of an object to reduce air/water resistance/drag
Describe a streamlined shape
- Low to the ground
- Smooth
- Have curves
What will an unbalanced force cause?
Acceleration
Explain how a seatbelt works in terms of Newtons Laws
- When a car travels at a constant velocity, the forces on it are balanced (Newtons 1st Law)
- If the brakes are applied, they provide an unbalanced force that stops the car (Newtons 2nd Law)
- The passengers continue to travel at a constant speed in a straight line (Newtons 1st Law)
- The seatbelt provides an unbalanced force to decelerate the passenger to zero without hitting the windscreen (Newtons 2nd Law)
What is work?
It is the measure of the energy used to move an object over a distance using a force
What is the difference between mass and weight?
- Mass is a measure of the particles that make up an object, in kg and cannot vary.
- Weight is a force caused by the gravitational field strength of a planet, in N and varies according to where in the universe the object is
What is meant by gravitational field strength?
Force per unit mass
An astronaut has a mass of 50kg. What will happen to the astronaut’s mass on the Moon?
It stays at 50kg. Mass does not change
An astronaut has a mass of 50kg. What will happen to the astronaut’s weight on the Moon?
The weight will decrease as there is a lower gravitational field strength on the Moon
How can a space rocket be slowed down in space?
Reverse the thrusters to provide an unbalanced force backwards and produce a deceleration
What is the weight of an object in freefall?
0 Newtons
Explain how a skydiver reaches terminal velocity
- When the skydiver jumps out of a plane, his initial vertical velocity is zero
- His weight causes him to accelerate towards the ground (Weight = Force = ma)
- As the skydivers velocity increases the air resistance upwards increases too
- So the unbalanced force downwards becomes less, so there is less acceleration towards the ground (Weight - air resistance = unbalanced force = ma)
- Eventually the weight downwards is equal and opposite to the air resistance upwards
- This is a balanced force so the skydiver reaches terminal velocity
What does the path of a projectile launched horizontally look like?
Curve
Explain why a projectile follows a curved trajectory
- It has a constant horizontal velocity
- It has a constant vertical acceleration downwards due to gravity
Describe how a cyclist can streamline themselves
- Crouch over the handlebars
- Wear tight Lycra clothing
- Specially shaped helmet
A car is travelling at a constant velocity of 25ms-1, the engine is producing a force of 4000N. What is the size of the frictional forces acting on the car?
4000N backwards
Which line on the graph represents the largest acceleration?
Blue line - it has the steepest gradient
For a projectile fired horizontally, what would the horizontal velocity-time graph look like?
Constant horizontal velocity - therefore a straight horizontal line
For a projectile fired horizontally, what would the vertical velocity-time graph look like?
It would show a constant acceleration, a line of gradient = 9.8ms-2
Explain how a satellite stays in orbit
- Satellite has a constant horizontal velocity
- Satellite has a constant vertical acceleration due to the gravitational field strength of the planet
- This gives a curved trajectory
- The planet is curved so the satellite remains in orbit
If an object is projected horizontally from a cliff, how can the height of the cliff be calculated?
- Sketch a vertical velocity-time graph
- Due to the gravitational field strength the velocity will increase vertically by 9.8ms-2
- Calculate the area under the velocity-time graph
- This is equal to the vertical height of the cliff
If an object is projected horizontally or dropped, how can the vertical velocity just before it hits the ground be calculated
- Use a = (v-u)/t
- u is 0ms-1
- a is 9.8ms-2
- t is the time it falls for
- a = (v-0)/t
- v = at
What does the gradient on a velocity-time graph represent?
The acceleration
What is conservation of energy?
Energy cannot be created or destroyed it can only be transferred from one form to another
When does an object have potential energy?
When it is lifted through a height
When does an object have kinetic energy?
When it is moving
When is work done?
When a force is exerted over a distance
Explain the energy change as a sledge slides down a hill
- At the start, the sledge has potential energy
- As it starts to slide down the hill, it loses potential energy and gains kinetic energy
- At the bottom of the hill, the sledge has no potential energy as it has all been transferred to kinetic energy
- At any point on the slope, the potential energy + the kinetic energy = potential energy at the top of the slope
