Dynamics Flashcards
What does this velocity-time graph show?
Constant velocity
How can you increases friction?
- Increase the area of the rubbing surfaces
- Make the rubbing surfaces rougher
- Push the rubbing surfaces togerther more
- Increase the speed that the surafces are rubbed together
Describe how a cyclist can streamline themself
- Crouch over the handlebars
- Wear tight lycra clothing
- Specially shaped helmet
What is a balanced force?
Equal forces acting in opposite directions
Newtons 1st Law is about balanced forces, what two things can happen if a balanced force acts on an object?
- Remains stationary
- Travels at a constant speed in a straight line
What is acceleration?
Acceleration is the rate of change of velocity or the change in velocity per second.
What is the difference between distance and displacement?
Distance is the length of the path you actually take and it is a scalar, displacement is the straight line distance between where you started and finished including a direction as it is a vector.
Which line on the graph represents the largest acceleration?
Blue Line - it has the steepest gradient
What is the difference between mass and weight?
Mass is a measure of the partcles that make an object up. Weight is a force caused by the gravitational field strength of a planet.
What is speed?
Distance travelled per second.
Name three vector quantities
- Displacement
- Velocity
- Acceleration
- Force
- Momentum
- Weight
An astronaut has a mass of 50kg. What will happen to the astronauts mass on the Moon?
It stays at 50kg. Mass does not change.
An astronaut has a mass of 50kg. What will happen to the astronauts weight on the Moon?
The weight will decrease as there is a lower gravitaional field strength on the Moon.
What is meant by gravitational field strength?
Force per unit mass
What is meant by an acceleration of 15ms-2 ?
The velocity increases by 15ms-1 every second.
Explain the difference between average and instantaneous speed.
Average speed is over a long period of time, Instantaneous speed is the speed over a very short period of time.
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 gravitational field strength the velocity will increase vertically by 9.8ms-1.
- Calculate the area under the velocity-time this is equal to the verical height of the cliff.
What will an unbalanced force cause?
Acceleration
Explain how to measure acceleration using the equipment shown below and a stop watch.
- When the cardboard passes through the first light gate the timer attached to it records this time.
- Measure the length of the card with 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 first light gate and when it reaches the second light gate stop the stop watch.
- This is t, the time for the change in velocity.
- When the cardboard passes through the second light gate the timer attached to it records this time.
- The final velocity, v = length of card / time on timer
- Then calculate acceleration, a = (v-u)/t
What does the area under this velocity time graph give?
If positive area and negative area are subtracted it gives the displacement. If the total area is calculated it gives the distance travelled.
What is work?
It is the measure of the energy used to move an object over a distance using a force.
How can a space rocket be slowed down in space?
Reverse the thrusters to provide an unbalanced force backwards to slow you down.
When does an object have kinetic energy?
When it is moving.
When is work done?
When a force is exterted over a certain distance.
What are the three effects a force can have?
- Change the speed of the object
- Change the direction of travel of the object
- Change the shape of the object
How can you decrease friction?
- Decrease the area of the rubbing surfaces
- Make the rubbing surfaces smoother
- Push the rubbing surfaces together less
- Decrease the speed that the surfaces are rubbed together
- Lubricate/oil/ wax the surfaces
- Streamline the object
What is conservation of energy?
Energy cannot be created or destroyed it can only be transferred from one type to another.
What happens to the object at point X?
It changes direction
What does this velocity-time graph show?
Constant acceleration
What is meant by streamlining?
Streamlining is changing the shape of an object to reduce air/water resistance
Name three scalar quantities
- Distance
- Time
- Speed
- Mass
- Energy
An object travels at a constant speed of 10ms-1. What can be said about the forces on it?
The forces are balanced
Explain why a projectile follows a curved trajectory.
- It has a constant horizontal velocity
- It has a constant vertical acceleration downwards due to gravity
Explain how a seatbelt works in terms of Newton’s 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 backwards force to decelerate the passenger to zero without hitting the windscreen. (Newtons 2nd Law)
What does the path a projectile takes look like?
Curve
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 the energy change as the sledge slides down the 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 sldge has no potential energy 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.
What causes friction?
Two surfaces rubbing together
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.
What does the gradient of the line on velocity - time graph represent?
The acceleration
What does this velocity - time graph show?
Constant de-acceleration
Describe a streamlined shape
- Low to the ground
- Smooth
- Have curves
What does the area under the velocity-time graph give?
The distance travelled
What is the difference between a vector and a scalar quantity?
A scalar needs size/magnitude to be described correctly. A vector needs size/ magnitude and direction to be described correctly.
Explain how to measure the acceleration of the car using the equipment shown below and a stop watch.
- Car starts from rest so u = 0ms-1.
- When the car is released start the stop watch when it reaches the light gate stop the stop watch.
- This is t, the time for the change in velocity.
- When the cardboard passes through the light gate the timer attached to it records this time.
- Measure the length of the card with a ruler.
- The final velocity, v = length of card / time on timer
- Then calculate acceleration, a = (v-u)/t
What is the weight of an object in freefall?
0 Newtons
For a projectile fired horizontally, what would the horizontal velocity time graph look like?
Constant horizontal velocity - therefore a straight horizontal line.
Describe how to calculate acceleration from a veocity - time graph.
- Pick two pints 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 to the final speed.
- Then use a = (v-u)/t
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
v you are calculating
t is the time it falls for
a = (v-0)/t
v = at
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.
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
When does an object have potential energy?
When it is lifted through a height.
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 down becomes less, so there is less aceleration 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.
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 cardboard breaks the first lightgate beam it starts the timer. When the cardboard breaks the second lightgate beam it stops the timer.
- The timer records the time taken to go from the first to the second light gate.
- Measure the distance between the two light gates using a metre stick.
- Calculate the speed = distance between light gates / time on timer
Explain how to use the equipment below to calculate the instantaneous speed of the car.
- Measure the length of the cardboard using a ruler.
- Roll car down the slope.
- When the cardboard breaks the beam it starts the timer connected to the light gate. When the cardboard has passed through the beam is remade and the timer stops.
- Speed = length of cardboard / time on timer
Describe how to measure average speed.
- Mark a start line and a finish line.
- Measure the distance between the start and finish line with a ruler/metre stick.
- Start a stopwatch when the object crosses the start line and stop it when the object crosses the finish line.
- Calculate the average speed = distance between start and finish / time on stopwatch
What is velocity?
Displacement travelled per second.
