Mod:3 motion

Question 1

Define acceleration

Answer 1

The rate of change of velocity

Question 2

What does a straight line show on a velocity/time graph

Answer 2

Constant velocity

Question 3

Define displacement

Answer 3

The distance of an object from its initial position in a given direction

Question 4

Define velocity

Answer 4

The rate of change of dispacement

Question 5

What is the the equation for velocity

Answer 5

Displacement/time

Question 6

What is an object average speed if it is accelerating steadily from rest

Answer 6

Half its max speed

Question 7

How do you find instantaneous speed on a distance time graph

Answer 7

Call the gradient of the tangent to the curve at the given time

Question 8

Explain the advantages of. Of using a data logger

Answer 8

Reduces human error so more accurate
Can get real time data
Can take more frequent readings

Question 9

Describe an experiment to derive the acceleration of free fall

Answer 9

Drop an object from a known height that has been measured using a ruler. Use the stopwatch to time how long it takes for the object to reach the ground. Then use the equation
G=2s/t^2
To improve use a video recording with a time setting or use light gates

Question 10

Explain why any experimental value of acceleration of free fall would only be an estimate

Answer 10

-inaccuracies in starting and stopping the stop watch due to reaction times
- air resistance so object will be accelerating at a slightly slower rate than the acceleration of free fall
- there could also be parallax arrow when measuring the time at which the ball hits the ground

Question 11

Define thinking distance

Answer 11

The distance travelled by the vehicle between the moment the driver sees the hazard to the time they apply the brakes

Question 12

Describe the relationship between reaction time and thinking distance

Answer 12

When travelling at a certain speed the reaction time is the time it takes to travel the thinking distance
Speed= thinking distance/ reaction time

Question 13

If velocity was tripled , how would braking distance change

Answer 13

Distance is directly proportional to velocity^2
Therefore distance would be 9 times larger

Question 14

Describe how a collision could be modelled using an air track

Answer 14

The air track will create a nearly frictionless surface
- place two pucks at each end of the air track
Use a metre ruler along side the track to measure the distance between
- push the puck forward so that that both collide
- then use light gates to measure the initial and final speed before and after the collision

Question 15

State the equation that can be used to find speed in the horizontal in projectile questions

Answer 15

Speed=distance/ time.
As the horizontal speed is constant in the horizontal as there is no driving force

Question 16

State the assumptions we make in projectile questions

Answer 16

The vertical acceleration is 9.81 ms^-2
There is no air resistance so the horizontal component is always constant

Question 17

A ball is fired from point A and reaches its maximum height at point B
Explain why the balls g.p.e energy gained by the ball as it move from A-B is not equal to its initial kinetic energy at A

Answer 17

The ball is still moving at B

Question 18

A ball is fired from point x, reaches its max height at point Y and reaches the ground at Z
Describe the balls motion from X-z

Answer 18

It’s vertical component is positive from X-Y and negative from Y-Z
The vertical component at Y=0

Question 19

How is kinetic energy related to speed

Answer 19

Kinetic energy is directly proportional to speed^2

Question 20

Why do astronauts feel weightless in space

Answer 20

There is no contact force between the austronaust and floor of space station

Question 21

Describe how kinetic energy changes during the journey of a bullet fired until it reaches its max height

Answer 21

Kinetic energy reduces with height and at maximum height the kinetic energy is 0

Question 22

Explain why the average speed of an object is not the same as the magnitude of its average velocity

Answer 22

The distance travelled does not have the same magnitude of the displacement as distance is always greater than displacement

Question 23

Explain why horizontal force remains constant as a ball is projected from the ground on a wall

Answer 23

There are no forces affecting the horizontal component of the ball therefore there is no horizontal components and weight only works vertically

Question 24

A student uses a steel ball attached to an electro magnet to calculate acceleration of free fall. They release the ball which drops and opens the trap door. State the possible error when timing the ball and how can the percentage uncertainty be minimised

Answer 24

The steel ball will not be released straight away due to the em , therefore increase the distance of the fall

Question 25

What is meant by thinking distance and how does it vary with the initial speed of a car

Answer 25

Distance travelled from the moment the driver sees the hazard until the brakes are applied. Distance is proportional to speed for constant thinking time

Question 26

How can you use a velocity time graph to find the acceleration of a skydiver and how does acceleration vary with time

Answer 26

Gradient of graph= acceleration, acceleration decreases with time

Question 27

A rocket is made from a 1.5 litre bottle which is only filled with 0.3 litres of water. A pump is used to create pressure and therefore cause the bottle to accelerate upwards. Discuss whether adding more water initially would enable the rocket to reach a great height

Answer 27

Height depends on the bottles acceleration and time taken to expel the water, more water means longer time to expel water, and more water means a decreased resultant force therefore a decreased acceleration

Question 28

An arrow is fired at 65 ms^- from the right to a target on the left. The target has a larger mass than the mass of the arrow. Using ideas of momentum, explain the velocity of the target immediately after the arrow sticks into the target

Answer 28

Due to the conservation of momentum, the velocity will be to the right and it will be much smaller than 65

Question 29

During the experiment of the acceleration of freefall, discuss the possible errors and improvements

Answer 29

There’s a source of random error due to the students reaction time when pressing the stopwatch this can be reduced using light gates. This is also a source of human error.
The experiment could be carried out using an electromagnet connected to a time and a trapdoor system to reduce human error and random errors
Resistance acting on the ball will prevent the student from measuring the true value of G. This can be reduced by using a smaller ball or an object which is more streamlined.
The height is a source of random error. This can be reduced by using a ruler or a measuring tape with a higher resolution
A systematic error on the height measurement due to the student measuring the heights from the top of the board instead of the bottom this can be removed by measuring the heights from the bottom of the wall