Observing and recording motion

Question 1

How do you calculate speed and what are the si units

Answer 1

Average Speed = Total distance traveled/ total time taken and is usually measured in metres per second (m/s)

It can also be measured in km/h and miles/h

Question 2

Explain the difference between a vector and scalar qualities giving examples.

Answer 2

Scalar qualities have a magnitude eg speed and distance. You cannot tell what direction is being travelled.

Vector qualities have both magnitude and direction, speed becomes velocity when direction is added direction becomes displacement.

You can maintain a constant speed but if you change direction then your velocity will change

Question 3

Distance time graphs

Answer 3

You could be asked plot or read from these type of graphs.

The speed between two points can be calculated from the gradient of the line by drawing a right angled triangle.

Remember as with all graphs check the scale and the units

Draw points in pencil, join them together using a ruler, remember you can get times when no distance is traveled, this would be a flat line.

Question 4

Define acceleration and give the formula and units

Answer 4

It is a change in velocity so a car accelerates as it speeds up. Deceleration is slowing down.

acceleration = change in velocity
———————–
time taken

a=∆v/t the delta sign indicates change

change in velocity is measured in m/s and time is measured in s therefore acceleration is measured in m/s2

Question 5

Velocity time graphs- important points

Answer 5

You could be asked to draw one of these from a table of information or read information and make calculations from one remember CHECK the SCALE

1. Time will be on the x axis and velocity on the y axis
2. The gradient of the graph shows acceleration, so a flat line shows constant velocity, forward slope acceleration backward slope deceleration
3. The area under the graph represents distance (displacement) this can be calculated by working out the area of the triangle (1/2base x height) or by the formula distance = average speed x time

Question 6

Give the equation for an object traveling at a constant velocity

Answer 6

v2-u2=2as
u = starting velocity  m/s
v = final velocity m/s
a = acceleration m/s2
s = distance traveled m.

you could be given any 3 of these variables and asked to calculate the fourth

Question 7

Calculating the distance traveled given a velocity time curve

Answer 7

When the graph is a curve rather than a straight line we cannot be quite as accurate calculating the distance- remember it is the area under the curve.

1. The graph will be drawn on squared paper.
2. Count the whole large squares, I would number them.
3. Estimate the part squares (so put together ones which are 1/3d with 2/3rds etc) eg there could be 20 part squares which you estimate at around 7 full ones
4. Add together the whole and part squares eg 23 +7+ 30
5. Now calculate the area of one square by multiplying the velocity by the time for that square. EG if the scale for one large square is 2m/s on the y axis and 4s on the x axis then the area will be 2 x 4 = 8m
6. Multiply the number of squares by the area of one square to get the the total distance eg 30 x 8m = 240m

Question 8

What would be the acceleration of a falling object due to the gravitational pull of the earth if there were no air resistance?

Answer 8

9.8m/s

The mass of the object does not affect acceleration however air resistance does.

Question 9

Which two factors affect the size of the air resistance of a falling object?

Answer 9

1. Area- the larger the area the greater the air resistance

2. Speed- the faster the speed the greater the air resistance

Question 10

Describe how the velocity of a sky diver changes between leaving the plane, opening her parachute and landing

Answer 10

1. She accelerates quickly to 9.8m/s on leaving the plane (steep upward curve)
2. The effect of air resistance slows her acceleration as she falls but she continues to accelerate as her weight is greater than the air resistance so the resultant force is downwards (shallower upward curve)
3. The air resistance increases as her speed increases until it is the same size as her weight, the resultant force is now 0 and she has reached terminal velocity. (Straight horizontal line)
4. She opens her parachute which increases the surface area and therefore the air resistance. She starts to decelerate and slows down, the resultant force is upwards. (steep downwards line)
5. The air resistance on the parachute is now the same as her weight so she continues at a slower terminal velocity until she hits the ground. (low horizontal line)
6. She lands (small drop to 0)

Question 11

Outline Newtons laws of motion

Answer 11

1st Law of motion called the law of Inertia- or balanced forces- a body will remain stationary or moving at a constant velocity unless acted on by a force.

2nd Law of Motion- sometimes called the law of unbalanced forces- a net force acting on a body will cause a change in acceleration or deceleration of a body
F = m x a
It can also cause a change in direction of the body (swerving)

3rd Law of motion- For every action there is a reaction which is equal and opposite

Question 12

Describe the relationship between mass force and acceleration according to newtons 2nd law of motion,

Answer 12

Acceleration is proportional to the resultant force acting on the object (a ∝ F)

Acceleration is inversely proportional to the mass of the object (a ∝ 1/m)

therefore 
F = m x a
F = resultant force in N
m = mass in kg
a = acceleration in m/s2

Question 13

Explain Newtons first law of balanced forces using examples

Answer 13

1 Newtons first law states that if the resultant forces are balanced a stationary object remains stationary, so if a box is lying on the ground it will stay where it is unless a force acts on it.
2. It also states that a moving object will continue in a straight line at a steady speed, eg a car with the driving forces forward balancing the air resistance backwards.

Question 14

Explain Newtons 2nd law of unbalanced forces using examples

Answer 14

Unbalanced forces cause acceleration or deceleration of an object and can cause a change of direction.

Eg a bowler throws a ball towards the batsman, the resultant force is forwards and the ball accelerates towards him. The batsman strikes the ball with the bat producing a force in the opposite direction which is greater than the forward force, the ball changes direction

A driver takes his foot off the accelerator of the car, this reduces the driving force and the air resistance backwards is now greater than driving force forwards and so the car slows down

Question 15

Define the term Inertia

Answer 15

Inertia is a state of inactivity. Newtons 1st law is often referred to as the law of inertia.
An object will remain at rest or moving with a constant speed unless acted on by an unbalanced force

Question 16

Explain Newtons 3rd law using practical examples

Answer 16

Newtons 3rd law states that every force has a paired force acting in the opposite direction.
For example, if you lean against a wall the wall exerts a force back towards you.
If you stretch an elastic band between two exactly equal toy trucks and then release it they will move towards each other at the same speed meeting exactly in the middle

Question 17

Explain the three different distances involved when a driver brakes on a car

Answer 17

1. Thinking distance, the distance travelled between the stimulus to brake and the when the action is initiated. the time taken is called reaction time.
2. Braking distance, the distance travelled before the car comes to a halt.
3. Stopping distance, the sum of the thinking and braking distances.

Question 18

Explain the main factors affecting braking distance in a car.

Answer 18

1. The speed that the car is travelling. The faster the car the longer the thinking as well as braking distance.
2. The size of the braking force- harder braking stops the car more quickly therefore reducing the distance. If the force is too large however it can cause the car to skid and loose control.
3. Weather conditions - wet or icy conditions reduce the frictional force between the tyres and the road which increases braking distance.
4. The condition of the car, worn tyres and brakes increase braking distance.
5. Road surface, the rougher the road surface the more friction and therefore the shorter the braking distance.

Question 19

Why does a cars brakes get hot when breaking hard?

Answer 19

The braking force reduces the kinetic energy of the vehicle, causing it slow down then stop. As the kinetic energy of the car goes down the temperature of the brakes goes up.

Question 20

Momentum is a property of moving objects, describe it in terms of mass and velocity.

Answer 20

Momentum is mass in motion therefore it is mass x velocity
Momentum (p) in kgm/s
Mass (m) in kg
velocity (v) in m/s

p = m x v

Question 21

Is momentum a vector or scalar quantity?

Answer 21

It is a vector because it always has a direction

Question 22

What is meant by a closed system when describing momentum

Answer 22

Closed means that there are no external forces acting.

Question 23

Describe what happens to the momentum when two objects collide with each other in a closed system

Answer 23

The total momentum is the same after the collision as it was before. This is called conservation of momentum.
Objects travelling the same direction combine momentum whereas objects travelling in different directs reduce momentum
EG two ice hockey players collide when moving in the same direction
red was moving at 3m/s and weighs 100kg, blue was moving at 5m/s and weighs 80kg, what was the final momentum?
3m/s x 100kg = 300 kgm/s 5m/s x 80kg = 400kgm/s total = 700kgm/s
If they collided head on with each other however it would be
400kgm/s - 300kgm/s = 100kgm/s which would be their final momentum and they would continue traveling in the direction of the blue player as he had the largest momentum.