SP1- Motion, forces and conservation of energy

Question 1

What does uncertainty mean?

Answer 1

When you repeat a measurement, you often get a slightly different figure each time you do it due to random error. This means that each result has some uncertainty attached to it.

Question 2

How do you calculate uncertainty?

Answer 2

Uncertainty= range/2

Question 3

What happens if the range is larger?

Answer 3

The less precise your results are and so the more uncertainty there will be in your results.

Question 4

How can you reduce uncertainty?

Answer 4

By measuring a greater amount of something. For example, in a speed experiment, measuring the distance travelled over a longer period compared to a longer period will reduce the percentage uncertainty of your results.

Question 5

What is a vector quantity?

Answer 5

A quantity that has both magnitude (size) and direction.

Question 6

Name 6 examples of a vector quantity?

Answer 6

Force, velocity, displacement, weight, acceleration and momentum.

Question 7

What is a scalar quantity?

Answer 7

A quantity that only has magnitude and not direction.

Question 8

Name 6 examples of a scalar quantity?

Answer 8

Speed, distance, mass, energy, temperature, time

Question 9

What is distance?

Answer 9

How far an object has moved. It’s a scalar quantity as it doesn’t require direction.

Question 10

What is displacement?

Answer 10

A vector quantity. It measures the distance and direction in a straight line from an object’s starting point to its finishing point.

Question 11

If you walk 5m North and 5m South, what is your distance and displacement?

Answer 11

Distance= 10m 
Displacement= 0m because they're at the same point as where they were when they started.

Question 12

What do speed and velocity both do?

Answer 12

Measure how fast an object is moving, but speed is scalar and velocity is a vector.

Question 13

What does this mean?

Answer 13

That you can have objects travelling at a constant speeds with a changing velocity. This happens when the object is constantly changing direction whilst staying at the same speed.

Question 14

What is the average walking, cycling, running and both car speed in a built-up area and cars on a motorway?

Answer 14

Walking- 1.4m/s
Cycling- 5.5m/s
Running- 3m/s
Car in a built-up area- 13m/s
Cars on a motorway- 31m/s

Question 15

What is acceleration?

Answer 15

The change in velocity of an object in a certain amount of time.

Question 16

How do you calculate acceleration?

Answer 16

(v-u)/t or final velocity-initial velocity over time.

Question 17

What is deceleration?

Answer 17

Negative acceleration (if something slows down, the change in velocity is negative).

Question 18

A car is travelling at 15m/s, when it collides with a tree and comes to a stop. Estimate the deceleration of the car. What steps should you take?

Answer 18

1) Estimate how long it would take the car to stop
2) Put these numbers into the acceleration equation.
3) As the car has slowed down, the change in velocity would be negative and so the acceleration would be negative- the car is decelerating.

Question 19

What does uniform acceleration mean?

Answer 19

Constant acceleration.

Question 20

Acceleration due to gravity is…

Answer 20

Uniform for objects in free fall. It’s roughly equal to 10m/s squared near the Earths surface and has the same value as gravitational field strength.

Question 21

What is the equation for uniform acceleration?

Answer 21

V squared - U squared= 2 x a x 𝑥 where 𝑥 is direction.

Question 22

How is speed represented on a distance/time graph?

Answer 22

It’s the gradient of a the line.

Question 23

If the line is curved, how do you find the gradient?

Answer 23

You need to draw a tangent to the curve at the point and then find the gradient of the tangent.

Question 24

What does the gradient show on a velocity/time graph?

Answer 24

Acceleration, since acceleration= change in velocity/ time.

Question 25

What do horizontal lines represent on a velocity/time graph?

Answer 25

Constant velocity.

Question 26

What does a curve mean in a velocity/time graph?

Answer 26

Changing acceleration. If the graph is curved, you can use a tangent to the curve at a point to find the acceleration at that point.

Question 27

How is distance represented in a velocity/time graph?

Answer 27

The area under any section of the graph is equal to the distance travelled in that time interval.
For bits of the graph where the acceleration is constant, you can split the area into rectangles and triangles to work it out.
You can also find the area under the graph by counting the squares under the line and multiplying the number by the value of one square.

Question 28

What is Newton’s first law of motion?

Answer 28

That an object remains in the same state of motion unless a resultant force acts on it.

Question 29

What does it mean if the resultant force on a object is 0?

Answer 29

• A stationary object stays stationary.

- A moving object continues to move at the same velocity.

Question 30

What are some examples of objects with uniform motion that can Newton’s first law can be used to explain?

Answer 30

Newton’s first law can be used to explain the movement of objects travelling with uniform motion (constant velocity). For example, when a car travels at a constant velocity, the driving force from the engine is balanced by the resistive forces such as air resistance and frictional forces in the car’s moving parts. The resultant force on the car is 0. Other examples include:

• A runner at their top speed experiences the same air resistance as their thrust.
• An object falling at terminal velocity experiences the same air resistance as its weight.

Question 31

How can newton’s first law be used to explain objects with non-uniform motion?

Answer 31

This includes situations when the speed changes, the direction changes or both change. For example, when a car accelerates, the driving force from the engine is greater than the resistive forces. The resultant force is not 0.
Other examples include:
-At the start of their run, a runner experiences less air resistance than their thrust, so they accelerate.
- An object that begins to fall experiences less air resistance than its weight, so it accelerates.

Question 32

What is Newton’s 2nd law?

Answer 32

Newton’s 2nd law can be described by this equation:
F=ma
This is when F is the resultant force and is measured in N
Mass is measured in kg
acceleration is measured in (m/s2) .

Question 33

What does F=ma show?

Answer 33

That the acceleration of an object is:

• Proportional to the resultant force on the object.
• Inversely proportional to the mass of the object.

Question 34

What is inertial mass?

Answer 34

The ratio of force over acceleration. It’s a measure of how difficult it is to change the velocity of an object.

Question 35

Explain the core practical that investigates the relationship between the force, mass and acceleration by varying the masses added to trolley.

Answer 35

1) Measure the mass of the trolley, the unit masses and the hanging hook. Measure the length of the piece of card which will interrupt the light gate beams. Then set up your apparatus with the trolley at one end of the ramp with some height to reduce friction and the 2 light gates above to measure initial and final velocity and the weights attached to the trolley at the other end of the ramp.
2) Adjust the height of the ramp until the trolley just starts to move.
3) Mark a line on the ramp just before the 1st light gate- this is to make sure he trolley travels the same distance each time. The 1st light gate will measure initial velocity of the trolley as it just begins to move.
4) Attach the trolley to the hanging mass by the string. Hold the trolley still at the starting line, and then let go of it as it starts to roll down the slope.
5) Each light gate will record the time when the trolley passes through it and the speed of the trolley at that time. The acceleration can then be found by using (v-u) over time.

Question 36

What is inertia the tendency for?

Answer 36

Motion to remain unchanged.

Question 37

How can inertial mass be found?

Answer 37

By rearranging f=ma to m= f/a, so inertial mass is just the ratio of force over acceleration.

Question 38

What is Newton’s 3rd law?

Answer 38

Whenever 2 objects interact, they exert equal and opposite forces on each other.

Question 39

What is important to remember about the 2 forces in Newton’s 3rd law?

Answer 39

• Act on 2 different objects.

- Are of the same type of force (eg both contact forces).

Question 40

Newton’s 3rd law can be applied to examples of equilibrium situations.
What are the forces between a cat sitting on the ground?

Answer 40

There are contact gravitational forces between Earth and the cat:
- The cat pulls the Earth up.
- The Earth pulls the cat down.
The forces are equal in size and opposite in direction.

Question 41

What is weight?

Answer 41

A force pushing against an object due to gravity.

Question 42

What is weight directly proportional to?

Answer 42

Mass of an object.

Question 43

How can weight be calculated?

Answer 43

Weight= mass x gravitational field strength

Question 44

What is momentum?

Answer 44

The product of the object’s mass and velocity.

P= mv.

Question 45

What is the conservation of momentum principle?

Answer 45

Total momentum before= total momentum after. IN a closed system, the total momentum before an event ( eg a collision) is the same after the event.

Question 46

What is a closed system?

Answer 46

Something that isn’t affected by external forces.

Question 47

What does force cause?

Answer 47

Changes in momentum.

Question 48

When a resultant force acts on an object for a certain amount of time, it causes a change in momentum. How can Newton’s 2nd law be used to explain this?

Answer 48

• A resultant force on an object causes it to accelerate: force= mass x acceleration.
• Acceleration is just a change in velocity overtime, so force: mass x change in velocity/ time.

Question 49

Mas x change in velocity is equal to change in momentum, so what is the final equation for change in momentum?

Answer 49

Force = change in momentum/time
F= (mv-mu)/t

Question 50

What happens the faster a given change in momentum?

Answer 50

The bigger the force causing the change in momentum.

Question 51

What is the equation that links stopping, thinking and braking distance?

Answer 51

Stopping distance= thinking distance+ braking distance.

Question 52

What is thinking distance? What is it affected by?

Answer 52

The thinking distance is the distance the car travels in the driver’s reaction time (the time taken between noticing the hazard and applying the brakes). It’s affected by 2 main factors:

1) Your reaction time- this is increased by tiredness, drugs, alcohol, distractions etc.
2) Your speed - the faster you’re going, the further you’ll travel during you’re reaction time.

Question 53

What is braking distance? What is it affected by?

Answer 53

The distance taken for the car to stop once the brakes have been applied. It’s affected by:

1) Your speed- the faster you’re going, the longer it takes to stop.
2) The mass of the car- a car full of people and luggage won’t stop as quickly as an empty car.
3) The condition of the brakes- worn or faulty brakes won’t be able o brake with as much force.
4) How much friction there is between your tyres and the road- you’re more likely to skid if the rad is dirty, if it’s wet or icy or if the tyres are bald (tyres must have a minimum tread depth of 1.6mm).

Question 54

Describe the ruler drop experiment and how it measures reaction times.

Answer 54

Everyone’s reaction time is different and many different factors affect it.

1) Sit with your arm resting on the edge of a table (this should stop you moving your arm up or down during the test). Get someone else to hold a ruler so it hangs between your thumb and finger, lined up with 0.
2) Without warning, the 2nd person should drop the ruler and the other person should catch it as quickly as they can.
3) The measurement on the ruler at the point where it was caught is how far the ruler dropped in the time it took you to react.
4) The longer the distance, the longer the reaction time.
5) You can calculate how long the ruler was falling for by using d-s/t so t = d/s as the acceleration is constant.
6) Do lots of repeats for accuracy and take an average for the distance the ruler fell.
7) Make sure it’s a fair test- keep the variables you’re not testing the same every time (eg using the same ruler for each repeat and the same person dropping the ruler for each repeat.
8) A typical reaction time is between 0.4s-0.9s.

Question 55

Energy in the car’s kinetic energy store= …

Answer 55

Work done by the brakes.

Work done = force x distance.

Question 56

What is the equation for this?

Answer 56

1/2 x m x v squared (equation for kinetic energy) = fd (equation for work done).