SP2 - Motion and Forces

Question 1

SP2a
1) What is the difference between scalar and vector quantities?
2) What is a resultant force?
3) How do you calculate the resultant force?

Answer 1

1) A scalar quantity only has magnitude, while a vector quantity has both magnitude and direction.
2) A resultant force is a single force that describes all the overall forces acting on an object.
3) If the forces are acting in the same direction, add them. If they are acting in opposite directions, subtract one from the other.

Question 2

SP2b
1) What does Newton’s First Law state?
2) How do you know if the forces are balanced or unbalanced from the resultant force?
3) Describe the effect of balanced forces on moving and stationary objects.
4) Describe the effect of a non-zero resultant force on moving and stationary objects.

Answer 2

1) Newton’s First Law of Motion states that objects with balanced forces acting on them will stay at rest or stay in constant motion.
2) If the resultant of the forces on an object is zero, we say that the forces are balanced. If there is a non-zero resultant force on an object, the forces are unbalanced.
3) Balanced forces (zero resultant force) will not change the velocity of an object, so the object will have a constant velocity. A moving object will keep moving at the same speed, and a stationary object will not move.
4) Unbalanced forces (non-zero resultant force) will change the velocity of an object, so the object will accelerate or decelerate.

Question 3

SP2b
1) What is the speed and velocity like in an object moving in a circle?
2) What is the force needed to keep the object moving in a circular path, and what direction does it act in?
3) What is an example of this force in a car on a roundabout?
4) Why must there be a centripetal force acting on an object in order to move in a circular motion?

Answer 3

1) Velocity is speed in a given direction. An object moving in a circle travels at constant speed, but it is always changing direction so its velocity is always changing.
2) The resultant force that causes the change of direction in a circular path is called the centripetal force, and acts towards the centre of a circle.
3) In a car moving in a circle, the centripetal force is provided by the friction from the road on the tyres.
4) Centripetal force changes direction of motion only and not speed. The direction of velocity changes, meaning there must be an acceleration. The object is accelerated, and moves in a circular motion.

Question 4

SP2c
1) What is weight?
2) List all the factors that determine the weight of an object?
3) What is the equation for calculating weight?

Answer 4

1) Weight is the force acting on a mass due to gravity.
2) The mass and gravitational field strength
3) Weight(N) = mass(kg) x gravitational field strength (N/kg)

Question 5

SP2c
1) What piece of equipment is used to measure weight?
2) Describe how the weight of an object is affected by gravitational field strength.
3) What is the difference between mass and weight?

Answer 5

1) Weight is measured using a newton metre. A mass suspended on the lower hook will pull the spring inside and move the indicator along the scale.
2) The bigger the gravitational field strength, the bigger the weight. The smaller the gravitational field strength is, the smaller the weight.
3) Mass is a measure of how much matter there is in an object, while weight is a measure of the size of the pull of gravity on the object.

Question 6

SP2d
1) What does Newton’s Second Law state?
2) What is acceleration?
3) List the factors that affect the acceleration of an object.
4) What is the equation that involves acceleration, mass and force?

Answer 6

1) Newton’s Second Law of Motion states that when an unbalanced force acts on an object, the object should accelerate or decelerate.
2) The acceleration of an object is the rate of change in the velocity.
3) The size of the force acting on it and the mass of an object.
4) Force (N) = mass (kg) x acceleration (m/s²)

Question 7

SP2e
1) Describe what Newton’s Third Law says
2) What is an equilibrium situation?
3) What is an action-reaction pair?

Answer 7

1) Newton’s Third Law states that whenever two objects interact, they exert equal and opposite forces on each other.
2) An equilibrium situation is when both sides are balanced, as both objects are pulling with the same force.
3) Action reaction pair is a pair of forces acting on two interactive objects. They are always the same size, in opposite directions, and the same type of force.

Question 8

SP2e
1) What is an action-reaction pair example in space?
2) What is the difference between action-reaction pairs and balanced forces?
3) Describe how objects affect each other when they collide

Answer 8

1) The earth and moon both exert the same force on each other, and are an action-reaction pair.
2) Action reaction forces act on different objects, while balanced forces act on the same object.
3) Action-reaction forces occur when things collide, and the forces are the same size. The two objects exert an equal and opposite force on each other. However, they do not always have the same effect as the objects are different masses.

Question 9

SP2f
1) What is inertial mass?
2) How does inertial mass affect velocity after a force is applied?
3) What is inertial mass defined as?

Answer 9

1) Inertial mass is a measure of how difficult it is to change the velocity of an object.
2) If the inertial mass is higher, then the velocity after the force has been applied will be lower. because the higher inertial mass will mean that the velocity is more difficult to change.
3) Inertial mass is defined as the ratio of force over acceleration.

Question 10

SP2f
1) What is momentum and describe the factors that affect the momentum of an object?
2) What is the formula and units for momentum?

Answer 10

1) Momentum is a measure of how difficult it is for an object to stop moving. Mass and velocity affect the momentum of an object.
2) Momentum (kg m/s) = Mass (kg) x Velocity (m/s)
Can be written as p = m x v

Question 11

SP2f
1) What is the conservation of momentum?
2) What equation is used to calculate the force needed to produce a change in momentum in a given time?

Answer 11

1) When moving objects collide, the total momentum of both objects is the same before the collision as it was after the collision.
2) Force = (mv-mu) / t
Force = (final momentum - initial momentum) / time

Question 12

SP2g
1) What is the average reaction time of a person driving a car?
2) What is the breaking distance?
3) How is the mass of a car related to its breaking distance?

Answer 12

1) The average reaction time of a driver is 0.5 seconds.
2) Breaking distance is the distance a vehicle travels under the breaking force of the vehicle.
3) The mass is proportional to the breaking distance. The bigger the mass, the bigger the breaking distance.

Question 13

SP2g
1) What tests can be used to investigate people’s reaction times?
2) How does the ruler drop test work?

Answer 13

1) The ruler drop test, and clicking a mouse when the computer screen changes colour.
2) Hold a ruler between the open forefinger and thumb of the person being tested. Align their finger to the zero line of the ruler. Drop the ruler without warning, and have the test subject close their finger and thumb to catch the ruler. The distance the ruler falls can be read from the ruler. The time taken for it to fall can be calculated, as the acceleration (due to gravity) is constant. This is the reaction time of the test subject.

Question 14

SP2g
1) Recall the factors that affect stopping distances and describe how they affect stopping distances.
2) What is the thinking distance?
3) What is the stopping distance?

Answer 14

1) Stopping time. A driver’s reaction time can be affected by: tiredness, drugs, alcohol and distractions.
The breaking distance can be increased by: poor road and weather conditions, poor vehicle conditions, larger vehicle speed, and larger vehicle mass.
2) Thinking distance is the distance travelled during a driver’s reaction time.
3) The stopping distance is the total of the thinking distance and the breaking distance together.

Question 15

SP2h
1) What is the formula to calculate breaking distance?
2) What is work done?
3) What is the formula for calculating work done?

Answer 15

1) Breaking distance = work done / force
2) Work done is the energy transferred by a force over a distance.
3) Work done (J) = force (N) x distance moved in the direction of the force (m)

Question 16

SP2h
1) What are the factors that affect kinetic energy?
2) What is the formula for kinetic energy?
3) How is the breaking distance affected if the speed doubles?
4) What is the work done by the brakes of a car equal to?

Answer 16

1) Mass and speed
2) Kinetic energy (J) = 1/2 x mass (kg) x (speed)² (m/s)²
KE = ½mv²
3) The breaking distance of a car depends on its kinetic energy, and so it depends on the square of its velocity. This means that if the velocity doubles, the breaking distance is multiplied by 2² which is 4.
4) The work done by the brakes of the car is equal to the kinetic energy store of the car before braking.

Question 17

SP2i
1) What is the equation involving velocity, acceleration and time?
2) What is the equation involving velocity, acceleration and distance (you are given this in the exam)?
3) What is a large deceleration?
4) What are the dangers caused by large deceleration?

Answer 17

1) acceleration = (v-u) / t
2) (final velocity)² – (initial velocity)² = 2 × acceleration × distance
v² - u² = 2ax
3) The faster a vehicle travels, the greater the braking force needed to stop it in a certain distance. A greater braking force produces a greater deceleration.
4) Very large deaccelerations can cause injuries. This is because a larger acceleration requires a larger force (since F = m x a).

Question 18

SP2i
1) What is an average mass of a car?
2) What is the average speed of a car and a lorry?
3) What is a crumple zone?
4) What does the force of a road collision depend on, and what formula can be used to calculate it?

Answer 18

1) An average mass of a car is 1000kg.
2) The average speed of a car is 15 m/s, and the average speed for lorry is 25 m/s.
3) Crumple zones crumple in the event of a crash, reducing the force of the impact.
4) The force in a road collision depends on the change of momentum as the car comes to a stop. The formula f = (mv - mu) / t can be used to calculate the force.

Question 19

SP2d - Core Practical
1) What is the aim of the investigating motion core practical?
2) What is the method for the investigating motion core practical?

Answer 19

1) Investigate the relationship between force, mass and acceleration by varying the masses added to trolleys.
2) A. Prop up one end of the ramp. Place a trolley on the ramp. Adjust the slope of the ramp until the trolley just starts to move on its own. Set up the light gates and the pulley.
B. Stick a piece of card to the top of the trolley. Measure the length of the card and write it down.
C. Find the mass of the trolley and write it down.
D. Put a mass on the end of the string. You will keep this mass the same for all your tests. You will have to decide what mass to use.
E. Release the trolley from the top of the ramp and write down the speed of the trolley (from the data logger) as it passes through each light gate. Also write down the time it takes for the trolley to go from one light gate to the other.
F. Put a mass on top of the trolley. Keep the masses on the end of the string as they were before. Repeat step E.
G. Repeat step E for other masses on top of the trolley. You will have to decide what masses to use, how many different masses you are going to test, and whether you need to repeat any of your tests.
H. The steps above are investigating how the mass of the trolley affects the acceleration. If you wish to investigate the effect of force on acceleration, you need to keep the mass the same. However, the masses on the end of the string are also accelerating, along with the trolley, and it is the overall mass that you need to keep the same. You can do this by starting with a stack of masses on the trolley. Take one mass off the trolley and hang it on the end of the string. Then follow step E to measure the acceleration.
I. Now transfer another mass from the trolley to the end of the string and find the acceleration again. Keep doing this until all the masses that started on the trolley have been transferred to the end of the string.

Question 20

SP2d - Core Practical
1) What are the safety precautions for the investigating motion core practical?
2) How do you use the datalogger to determine the acceleration of the trolley?
3) How can you find the uncertainty of one of your mean results?
4) Explain why a light gate is needed at the top of the ramp as well as the bottom

Answer 20

1) Place something soft below the falling mass at the end of the string to break the fall.
Do not stand next to the end of the ramp as to ensure that you do not get hit by falling mass or the trolley.
Attach the masses to the trolley using something that will keep them in place such as tape or sticky tack so that they do not fall off and cause injury.
Make sure the trolley does not fall off the table.
2) You calculate the difference in the speed of the trolley between the two light gates and divide this by the time it took the trolley to travel between the two light gates.
3) By finding the range of the results used to calculate that mean and them using uncertainty = range / 2.
4) To measure the time the trolley takes to move between the two light gates (1) as acceleration is calculated from a change in velocity divided by time (or equivalent explanation). (1) Do not accept ‘to measure the speed/velocity at the beginning’.

Question 21

SP2d - Core Practical
1) Give a potential source of error in the method where a person uses a stopwatch instead of light gates, and how the data could be more accurate
2) Explain why a pulley is used in the experiment
3) Explain why a graph plotting the acceleration and force of the trolley may be different values to the predicted values

Answer 21

1) It is difficult to time exactly when the trolley passes the line. Therefore, you can record video of experiment with timer and play back.
2) The pulley in the experiment is used to reduce frictions.
3) Theoretical values don’t account for friction resultant force smaller than the force pulling on the trolley. Therefore acceleration is not as great.

Question 22

Explain how momentum conserved in a collision, using Newton’s second and third law

Answer 22

Momentum = mass x velocity
Force = change in momentum / time
Action reaction pairs exert equal and opposite forces on each other. Therefore, the change in momentum over time for one object is equal to negative the change in momentum over time for the other object.
The time of the collision is the same for both objects, so there is no overall change in momentum. The object that is being hit accelerates because of the force from the object hitting it, so there is a transfer of momentum between the the two objects.