5: Forces

Question 1

What is the difference between a scalar and a vector?

Answer 1

Scalar quantities have magnitude only.

Vector quantities have magnitude and an associated direction.

Question 2

What is the difference between a contact and a non-contact force?

Answer 2

Contact forces: the objects are physically touching
- Friction
- Air resistance
- Tension
- Normal contact

Non-contact forces: the objects are physically separated
- Electrostatic
- Weight
- Magnetism

Question 3

What is the equation for calculating weight?

Answer 3

W = mg

W: weight (newtons, N)

m: mass (kilograms, kg)

g: gravitational field strength (Newtons per kilogram, N/kg)

Question 4

What is the resultant force?

Answer 4

A number of forces acting on an object may be replaced by a force that has the same effect as all the original forces acting on together. This single force is called the resultant force.

A single force can be resolved into two components acting at right angles to each other. These two component forces together have the same effect as a single force.

Question 5

What is the equation for calculating mechanical work done?

Answer 5

W = Fs

W: work done (joules, J)

F: force (newtons, N)

s: distance (metres, m)

Question 6

What is the difference between plastic and elastic deformation.

Answer 6

If the object, after being deformed, does not return to its original shape, is plastic deformation. If the object returns to its original shape, it is elastic deformation.

Question 7

State Hooke’s law

Answer 7

The extension of an elastic object, such as a spring, is directly proportional to the force applied, providing that the limit of proportionality is not exceeded.

F = ke

F: force (newtons, N)

k: spring constant (newtons per metre, N/m)

e: extension: (metres, m)

Question 8

What is the equation for calculating elastic potential energy?

Answer 8

E = 1/2ke²

E: elastic potential energy (joules, J)

k: spring constant (newtons per metre, N/m)

e: extension (metres, m)

Question 9

What is the difference between distance and displacement?

Answer 9

Distance is how far an object moves. It does not involve direction. Distance is a scalar quantity.

Displacement involves both the distance the object moves and the direction of the straight line. Displacement is a vector quantity.

Question 10

What is the equation for calculating distance travelled from speed?

Answer 10

s = vt

s: distance (metres, m)

v: speed (metres per second, m/s)

t: time (seconds, s)

Question 11

What is the equation for calculating average acceleration?

Answer 11

a = Δv/t

a: acceleration (metres per second squared, m/s²)

Δv: change in velocity (metres per second, m/s)

t: time taken (seconds, s)

Question 12

What is the equation that can be used to calculate uniform acceleration?

Answer 12

v² - u² = 2as

v: final velocity (metres per second, m/s)

u: initial velocity (metres per second, m/s)

a: acceleration (metres per second squared, m/s²)

s: distance (metres, m)

Question 13

State Newton’s first law

Answer 13

If the resultant force acting upon an object is zero and…

…the object is stationary, the object remains stationary

…the object is moving, the object continues to move at the same speed and in the same direction, so the object continues to move at the same velocity.

Question 14

Define inertia.

Answer 14

The tendency of objects to continue in their state of uniform motion.

Question 15

State Newton’s second law.

Answer 15

The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object.

As an equation:

F = ma

F: force (newtons, N)

m: mass (kilograms; kg)

a: acceleration (metres per second squared, m/s²)

Question 16

What is inertial mass?

Answer 16

Inertial mass is a measure of how difficult it is to change the velocity of an object. It is defined as the ratio of force over acceleration.

Question 17

State Newton’s third law.

Answer 17

Whenever two objects interact, the forces they exert on each other are equal and opposite.

Question 18

What is the stopping distance of a vehicle?

Answer 18

The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the breaking force.

Question 19

What is the equation for calculating momentum?

Answer 19

p = mv

p: momentum (kilograms metre per second, kgm/s)

m: mass (kilograms, kg)

v: velocity (metres per second, m/s)

Question 20

State the law of conservation of momentum.

Answer 20

In a closed system, the total momentum before an event is equal to the total momentum after the event.

Question 21

Required practical 18: What apparatus is required to investigate the relationship between force and extension of a spring.

Answer 21

• a spring
• a metre ruler
• a splint and tape to act as a pointer
• a 10N weight stack
• two clamps and bosses
• a heavy weight and G clamp to prevent the apparatus tipping over
• safety goggles

Question 22

Required practical 18: State a method you could use to investigate the relationship between force and extension of a spring.

Answer 22

1: Set up the apparatus as shown in the diagram.

—(diagram WILL be provided)—

• attach two clamps and bosses to the clamp stand, one above the other, pointing in the same direction
• place the heavy weight at the base
• attach the zero end of the ruler to the lower clamp, making sure it is vertical
• attach one end of the spring to the upper clamp.
• attach the splint to the bottom end of the spring with the tape, so that it is perpendicular and is pointing towards the zero end of the ruler.

—(This information is not required, )—
—(it is just here for reference)—

2) Take a reading on the ruler - this is the length of the unstretched spring. Record this reading in your results table under 0N of weight added.

3) Carefully hook the base of the weight stack onto the bottom of the spring. This weighs 1.0 Newton (1.0N). Don’t forget that the mass added will have to be converted to newtons.

4) Take a reading on the ruler - this is the length of the spring when a force on 1.0N is applied to it.

5) Add further weights. Measure and record the length each time.

6) Calculate the extension for each weight and record it in the table.

Question 23

Required Practical Activity 19.1: What apparatus is required to measure the effect of force on acceleration at constant mass?

Answer 23

• a toy car (or trolley)
• a metre ruler
• pencil, chalk or masking tape to mark the intervals
• a bench pulley
• string
• a small weight stack
• a stopwatch
• Blu-tac

Question 24

Required Practical Activity 19.1: State a method you could use to measure the effect of force on acceleration at constant mass?

Answer 24

1: Use the ruler to measure intervals on the bench and draw straight lines or place tape across the bench at these intervals.

2: Attach the bench pulley to the end of the bench

3: Tie a length of string to the toy car or trolley. Pass the string over the pulley and attach the weight stack to the other end of the string.

4: Make sure the string is horizontal and in line with the toy car or trolley

5: Hold the toy car or trolley at the start point

6: Attach the full weight stack (1.0 N) to the end of the string

7: Release the toy car or trolley at the same time as you start the stopwatch, press the stop watch (lap mode) at each measured interval on the bench and for the final time at 100 cm

8: Record the results in a table

9: Repeat steps 5-8 for decreasing weights on the stack for example, 0.8N, 0.6N, 0.4N, 0.2N.

Question 25

Required Practical Activity 19.2: What apparatus is required to measure the effect of mass on acceleration with a constant force.

Answer 25

The same apparatus as you used in activity 1

Question 26

Required practical activity 19.2: State a method you could use to measure the effect of mass on acceleration with a constant force.

Answer 26

1: Setup the bench, pulley, weight stack and car as in steps 1-5 of activity 1

2: Use your results from activity 1 to select a weight for the weight stack that will just accelerate the car along the bench.

3: Put a 200g mass on the car

4: Hold the car at the start point.

5: Attach your chosen weight stack to the end of the string

6: Release the car at the same time as you start the stopwatch, press the stopwatch (lap mode) at each measured interval on the bench and for the final time at 100cm

7: Record the results in the table outline below.

8: Repeat steps 5-8 for increasing more masses on the car.