Newton's Laws and Work, Energy and Power

Question 1

What is Newton’s 1st Law of Motion

Answer 1

An object will remain at rest or travelling at a constant velocity, until it experiences a resultant force causing it to change speed or direction

Question 2

What is Newton’s 2nd Law of Motion

Answer 2

F = Δmv / t

Force = Mass x Velocity / Time

Force is the rate of change of momentum

Question 3

What is Newton’s 3rd Law of Motion

Answer 3

For each force experienced by an object, the object exerts an equal and opposite force

Question 4

Describe an example of a free body diagram of a man climbing a ladder

Answer 4

Rw, horizontal force from the wall

RF, a vertical force from the floor

F, a horizontal force from the floor

W, weight of the ladder

Rm, contact force from tha man (not the man’s weight but the weight that acts on him)

Question 5

What is a free body diagram

Answer 5

A diagram which shows all the forces that act on an object

Question 6

What are the Work Done Equations

Answer 6

• W = f x d

Work done (J) = force (N) x distance moved in the direction of the force (m)

• W = Fs cos θ

Work done (J) = force (N) x distance travelled x cos (angle between direction of force and direction of motion)

Question 7

What is the principle of conservation of energy

Answer 7

Energy cannot be created or destroyed, but can be transferred from one form to another. Therefore, the total energy in a closed system stays constant

Question 8

What is the equation for gravitational potential energy

Answer 8

ΔEp = mgh

Gravitational Potential energy(J) = Mass (kg) x Gravitational Field Strength (N/kg) x Height (m)

Question 9

What is the equation for kinetic energy

Answer 9

Ek = 1/2 x m x v²

Kinetic energy (J) = ½ x Mass (kg) x Velocity2 (m/s)

Question 10

Explain the Gravitational Potential and Kinetic Energy while thowing a ball upwards

Answer 10

• As a ball is thrown up into the air, the thrower gives the ball kinetic energy moving it upwards
• The ball slows down because kinetic energy is transferred to gravitational potential energy
• Eventually, all of the kinetic energy is transferred into gravitational potential energy so the ball stops momentarily
• Afterwards, the ball’s gravitational potential energy is converted back into kinetic energy and falls to the ground

Question 11

Explain what happens to the gravitational potential energy of a skateboarder sliding down a slope as they come to a stop

Answer 11

• The gravitational potential energy is transferred into kinetic energy
• Since work is being done against the resistive forces (air resistance and friction)
• Eventually all of the gravitational potential energy is dissipated as heat

Question 12

Explain why the initial kinetic energy given to a ball durig projectile motion is not equal to the maximum gravitational potential energy

Answer 12

• Work is being done by the ball to work against air resistance and friction
• Meaning some of the kinetic energy of the ball is dissipated to the enivroment in the form of heat and sound
• Therefore the initial kinetic energy given to the ball is not equal to the maximum gravitational potential when the ball has stopped in mid-air

Question 13

How would you approach a question where you have to calculate the maximum speed of a moving pendulum

Answer 13

• Find the maximum gravitational potential energy
• Equate this to the kinetic energy formula
• Rearrange to find velocity

Question 14

What are the equations for power in regards to speed

Answer 14

• P = ∆ W / ∆ t

Power (W) = Work Done(J) / Time (s)

• P = Fv

Power (W) = Force (N) x Velocity (m/s)

Question 15

What is the area under a force-displacement graph equal to

Answer 15

The work done

Question 16

What is efficiency

Answer 16

A measure of how efficiently a system transfers energy

Question 17

What is the Efficiency Equation

Answer 17

useful output power / input power

Question 18

Describe an experiment to check whether the speed of the rising mass is constant

Answer 18

• Mark out (equal) distances along height being raised
• Measure time taken to travel each of these distances
• The times should be equal
• Or use a light gate to measure measure distance or speeds at regular intervals and the increase in distance or speeds should be constant

Question 19

Describe how the speed of a trolley may be measured experimentally

Answer 19

• Use light gates by place a piece of card on trolley of measured length
• The card will obscure the light gate
• Calculate speed from length of card/time obscured