Physics Grade 9 – Work, Energy, Power & Gravitation Review

These flashcards cover the main learning objectives (LO-1 to LO-20) including work, energy, power, mechanical and gravitational concepts, formulas, and example applications.

What is the definition of work in physics?

Work is the transfer of energy that occurs when a force is applied through a distance in the direction of the displacement (W = F d).

Is work a scalar or vector quantity and what is its SI unit?

Work is a scalar quantity measured in Joules (J), where 1 J = 1 N·m.

How much work is done when a 1 N force moves an object 1 m in the direction of the force?

1 Joule of work is done.

Define energy in the context of physics.

Energy is the ability of a system to do work or produce a change in itself or the surrounding world, measured in Joules.

How do you find the change in energy when three different forces act on a system simultaneously?

Calculate the work done by each force (W₁, W₂, W₃) and sum them: ΔE = W₁ + W₂ + W₃.

Write the formula that relates power to work and time.

P = W⁄t

A pump lifts 35 L of water (35 kg) up 110 m in 60 s. What power does it develop?

Approximately 630 W (0.630 kW).

An elevator motor delivers 65 kW while raising an elevator 17.5 m in 35 s. What force does the motor exert?

1.3 × 10⁵ N.

State the law of conservation of energy.

In a closed system, energy is neither created nor destroyed; it is conserved.

Give the equation for translational kinetic energy.

KE = ½ m v²

If a car’s speed doubles, how does its kinetic energy change?

Kinetic energy becomes four times larger because KE ∝ v².

Doubling an object’s kinetic energy changes its speed by what factor?

The speed increases by √2 ≈ 1.4.

Calculate the kinetic energy of a 70 kg cheetah running at 32 m/s.

KE = ½ (70 kg)(32 m/s)² ≈ 3.58 × 10⁴ J.

Write the formula for rotational kinetic energy.

KE_rot = ½ I ω²

If the angular speed of a merry-go-round doubles, how do angular momentum and rotational kinetic energy change?

Angular momentum doubles; rotational kinetic energy quadruples.

What is the work done by a force acting perpendicular to the displacement?

Zero joules; perpendicular forces do no work.

State the general work formula when force and displacement are at an angle θ.

W = F d cos θ

A 628 N force pulls a box 15 m with the rope 46° above the floor. How much work is done?

≈ 6.54 × 10³ J.

What is meant by mechanical energy?

Mechanical energy is the sum of all kinetic and potential energies of a system.

Write the formula for gravitational potential energy.

GPE = m g h

A 2.2 kg book is lifted from 0.80 m to 2.10 m above a desk. What is its GPE relative to the desk?

≈ 28 J.

State Newton’s law of universal gravitation.

F_g = G (m₁ m₂)⁄r², where G = 6.67 × 10⁻¹¹ N·m²/kg².

How does gravitational force depend on the masses of two objects and the distance between them?

Directly proportional to the product of the masses and inversely proportional to the square of the distance between their centers.

Give the formula for the orbital period of a planet or satellite.

T = 2π √(r³⁄G M) where r is orbital radius and M is the central mass.

Neptune’s orbital period is calculated to be about how many Earth years?

≈ 165 years.

How do you compute gravitational field strength (surface gravity) of a spherical body?

g = G M⁄r²

What is the surface gravity of the Moon (M = 7.3 × 10²² kg, r = 1.738 × 10⁶ m)?

≈ 1.6 m/s².

Which has the larger orbital period: a satellite 150 km or 160 km above Earth?

The 160 km satellite (greater altitude → longer period).

Which satellite moves faster: 150 km or 160 km above Earth?

The 150 km satellite (lower altitude → higher speed).

Define pressure in physics.

Pressure is the perpendicular force per unit area, P = F⁄A.

A 364 N child and 41 N stool rest on legs covering 19.3 cm². What average pressure is exerted?

≈ 2.10 × 10² kPa.

What happens to the pressure when the stool tips so only two legs touch the floor?

The pressure increases to ≈ 3.14 × 10² kPa because the supporting area is halved.

How can work be found graphically from a force–displacement graph?

Work equals the area under the F-vs-d curve.

What is the work represented by a 1.50 m wide rectangle under a 20 N force?

30 J (area = length × width).

What power is produced when a 1.20 × 10⁴ N force lifts an elevator 9 m in 15 s?

7.20 kW.

Two 15 kg balls are 0.35 m apart. What gravitational force exists between them?

≈ 1.22 × 10⁻⁷ N.

How does the gravitational attraction between the two 15 kg balls compare with the weight of one ball?

It is about 2.5 × 10⁻⁹ of the ball’s weight; the weight is roughly two billion times larger.

Give an everyday example where conservation of mechanical energy is applied.

Roller-coaster cars converting potential energy at the top of a hill into kinetic energy at the bottom (KE + GPE = constant, neglecting friction).