General Physics I Midterm

What equation should be used when given v_f, v_i, and a to find t?

v_f = v_i + a(Δt)

Constant acceleration kinematics equation to find position (x or y)?

x_f = x_i + v_iΔt + ½aΔt²

What equation should be used when given v_f, v_i, and a to find Δx?

v_f² = v_i² + 2a(Δx)

A runner accelerates at 2.5 m/s² until reaching a top speed of 12 m/s. He continues at that speed. How long does it take for him to run the 100 m dash?

Based on this question, what formulas should be used to solve?

phase 1: v = a(t) & x_f = x_i + v_iΔt + ½aΔt²

phase 2: d = vt

How to find displacement from a velocity v. time diagram?

find the area under the graph

What is the distance formula?

d = vt

What is Newton’s Universal Law of Gravity?

Fg = -G (m₁m₂ / r_n²)

Define the variables in Newton’s Universal Law of Gravity

Fg = force of gravity

G = gravitational constant

m₁ = mass of object 1

m₂ = mass of object 2

r = radius from center of mass to point

A 500g rocket with a weight of 4.90 N launches. The small rocket burns for 5.0 s and has a steady thrust of 20.0 N. What is the rocket’s max altitude?

Solve.

F_net = F_thrust - Fg

y₁ = y₀ + v₀Δt + ½a₀(Δt)²

v₁ = v₀ + a₀Δt

v_f² = v_i² + 2aΔy

y₁ + v₁/2(Fg)

1,541 m

What is the formula for weight?

w = mg

Formula for static friction force

f_s= μ_sn

Formula for kinetic friction force

f_k= μ_kn

Formula for kinetic friction force

f_k= μ_kn

Which type(s) of friction always oppose motion?

kinetic, rolling

What does the Reynold’s Number describe?

the ratio of inertial forces to viscous forces on an object

Reynold’s Number formula

Re = (ρvL/n) where ρ = fluid density and n = viscosity

What is the formula for drag?

F = (½C_dρAv²)

Define the variables in the formula for drag.

C_d = dimensionless drag coefficient

ρ = density of fluid

A = cross-section area of object

What is the relationship between drag and an object’s speed?

proportional

What is the formula for Stokes Law?

b = 6πnr

What does Stokes Law describe?

viscous drag on a sphere

What is Newton’s First Law?

F_net = 0 ∴ a = 0

What is Newton’s Second Law?

F_net = ma ∴ F ∝ a

What is Newton’s Third Law?

F_1→2 = -F_2→1

What is propulsion?

the force that a system with an internal source of energy uses to drive itself forward

A constant force is applied to an object, causing the object to accelerate at 8 m/s². What will the acceleration be if the force is doubled?

F = ma

2F = m(2a)

2(8 m/s²) = 16 m/s²

A constant force is applied to an object, causing the object to accelerate at 8 m/s². What will the acceleration be if the mass is doubled?

F = ma

F/m = a

F/2m = a/2

(8 m/s²)/2 = 4 m/s²

A constant force is applied to an object, causing the object to accelerate at 8 m/s². What will the acceleration be if the force and object’s mass are both doubled?

F = ma

2F = 2ma

2F/2m = a = 8 m/s²

A constant force is applied to an object, causing the object to accelerate at 8 m/s². What will the acceleration be if the force is doubled and object’s mass is halved?

F = ma

2F = (m/2)a

4F = ma

4a = 32 m/s²

A horizontal rope is tied to a 50 kg box on frictionless ice. What is the tension in the rope if the box is at rest?

0 N

A horizontal rope is tied to a 50 kg box on frictionless ice. What is the tension in the rope if the box is moving at a steady 5.0 m/s?

0 N

What is the tension formula?

T = m(g±a)

A 8.0 × 10⁴ kg spaceship is at rest in deep space. Its thrusters provide a force of 1200 kN. The spaceship fires its thrusters for 20 s, then coasts for 12 km. How long does it take the spaceship to coast this distance?

40 s

A car in neutral is rolling freely down a hill. What is the net force on the car?

down the hill and less than the weight of the car

A stalled car is being pushed up a hill at constant velocity by three people. What is the net force on the car?

zero

A 615 N student standing on a scale in an elevator notices that the scale reads 645 N. From this information, the student knows that the elevator must be moving in what direction?

You cannot tell if it is moving upward or downward

A 615 N student standing on a scale in an elevator notices that the scale reads 645 N. From this information, the student knows that the elevator must be accelerating in what direction?

upward

A 615 N student standing on a scale in an elevator notices that the scale reads 595 N. From this information, the student knows that the elevator must be accelerating in what direction?

downward

The net force on a ball thrown into the air, when it is on the way up after the throw, is in what direction?

downward

The net force on a ball thrown into the air, when it is at its highest point, is in what direction?

downward

The net force on a ball thrown into the air, when it is on the way down before the catch, is in what direction?

downward

Static friction must point in what direction relative to the velocity of an object?

Opposite direction

In order to get an object moving, you must push harder on it than it pushes back on you.

True

The Newton’s 3rd Law pair force to your weight is the normal force of the floor pushing up on you.

True

When there is no force acting on an object, what shape will the velocity v. time diagram take?

Flat line

What shape will the velocity v. time graph take on when there is a net positive constant force acting on the object?

Positively sloped line

What shape will the velocity v. time graph take on when there is a net negative constant force acting on the object?

Negatively sloped line

What shape will the velocity v. time graph take on when there is a variable force acting on the object?

It will be curved

When an object is sitting still, the velocity v. time graph takes on a shape that is a flat line on the x-axis.

True

When does a velocity v. time graph demonstrate an object changing direction?

When it crosses the x-axis