Forces

A 60.-kilogram astronaut weighs 96 newtons on the surface of the Moon. The acceleration due to gravity on the Moon is

• 9.8 m/s²

• 1.6 m/s²

• 4.9 m/s²

• 0.0 m/s²

Answer explanation

a = F/m

a = 96N/60kg

Which graph best represents the motion of an object that has no unbalanced force acting on it?

• 
• 
• 
• 

Answer explanation

No unbalanced force means the object is in equilibrium. If it is moving, then it must be moving with a constant velocity. Only the displacement-time graph shows constant velocity.

A table exerts a 2.0-newton force on a book lying on the table. The force exerted by the book on the table is

• 0 N

• 2.0 N

• 0.20 N

• 20. N

Answer explanation

An object at rest on a flat surface, the normal force equals the weight. Also, Newton's 3rd Law.

A 1.0 x 10²-kilogram box rests on the bed of a truck that is accelerating at 2.0 meters per second². What is the magnitude of the force of friction on the box as it moves with the truck without slipping?

• 2.0 x 10² N

• 0.0 N

• 1.0 x 10³ N

• 5.0 x 10² N

Answer explanation

If the box is not moving, friction must be balancing the forwards for the truck is exerting on the book. The forwards force is equal to mass x acceleration, 1.0 x 10² kg x 2 = 2 x 10²N

The weight of an apple is closest to

• 10^-2 N

• 10⁰ N

• 10⁴ N

• 10² N

Answer explanation

You must remember the weight of an apple is about 100g = 0.1kg. Weight = mg = (0.1kg)(9.81m/s²) = 0.981N, about 1N. 10⁰ is equal to 1.

In the diagram below, the weight of a box on a plane inclined at 30.^o is represented by the vector W. What is the magnitude of the component of the weight (W) that acts parallel to the incline?

• 1.5W

• W

• 0.50W

• 0.87W

Answer explanation

Parallel component is F_gsinθ = Wsin30⁰ = 0.50W

The diagram below represents a car resting on a hill. Which vector best represents the weight of the car?

• A

• B

• C

• D

Answer explanation

Weight always acts straight down

Four forces are acting on an object as shown in the diagram below. If the object is moving with a constant velocity, the

magnitude of force F must be

• 0 N

• 100 N

• 40 N

• 20 N

Answer explanation

Constant velocity means forces are balanced. So F must be 40N

What is an essential characteristic of an object in equilibrium?

• zero velocity

• zero kinetic energy

• zero acceleration

• zero potential energy

Answer explanation

An object in equilibrium can be at rest OR moving with a constant velocity, but will ALWAYS have ZERO ACCELERATION

Which statement explains why a book resting on a table is in equilibrium?

• The weight of the book equals the weight of the table.

• The weight of the book and the table's upward force on the book are equal in magnitude, but opposite in direction.

• There is a net force acting downward on the book.

• The acceleration due to gravity is 9.8 m/s2 for both the book and the table.

Answer explanation

Newton's 3rd Law. F_N = F_g