Topic 4 - Forces Review

A box is pushed toward the right across a classroom floor. The force of friction on the box is directed toward the

• ceiling

• floor

• left

• right

Answer explanation

Friction always acts OPPOSITE the direction of motion

Tags

NGSS.HS-PS2-1

The force required to start an object sliding across a uniform horizontal surface is larger than the force required to keep the object sliding at a constant velocity. The magnitudes of the required forces are different in

these situations because the force of kinetic friction

• decreases as the speed of the object relative to the surface

  increases

• increases as the speed of the object relative to the surface

  increases

• is greater than the force of static friction

• is less than the force of static friction

Answer explanation

Kinetic friction is ALWAYS less than static friction

In the diagram a box is at rest on an inclined plane. Which vector best represents the direction of the normal force acting on the box?

• A

• B

• C

• D

Answer explanation

Normal force always acts PERPENDICULAR to the surface

Tags

NGSS.HS-PS2-1

The diagram below shows a 10.0-kilogram mass held at rest on a frictionless 30.0 degree incline by force F. What is the approximate magnitude of force F?

• 9.81 N

• 49.1 N

• 98.1 N

• 85.0 N

Answer explanation

If the box is at rest, the force F must be equal to the parallel component of gravity which is pulling the box down the incline. F_gII = F_gsin = (10*9.81)sin(30^o)= 49.1 N

Tags

NGSS.HS-PS2-1

Three forces act on a box on an inclined plane as shown in the diagram. [Vectors are not drawn to scale.] If the box is at rest, the net force acting on it is equal to

• the weight

• zero

• the normal force

• friction

Answer explanation

If an object is moving with a constant velocity OR at rest, the net force MUST EQUAL ZERO.

Tags

NGSS.HS-PS2-1

A 50.-newton horizontal force is needed to keep an object weighing 500. newtons moving at a constant velocity of 2.0 meters per second across a horizontal surface. What is the magnitude of the frictional force acting on the object?

• 500 N

• 50 N

• 0 N

• 450 N

Answer explanation

If the object is moving at a constant velocity, forces must be BALANCED. So if it is being pushed with a force of 50N, then the frictional force must be 50N.

Tags

NGSS.HS-PS2-1

A wooden block is at rest on a horizontal steel surface. If a 10.-newton force applied parallel to the surface is required to set the block in motion, how much force is required to keep the block moving at constant velocity?

• less than 10 N

• 10 N

• greater than 10 N

Answer explanation

Once in motion, you will be dealing with kinetic friction, which is always LESS than static friction.

Tags

NGSS.HS-PS2-1

The diagram represents a block sliding down an incline. Which vector best represents the frictional force acting on the block?

• A

• B

• C

• D

Answer explanation

Friction always acts opposite the direction of motion - since they told you the block was sliding DOWN the incline, friction must act UP the incline.

Tags

NGSS.HS-PS2-1

A different force is applied to each of four 1-kilogram blocks to slide them across a uniform steel surface at constant speed as shown below. In which diagram is the coefficient of friction between the block and steel smallest?

• 
• 
• 
• 

Answer explanation

If block is moving at constant speed, forces are balances, and applied force in diagram must equal frictional force.

Since F_f = μF_N, μ and F_f are directly related; so the smaller the F_f, the smaller the coefficient of friction, μ. Therefore the smallest force, 2 N, must have the smallest coefficient.

Tags

NGSS.HS-PS2-1

A 60-kilogram skydiver is falling at a constant speed near the surface of Earth. What is the approximate magnitude of the force of air friction acting on the skydiver?

• 6 N

• 0 N

• 60 N

• 600 N

Answer explanation

Constant speed means forces are balanced! F_g pulling down must be balanced by the force of air friction acting up. So F_g = mg = (60kg)(9.81)= about 600N.

Tags

NGSS.HS-PS2-1

NGSS.HS-PS2-4