Newton's second law questions

A ball of mass m is projected vertically upwards. An air resistance F acts on the ball at all times. Which of the following equations would apply to the ball as it is travelling upwards?

• mg − F = 0

• mg + F = 0

• mg − F = ma

• mg + F = ma

A van travels along a straight, horizontal road at a constant velocity. Which of the following statements is correct?

• The frictional force of the road on the tyres can be ignored.

• The frictional force of the road on the tyres is equal to the resultant force on the van.

• The frictional force of the road on the tyres is in the direction of motion of the van.

• The frictional force of the road on the tyres is in the opposite direction to the motion of the van.

A rocket is accelerating horizontally due to a constant resultant force. The mass of the rocket decreases steadily as it uses up its fuel. Which graph shows how the acceleration of the rocket could change?

• A

• B

• C

• D

A length of string is attached between two vertical posts. A bird of weight W stands on the middle of the string causing tension T in the string. The string makes an angle of 80° with each post, as shown. Which of the following expressions is correct?

• 
• 
• 
• 

Some students were shown a velocity-time graph for the motion of an object falling through a liquid. The students suggested possible explanations for the decrease in gradient of the graph. Which explanation could be a reason for the gradient of the graph decreasing?

• The drag force increases.

• The gravitational force increases.

• The temperature of the liquid increases.

• The upthrust on the object decreases.

The diagram shows a rocket of mass m accelerating upwards with acceleration a. The diagram represents the forces acting on the rocket. Which of the following equations gives the value of D?

• D = T + m (g - a)

• D = T + m (a + g)

• D = T - m (g - a)

• D = T - m (g + a)