Resultant forces and their effects | Exit Quiz | Oak National Academy

The diagram shows four forces acting on an object. What is the magnitude of the resultant force?

• 1 N

• 5 N

• 6 N

• 7 N

• 13 N

Answer explanation

The sum of forces acting to the left is 4 + 3 = 7 N. The sum of forces acting to the right is 1 + 5 = 6 N. Find the difference: 7 – 6 = 1 N. The magnitude of the resultant force is 1 N. (It acts in the direction of the larger total: to the left.)

Which of the following is not caused by a resultant force acting on an object?

• moving at constant velocity

• changing direction

• start moving

• stop moving

• decelerating and changing direction

Answer explanation

If a resultant force acts on an object, the object’s velocity changes. This can mean a change in speed, a change in direction, or both. A change in speed can make an object start or stop moving.

Which of the following statements are included in Newton’s First Law of motion?

• A stationary object remains stationary if there is no resultant force.

• Every action has an equal and opposite reaction.

• A moving object’s velocity stays the same if no resultant force acts.

• When object A exerts a force on object B, B also exerts a force on A.

Answer explanation

Newton’s First Law states that an object will remain at rest, or in motion in a straight line at constant speed, unless a resultant force acts on it. (The other two statements are descriptions of Newton’s Third Law, which is about pairs of forces.)

A ship travels at a constant speed without changing direction. Which of the following statements are correct?

• There is a steady resultant force acting on the ship.

• There is no resultant force acting on the ship.

• The force pushing it forwards is equal to the force pushing it backwards.

• The force pushing it forwards is greater than the force pushing it backwards.

Answer explanation

Since the motion of the ship is not changing, there must be no resultant force acting. This can only happen if the forwards force equals the backwards force.

A person pushes a stone across a frozen lake. After they let go, the stone slides across the ice, slowing and stopping before it reaches the other side of the lake. Why does the stone slow and stop?

• The person’s pushing force runs out.

• The person’s pushing force transfers to the thermal store.

• There is a resultant force acting in the opposite direction to the motion.

• Other forces cancel out the person’s pushing force.

Answer explanation

There must be a resultant force to make the stone change its motion. The person has stopped pushing, so that pushing force is no longer present. Friction causes the resultant force opposite to the direction of motion. This makes the stone decelerate.

Which of the following vans is decelerating?

• 
• 
• 
• 

Answer explanation

The first van has a resultant force acting opposite to its motion so it is decelerating. The second van has no resultant force so its speed is constant. The last two vans resultant force is in the same direction as their motion so are accelerating.