Sec 3 Combined Science (Physics) Dynamics topical questions

State what a resultant force of zero means for the velocity of a body.

Name the force that (a) opposes a box sliding on a floor, (b) pulls a dropped apple downwards, (c) is transmitted along a pulled rope.

Calculate the resultant force that gives a 4.0 kg mass an acceleration of 2.5 m/s².

State one situation where friction is useful and one where it is a nuisance.

A 900 kg car accelerates from rest to 27 m/s in 9.0 s. (a) Calculate the acceleration. (b) Calculate the resultant force. (c) The engine provides 3600 N of drive. Explain why the resultant in (b) is smaller.

Draw (in words) the free-body diagram of a hot-air balloon hovering (motionless) in the air. Name each force, its direction, and state what the diagram shows about their sizes.

A 0.20 kg ball is hit with a resultant force of 15 N. Calculate its acceleration, and its speed 0.10 s after the hit if it starts from rest.

A student says: "The engine makes the force that moves the car forward, so the forward force on the car is the engine pushing the road." Identify the errors and give the correct action–reaction description of a car’s forward drive.

A 70 kg sprinter accelerates from rest to 9.0 m/s in 1.5 s. (a) Calculate the horizontal resultant force. (b) The sprinter’s shoes grip the track by friction; state the direction of the friction force on the sprinter and explain why it points that way. (c) The sprinter’s mass is unchanged but a loaded backpack slows the same acceleration run — explain using F = ma.

A boat’s motor drives it forward with 2000 N while water drag is 1500 N and air resistance is 100 N. The boat and occupants have mass 800 kg. (a) Calculate the acceleration. (b) The boat reaches a steady speed later; state the drag at that moment and explain the change.

A lift of mass 500 kg carries a passenger of mass 70 kg. The lift cable pulls upwards with 6200 N. (a) Calculate the total weight (g = 10 N/kg). (b) Calculate the acceleration of the lift, stating its direction. (c) The lift now rises at constant speed. State the cable tension.

Two students debate: "Friction is a useless force — engineers should try to remove it everywhere." Write a balanced response: give two situations where reducing friction helps, two where friction is essential, and one method of reducing it.