Forces, mass and acceleration — GCSE Physics practice questions

Question 1

A team of dogs pulls a sled along flat snow with a total horizontal force of 460 N.
The snow exerts a friction force of 85 N on the sled in the opposite direction.

What is the size of the resultant horizontal force on the sled?

Accepted Answer

B: 375 N. The pull of the dogs and the friction act along the same line in opposite directions, so the resultant is their difference: 460 − 85 = 375 N, in the direction the dogs pull.

Question 2

A sycamore seed spins as it falls from a tree. After a short time it falls at a constant speed.

Which statement explains why its speed becomes constant?

Accepted Answer

B: The air resistance on it has increased to equal its weight.. As the seed speeds up, the upward air resistance increases while its weight stays the same. When the air resistance equals the weight, the resultant force is zero, so the acceleration is zero and the speed stays constant (terminal velocity).

Question 3

A survey robot has a weight of 296 N on the surface of Mars, where g = 3.7 N / kg.
The robot is brought back to the Earth, where g = 9.8 N / kg.

What is the weight of the robot on the Earth?

Accepted Answer

C: 780 N. The mass does not change: m = 296 / 3.7 = 80 kg. On the Earth, W = 80 × 9.8 = 784 N ≈ 780 N. The weight is larger because g is larger on the Earth.